Add dependency on mailbox from Maven local

Tolerate 404 responses due to missing folders

See merge request briar/briar!1692

.gitlab-ci.yml

@@ -118,10 +118,3 @@ mailbox integration test:
     - cd "$CI_PROJECT_DIR"
     - bramble-core/src/test/bash/wait-for-mailbox.sh
     - OPTIONAL_TESTS=org.briarproject.bramble.mailbox.MailboxIntegrationTest ./gradlew --info bramble-core:test --tests MailboxIntegrationTest
-
-pre_release_tests:
-  extends: .optional_tests
-  script:
-    - OPTIONAL_TESTS=org.briarproject.bramble.plugin.tor.BridgeTest ./gradlew --info bramble-java:test --tests BridgeTest
-  only:
-    - tags

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+Folder-Description:
+===================
+* `briar-*`: Specifically for the Briar app (Phone/Desktop/Headless)
+* `bramble-*`: The protocol stack - not necessarily Briar-dependent
+
+---
+
+* `*-api`: public stuff that can be referenced from other packages and modules - mostly interfaces + a few utility classes
+* `*-core`: implementations of api that are portable across Android/Desktop/Headless
+* `*-java`: implementations of api that are specific to Desktop & Headless

bramble-android/build.gradle

@@ -15,8 +15,8 @@ android {
 	defaultConfig {
 		minSdkVersion 16
 		targetSdkVersion 30
-		versionCode 10406
-		versionName "1.4.6"
+		versionCode 10410
+		versionName "1.4.10"
 		consumerProguardFiles 'proguard-rules.txt'
 
 		testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

bramble-android/src/main/java/org/briarproject/bramble/network/AndroidNetworkManager.java

@@ -11,7 +11,10 @@ import android.net.LinkAddress;
 import android.net.LinkProperties;
 import android.net.Network;
 import android.net.NetworkInfo;
+import android.net.wifi.WifiInfo;
+import android.net.wifi.WifiManager;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.event.EventBus;
 import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.lifecycle.Service;
@@ -21,7 +24,6 @@ import org.briarproject.bramble.api.network.event.NetworkStatusEvent;
 import org.briarproject.bramble.api.nullsafety.MethodsNotNullByDefault;
 import org.briarproject.bramble.api.nullsafety.ParametersNotNullByDefault;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 
 import java.net.Inet4Address;
 import java.net.InetAddress;
@@ -38,6 +40,7 @@ import javax.annotation.Nullable;
 import javax.inject.Inject;
 
 import static android.content.Context.CONNECTIVITY_SERVICE;
+import static android.content.Context.WIFI_SERVICE;
 import static android.content.Intent.ACTION_SCREEN_OFF;
 import static android.content.Intent.ACTION_SCREEN_ON;
 import static android.net.ConnectivityManager.CONNECTIVITY_ACTION;
@@ -111,15 +114,37 @@ class AndroidNetworkManager implements NetworkManager, Service {
 
 	@Override
 	public NetworkStatus getNetworkStatus() {
-		NetworkInfo net = connectivityManager.getActiveNetworkInfo();
-		boolean connected = net != null && net.isConnected();
-		boolean wifi = false, ipv6Only = false;
-		if (connected) {
-			wifi = net.getType() == TYPE_WIFI;
-			if (SDK_INT >= 23) ipv6Only = isActiveNetworkIpv6Only();
-			else ipv6Only = areAllAvailableNetworksIpv6Only();
+		// https://issuetracker.google.com/issues/175055271
+		try {
+			NetworkInfo net = connectivityManager.getActiveNetworkInfo();
+			boolean connected = net != null && net.isConnected();
+			boolean wifi = false, ipv6Only = false;
+			if (connected) {
+				wifi = net.getType() == TYPE_WIFI;
+				if (SDK_INT >= 23) ipv6Only = isActiveNetworkIpv6Only();
+				else ipv6Only = areAllAvailableNetworksIpv6Only();
+			}
+			return new NetworkStatus(connected, wifi, ipv6Only);
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
+			// Without the ConnectivityManager we can't detect whether we have
+			// internet access. Assume we do, which is probably less harmful
+			// than assuming we don't. Likewise, assume the connection is
+			// IPv6-only. Fall back to the WifiManager to detect whether we
+			// have a wifi connection.
+			LOG.info("ConnectivityManager is broken, guessing connectivity");
+			boolean connected = true, wifi = false, ipv6Only = true;
+			WifiManager wm = (WifiManager) app.getSystemService(WIFI_SERVICE);
+			if (wm != null) {
+				WifiInfo info = wm.getConnectionInfo();
+				if (info != null && info.getIpAddress() != 0) {
+					LOG.info("Connected to wifi");
+					wifi = true;
+					ipv6Only = false;
+				}
+			}
+			return new NetworkStatus(connected, wifi, ipv6Only);
 		}
-		return new NetworkStatus(connected, wifi, ipv6Only);
 	}
 
 	/**
@@ -130,23 +155,29 @@ class AndroidNetworkManager implements NetworkManager, Service {
 	 */
 	@TargetApi(23)
 	private boolean isActiveNetworkIpv6Only() {
-		Network net = connectivityManager.getActiveNetwork();
-		if (net == null) {
-			LOG.info("No active network");
+		// https://issuetracker.google.com/issues/175055271
+		try {
+			Network net = connectivityManager.getActiveNetwork();
+			if (net == null) {
+				LOG.info("No active network");
+				return false;
+			}
+			LinkProperties props = connectivityManager.getLinkProperties(net);
+			if (props == null) {
+				LOG.info("No link properties for active network");
+				return false;
+			}
+			boolean hasIpv6Unicast = false;
+			for (LinkAddress linkAddress : props.getLinkAddresses()) {
+				InetAddress addr = linkAddress.getAddress();
+				if (addr instanceof Inet4Address) return false;
+				if (!addr.isMulticastAddress()) hasIpv6Unicast = true;
+			}
+			return hasIpv6Unicast;
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
 			return false;
 		}
-		LinkProperties props = connectivityManager.getLinkProperties(net);
-		if (props == null) {
-			LOG.info("No link properties for active network");
-			return false;
-		}
-		boolean hasIpv6Unicast = false;
-		for (LinkAddress linkAddress : props.getLinkAddresses()) {
-			InetAddress addr = linkAddress.getAddress();
-			if (addr instanceof Inet4Address) return false;
-			if (!addr.isMulticastAddress()) hasIpv6Unicast = true;
-		}
-		return hasIpv6Unicast;
 	}
 
 	/**

bramble-android/src/main/java/org/briarproject/bramble/plugin/file/AndroidRemovableDrivePlugin.java

@@ -32,13 +32,22 @@ class AndroidRemovableDrivePlugin extends RemovableDrivePlugin {
 	InputStream openInputStream(TransportProperties p) throws IOException {
 		String uri = p.get(PROP_URI);
 		if (isNullOrEmpty(uri)) throw new IllegalArgumentException();
-		return app.getContentResolver().openInputStream(Uri.parse(uri));
+		try {
+			return app.getContentResolver().openInputStream(Uri.parse(uri));
+		} catch (SecurityException e) {
+			throw new IOException(e);
+		}
 	}
 
 	@Override
 	OutputStream openOutputStream(TransportProperties p) throws IOException {
 		String uri = p.get(PROP_URI);
 		if (isNullOrEmpty(uri)) throw new IllegalArgumentException();
-		return app.getContentResolver().openOutputStream(Uri.parse(uri), "wt");
+		try {
+			return app.getContentResolver()
+					.openOutputStream(Uri.parse(uri), "wt");
+		} catch (SecurityException e) {
+			throw new IOException(e);
+		}
 	}
 }

bramble-android/src/main/java/org/briarproject/bramble/plugin/tcp/AndroidLanTcpPlugin.java

@@ -175,16 +175,24 @@ class AndroidLanTcpPlugin extends LanTcpPlugin {
 	@TargetApi(21)
 	@Nullable
 	private InetAddress getWifiClientIpv6Address() {
-		for (Network net : connectivityManager.getAllNetworks()) {
-			NetworkCapabilities caps =
-					connectivityManager.getNetworkCapabilities(net);
-			if (caps == null || !caps.hasTransport(TRANSPORT_WIFI)) continue;
-			LinkProperties props = connectivityManager.getLinkProperties(net);
-			if (props == null) continue;
-			for (LinkAddress linkAddress : props.getLinkAddresses()) {
-				InetAddress addr = linkAddress.getAddress();
-				if (isIpv6LinkLocalAddress(addr)) return addr;
+		// https://issuetracker.google.com/issues/175055271
+		try {
+			for (Network net : connectivityManager.getAllNetworks()) {
+				NetworkCapabilities caps =
+						connectivityManager.getNetworkCapabilities(net);
+				if (caps == null || !caps.hasTransport(TRANSPORT_WIFI)) {
+					continue;
+				}
+				LinkProperties props =
+						connectivityManager.getLinkProperties(net);
+				if (props == null) continue;
+				for (LinkAddress linkAddress : props.getLinkAddresses()) {
+					InetAddress addr = linkAddress.getAddress();
+					if (isIpv6LinkLocalAddress(addr)) return addr;
+				}
 			}
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
 		}
 		return null;
 	}
@@ -227,12 +235,17 @@ class AndroidLanTcpPlugin extends LanTcpPlugin {
 	// network's socket factory may try to connect via another network
 	private SocketFactory getSocketFactory() {
 		if (SDK_INT < 21) return SocketFactory.getDefault();
-		for (Network net : connectivityManager.getAllNetworks()) {
-			NetworkCapabilities caps =
-					connectivityManager.getNetworkCapabilities(net);
-			if (caps != null && caps.hasTransport(TRANSPORT_WIFI)) {
-				return net.getSocketFactory();
+		// https://issuetracker.google.com/issues/175055271
+		try {
+			for (Network net : connectivityManager.getAllNetworks()) {
+				NetworkCapabilities caps =
+						connectivityManager.getNetworkCapabilities(net);
+				if (caps != null && caps.hasTransport(TRANSPORT_WIFI)) {
+					return net.getSocketFactory();
+				}
 			}
+		} catch (SecurityException e) {
+			logException(LOG, WARNING, e);
 		}
 		LOG.warning("Could not find suitable socket factory");
 		return SocketFactory.getDefault();

bramble-android/src/main/java/org/briarproject/bramble/plugin/tor/AndroidTorPluginFactory.java

@@ -11,62 +11,35 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.Backoff;
 import org.briarproject.bramble.api.plugin.BackoffFactory;
 import org.briarproject.bramble.api.plugin.PluginCallback;
-import org.briarproject.bramble.api.plugin.TorConstants;
 import org.briarproject.bramble.api.plugin.TorControlPort;
 import org.briarproject.bramble.api.plugin.TorDirectory;
 import org.briarproject.bramble.api.plugin.TorSocksPort;
-import org.briarproject.bramble.api.plugin.TransportId;
-import org.briarproject.bramble.api.plugin.duplex.DuplexPlugin;
-import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
 import org.briarproject.bramble.api.system.AndroidWakeLockManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.LocationUtils;
 import org.briarproject.bramble.api.system.ResourceProvider;
 import org.briarproject.bramble.api.system.WakefulIoExecutor;
-import org.briarproject.bramble.util.AndroidUtils;
 
 import java.io.File;
 import java.util.concurrent.Executor;
-import java.util.logging.Logger;
 
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 import javax.net.SocketFactory;
 
+import static org.briarproject.bramble.util.AndroidUtils.getSupportedArchitectures;
+
 @Immutable
 @NotNullByDefault
-public class AndroidTorPluginFactory implements DuplexPluginFactory {
+public class AndroidTorPluginFactory extends TorPluginFactory {
 
-	private static final Logger LOG =
-			Logger.getLogger(AndroidTorPluginFactory.class.getName());
-
-	private static final int MAX_LATENCY = 30 * 1000; // 30 seconds
-	private static final int MAX_IDLE_TIME = 30 * 1000; // 30 seconds
-	private static final int MIN_POLLING_INTERVAL = 60 * 1000; // 1 minute
-	private static final int MAX_POLLING_INTERVAL = 10 * 60 * 1000; // 10 mins
-	private static final double BACKOFF_BASE = 1.2;
-
-	private final Executor ioExecutor, wakefulIoExecutor;
 	private final Application app;
-	private final NetworkManager networkManager;
-	private final LocationUtils locationUtils;
-	private final EventBus eventBus;
-	private final SocketFactory torSocketFactory;
-	private final BackoffFactory backoffFactory;
-	private final ResourceProvider resourceProvider;
-	private final CircumventionProvider circumventionProvider;
-	private final BatteryManager batteryManager;
 	private final AndroidWakeLockManager wakeLockManager;
-	private final Clock clock;
-	private final File torDirectory;
-	private int torSocksPort;
-	private int torControlPort;
-	private final CryptoComponent crypto;
 
 	@Inject
 	AndroidTorPluginFactory(@IoExecutor Executor ioExecutor,
 			@WakefulIoExecutor Executor wakefulIoExecutor,
-			Application app,
 			NetworkManager networkManager,
 			LocationUtils locationUtils,
 			EventBus eventBus,
@@ -75,80 +48,43 @@ public class AndroidTorPluginFactory implements DuplexPluginFactory {
 			ResourceProvider resourceProvider,
 			CircumventionProvider circumventionProvider,
 			BatteryManager batteryManager,
-			AndroidWakeLockManager wakeLockManager,
 			Clock clock,
+			CryptoComponent crypto,
 			@TorDirectory File torDirectory,
 			@TorSocksPort int torSocksPort,
 			@TorControlPort int torControlPort,
-			CryptoComponent crypto) {
-		this.ioExecutor = ioExecutor;
-		this.wakefulIoExecutor = wakefulIoExecutor;
+			Application app,
+			AndroidWakeLockManager wakeLockManager) {
+		super(ioExecutor, wakefulIoExecutor, networkManager, locationUtils,
+				eventBus, torSocketFactory, backoffFactory, resourceProvider,
+				circumventionProvider, batteryManager, clock, crypto,
+				torDirectory, torSocksPort, torControlPort);
 		this.app = app;
-		this.networkManager = networkManager;
-		this.locationUtils = locationUtils;
-		this.eventBus = eventBus;
-		this.torSocketFactory = torSocketFactory;
-		this.backoffFactory = backoffFactory;
-		this.resourceProvider = resourceProvider;
-		this.circumventionProvider = circumventionProvider;
-		this.batteryManager = batteryManager;
 		this.wakeLockManager = wakeLockManager;
-		this.clock = clock;
-		this.torDirectory = torDirectory;
-		this.torSocksPort = torSocksPort;
-		this.torControlPort = torControlPort;
-		this.crypto = crypto;
 	}
 
+	@Nullable
 	@Override
-	public TransportId getId() {
-		return TorConstants.ID;
-	}
-
-	@Override
-	public long getMaxLatency() {
-		return MAX_LATENCY;
-	}
-
-	@Override
-	public DuplexPlugin createPlugin(PluginCallback callback) {
-
-		// Check that we have a Tor binary for this architecture
-		String architecture = null;
-		for (String abi : AndroidUtils.getSupportedArchitectures()) {
-			if (abi.startsWith("x86_64")) {
-				architecture = "x86_64";
-				break;
-			} else if (abi.startsWith("x86")) {
-				architecture = "x86";
-				break;
-			} else if (abi.startsWith("arm64")) {
-				architecture = "arm64";
-				break;
-			} else if (abi.startsWith("armeabi")) {
-				architecture = "arm";
-				break;
-			}
+	String getArchitectureForTorBinary() {
+		for (String abi : getSupportedArchitectures()) {
+			if (abi.startsWith("x86_64")) return "x86_64_pie";
+			else if (abi.startsWith("x86")) return "x86_pie";
+			else if (abi.startsWith("arm64")) return "arm64_pie";
+			else if (abi.startsWith("armeabi")) return "arm_pie";
 		}
-		if (architecture == null) {
-			LOG.info("Tor is not supported on this architecture");
-			return null;
-		}
-		// Use position-independent executable
-		architecture += "_pie";
+		return null;
+	}
 
-		Backoff backoff = backoffFactory.createBackoff(MIN_POLLING_INTERVAL,
-				MAX_POLLING_INTERVAL, BACKOFF_BASE);
-		TorRendezvousCrypto torRendezvousCrypto =
-				new TorRendezvousCryptoImpl(crypto);
-		AndroidTorPlugin plugin = new AndroidTorPlugin(ioExecutor,
+	@Override
+	TorPlugin createPluginInstance(Backoff backoff,
+			TorRendezvousCrypto torRendezvousCrypto, PluginCallback callback,
+			String architecture) {
+		return new AndroidTorPlugin(ioExecutor,
 				wakefulIoExecutor, app, networkManager, locationUtils,
 				torSocketFactory, clock, resourceProvider,
 				circumventionProvider, batteryManager, wakeLockManager,
 				backoff, torRendezvousCrypto, callback, architecture,
 				MAX_LATENCY, MAX_IDLE_TIME, torDirectory, torSocksPort,
 				torControlPort);
-		eventBus.addListener(plugin);
-		return plugin;
 	}
 }

bramble-android/src/main/java/org/briarproject/bramble/system/AndroidTaskScheduler.java

@@ -8,6 +8,7 @@ import android.content.Intent;
 import android.os.Process;
 import android.os.SystemClock;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.lifecycle.Service;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.AlarmListener;
@@ -116,10 +117,12 @@ class AndroidTaskScheduler implements TaskScheduler, Service, AlarmListener {
 		long dueMillis = now + MILLISECONDS.convert(delay, unit);
 		Runnable wakeful = () ->
 				wakeLockManager.executeWakefully(task, executor, "TaskHandoff");
-		Future<?> check = scheduleCheckForDueTasks(delay, unit);
-		ScheduledTask s = new ScheduledTask(wakeful, dueMillis, check,
-				cancelled);
+		// Acquire the lock before scheduling the check to ensure the check
+		// doesn't access the task queue before the task has been added
+		ScheduledTask s;
 		synchronized (lock) {
+			Future<?> check = scheduleCheckForDueTasks(delay, unit);
+			s = new ScheduledTask(wakeful, dueMillis, check, cancelled);
 			tasks.add(s);
 		}
 		return s;
@@ -136,6 +139,7 @@ class AndroidTaskScheduler implements TaskScheduler, Service, AlarmListener {
 		return schedule(wrapped, executor, delay, unit, cancelled);
 	}
 
+	@GuardedBy("lock")
 	private Future<?> scheduleCheckForDueTasks(long delay, TimeUnit unit) {
 		Runnable wakeful = () -> wakeLockManager.runWakefully(
 				this::runDueTasks, "TaskScheduler");
@@ -206,7 +210,7 @@ class AndroidTaskScheduler implements TaskScheduler, Service, AlarmListener {
 		private final Future<?> check;
 		private final AtomicBoolean cancelled;
 
-		public ScheduledTask(Runnable task, long dueMillis,
+		private ScheduledTask(Runnable task, long dueMillis,
 				Future<?> check, AtomicBoolean cancelled) {
 			this.task = task;
 			this.dueMillis = dueMillis;

bramble-android/src/main/java/org/briarproject/bramble/util/AndroidUtils.java

@@ -4,6 +4,7 @@ import android.annotation.SuppressLint;
 import android.bluetooth.BluetoothAdapter;
 import android.content.Context;
 import android.os.Build;
+import android.os.Looper;
 import android.provider.Settings;
 
 import org.briarproject.bramble.api.Pair;
@@ -134,4 +135,8 @@ public class AndroidUtils {
 			return null;
 		}
 	}
+
+	public static boolean isUiThread() {
+		return Looper.myLooper() == Looper.getMainLooper();
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/Cancellable.java

@@ -0,0 +1,6 @@
+package org.briarproject.bramble.api;
+
+public interface Cancellable {
+
+	void cancel();
+}

bramble-api/src/main/java/org/briarproject/bramble/api/StringMap.java

@@ -1,8 +1,14 @@
 package org.briarproject.bramble.api;
 
+import org.briarproject.bramble.util.StringUtils;
+
+import java.util.ArrayList;
 import java.util.Hashtable;
+import java.util.List;
 import java.util.Map;
 
+import javax.annotation.Nullable;
+
 public abstract class StringMap extends Hashtable<String, String> {
 
 	protected StringMap(Map<String, String> m) {
@@ -52,4 +58,31 @@ public abstract class StringMap extends Hashtable<String, String> {
 	public void putLong(String key, long value) {
 		put(key, String.valueOf(value));
 	}
+
+	@Nullable
+	public int[] getIntArray(String key) {
+		String s = get(key);
+		if (s == null) return null;
+		// Handle empty string because "".split(",") returns {""}
+		if (s.length() == 0) return new int[0];
+		String[] intStrings = s.split(",");
+		int[] ints = new int[intStrings.length];
+		try {
+			for (int i = 0; i < ints.length; i++) {
+				ints[i] = Integer.parseInt(intStrings[i]);
+			}
+		} catch (NumberFormatException e) {
+			return null;
+		}
+		return ints;
+	}
+
+	public void putIntArray(String key, int[] value) {
+		List<String> intStrings = new ArrayList<>();
+		for (int integer : value) {
+			intStrings.add(String.valueOf(integer));
+		}
+		// Puts empty string if input array value is empty
+		put(key, StringUtils.join(intStrings, ","));
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/client/ClientHelper.java

@@ -9,7 +9,8 @@ import org.briarproject.bramble.api.data.BdfList;
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.identity.Author;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportProperties;
@@ -19,10 +20,9 @@ import org.briarproject.bramble.api.sync.MessageId;
 
 import java.security.GeneralSecurityException;
 import java.util.Collection;
+import java.util.List;
 import java.util.Map;
 
-import javax.annotation.Nullable;
-
 @NotNullByDefault
 public interface ClientHelper {
 
@@ -127,16 +127,17 @@ public interface ClientHelper {
 			BdfDictionary properties) throws FormatException;
 
 	/**
-	 * Parse and validate the property dictionary of a Mailbox property update
-	 * message.
+	 * Parse and validate the elements of a Mailbox update message.
 	 *
-	 * @return the properties for using the Mailbox, or null if there is no
-	 * Mailbox available
-	 * @throws FormatException if the properties are not valid
+	 * @return the parsed update message
+	 * @throws FormatException if the message elements are invalid
 	 */
-	@Nullable
-	MailboxPropertiesUpdate parseAndValidateMailboxPropertiesUpdate(
-			BdfDictionary properties) throws FormatException;
+	MailboxUpdate parseAndValidateMailboxUpdate(BdfList clientSupports,
+			BdfList serverSupports, BdfDictionary properties)
+			throws FormatException;
+
+	List<MailboxVersion> parseMailboxVersionList(BdfList bdfList)
+			throws FormatException;
 
 	/**
 	 * Retrieves the contact ID from the group metadata of the given contact

bramble-api/src/main/java/org/briarproject/bramble/api/connection/ConnectionManager.java

@@ -7,6 +7,7 @@ import org.briarproject.bramble.api.plugin.TransportConnectionReader;
 import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
 
 @NotNullByDefault
 public interface ConnectionManager {
@@ -16,6 +17,17 @@ public interface ConnectionManager {
 	 */
 	void manageIncomingConnection(TransportId t, TransportConnectionReader r);
 
+	/**
+	 * Manages an incoming connection from a contact via a mailbox.
+	 * <p>
+	 * This method does not mark the tag as recognised until after the data
+	 * has been read from the {@link TransportConnectionReader}, at which
+	 * point the {@link TagController} is called to decide whether the tag
+	 * should be marked as recognised.
+	 */
+	void manageIncomingConnection(TransportId t, TransportConnectionReader r,
+			TagController c);
+
 	/**
 	 * Manages an incoming connection from a contact over a duplex transport.
 	 */
@@ -34,6 +46,14 @@ public interface ConnectionManager {
 	void manageOutgoingConnection(ContactId c, TransportId t,
 			TransportConnectionWriter w);
 
+	/**
+	 * Manages an outgoing connection to a contact via a mailbox. The IDs of
+	 * any messages sent or acked are added to the given
+	 * {@link OutgoingSessionRecord}.
+	 */
+	void manageOutgoingConnection(ContactId c, TransportId t,
+			TransportConnectionWriter w, OutgoingSessionRecord sessionRecord);
+
 	/**
 	 * Manages an outgoing connection to a contact over a duplex transport.
 	 */
@@ -46,4 +66,21 @@ public interface ConnectionManager {
 	 */
 	void manageOutgoingConnection(PendingContactId p, TransportId t,
 			DuplexTransportConnection d);
+
+	/**
+	 * An interface for controlling whether a tag should be marked as
+	 * recognised.
+	 */
+	interface TagController {
+		/**
+		 * This method is only called if a tag was read from the corresponding
+		 * {@link TransportConnectionReader} and recognised.
+		 *
+		 * @param exception True if an exception was thrown while reading from
+		 * the {@link TransportConnectionReader}, after successfully reading
+		 * and recognising the tag.
+		 * @return True if the tag should be marked as recognised.
+		 */
+		boolean shouldMarkTagAsRecognised(boolean exception);
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseComponent.java

@@ -33,11 +33,18 @@ import java.util.List;
 import java.util.Map;
 
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
 
 /**
  * Encapsulates the database implementation and exposes high-level operations
  * to other components.
+ * <p>
+ * With the exception of the {@link #open(SecretKey, MigrationListener)} and
+ * {@link #close()} methods, which must not be called concurrently, the
+ * database can be accessed from any thread. See {@link TransactionManager}
+ * for locking behaviour.
  */
+@ThreadSafe
 @NotNullByDefault
 public interface DatabaseComponent extends TransactionManager {
 
@@ -119,16 +126,11 @@ public interface DatabaseComponent extends TransactionManager {
 			TransportKeys k) throws DbException;
 
 	/**
-	 * Returns true if there are any acks or messages to send to the given
-	 * contact over a transport with the given maximum latency.
+	 * Returns true if there are any acks to send to the given contact.
 	 * <p/>
 	 * Read-only.
-	 *
-	 * @param eager True if messages that are not yet due for retransmission
-	 * should be included
 	 */
-	boolean containsAnythingToSend(Transaction txn, ContactId c,
-			long maxLatency, boolean eager) throws DbException;
+	boolean containsAcksToSend(Transaction txn, ContactId c) throws DbException;
 
 	/**
 	 * Returns true if the database contains the given contact for the given
@@ -154,6 +156,18 @@ public interface DatabaseComponent extends TransactionManager {
 	 */
 	boolean containsIdentity(Transaction txn, AuthorId a) throws DbException;
 
+	/**
+	 * Returns true if there are any messages to send to the given contact
+	 * over a transport with the given maximum latency.
+	 * <p/>
+	 * Read-only.
+	 *
+	 * @param eager True if messages that are not yet due for retransmission
+	 * should be included
+	 */
+	boolean containsMessagesToSend(Transaction txn, ContactId c,
+			long maxLatency, boolean eager) throws DbException;
+
 	/**
 	 * Returns true if the database contains the given pending contact.
 	 * <p/>
@@ -193,26 +207,15 @@ public interface DatabaseComponent extends TransactionManager {
 			throws DbException;
 
 	/**
-	 * Returns a batch of messages for the given contact, with a total length
-	 * less than or equal to the given length, for transmission over a
-	 * transport with the given maximum latency. Returns null if there are no
-	 * sendable messages that fit in the given length.
+	 * Returns a batch of messages for the given contact, for transmission over
+	 * a transport with the given maximum latency. The total length of the
+	 * messages, including record headers, will be no more than the given
+	 * capacity. Returns null if there are no sendable messages that would fit
+	 * in the given capacity.
 	 */
 	@Nullable
 	Collection<Message> generateBatch(Transaction txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException;
-
-	/**
-	 * Returns a batch of messages for the given contact containing the
-	 * messages with the given IDs, for transmission over a transport with
-	 * the given maximum latency.
-	 * <p/>
-	 * If any of the given messages are not in the database or are not visible
-	 * to the contact, they are omitted from the batch without throwing an
-	 * exception.
-	 */
-	Collection<Message> generateBatch(Transaction txn, ContactId c,
-			Collection<MessageId> ids, long maxLatency) throws DbException;
+			long capacity, long maxLatency) throws DbException;
 
 	/**
 	 * Returns an offer for the given contact for transmission over a
@@ -232,15 +235,16 @@ public interface DatabaseComponent extends TransactionManager {
 			throws DbException;
 
 	/**
-	 * Returns a batch of messages for the given contact, with a total length
-	 * less than or equal to the given length, for transmission over a
-	 * transport with the given maximum latency. Only messages that have been
-	 * requested by the contact are returned. Returns null if there are no
-	 * sendable messages that fit in the given length.
+	 * Returns a batch of messages for the given contact, for transmission over
+	 * a transport with the given maximum latency. Only messages that have been
+	 * requested by the contact are returned. The total length of the messages,
+	 * including record headers, will be no more than the given capacity.
+	 * Returns null if there are no sendable messages that have been requested
+	 * by the contact and would fit in the given capacity.
 	 */
 	@Nullable
 	Collection<Message> generateRequestedBatch(Transaction txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException;
+			long capacity, long maxLatency) throws DbException;
 
 	/**
 	 * Returns the contact with the given ID.
@@ -344,6 +348,30 @@ public interface DatabaseComponent extends TransactionManager {
 	Collection<MessageId> getMessageIds(Transaction txn, GroupId g,
 			Metadata query) throws DbException;
 
+	/**
+	 * Returns the IDs of some messages received from the given contact that
+	 * need to be acknowledged, up to the given number of messages.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<MessageId> getMessagesToAck(Transaction txn, ContactId c,
+			int maxMessages) throws DbException;
+
+	/**
+	 * Returns the IDs of some messages that are eligible to be sent to the
+	 * given contact over a transport with the given maximum latency. The total
+	 * length of the messages including record headers will be no more than the
+	 * given capacity.
+	 * <p/>
+	 * Unlike {@link #getUnackedMessagesToSend(Transaction, ContactId)} this
+	 * method does not return messages that have already been sent unless they
+	 * are due for retransmission.
+	 * <p/>
+	 * Read-only.
+	 */
+	Collection<MessageId> getMessagesToSend(Transaction txn, ContactId c,
+			long capacity, long maxLatency) throws DbException;
+
 	/**
 	 * Returns the IDs of any messages that need to be validated.
 	 * <p/>
@@ -460,15 +488,30 @@ public interface DatabaseComponent extends TransactionManager {
 	MessageStatus getMessageStatus(Transaction txn, ContactId c, MessageId m)
 			throws DbException;
 
+	/**
+	 * Returns the message with the given ID for transmission to the given
+	 * contact over a transport with the given maximum latency. Returns null
+	 * if the message is no longer visible to the contact.
+	 *
+	 * @param markAsSent True if the message should be marked as sent.
+	 * If false it can be marked as sent by calling
+	 * {@link #setMessagesSent(Transaction, ContactId, Collection, long)}.
+	 */
+	@Nullable
+	Message getMessageToSend(Transaction txn, ContactId c, MessageId m,
+			long maxLatency, boolean markAsSent) throws DbException;
+
 	/**
 	 * Returns the IDs of all messages that are eligible to be sent to the
-	 * given contact, together with their raw lengths. This may include
-	 * messages that have already been sent and are not yet due for
-	 * retransmission.
+	 * given contact.
+	 * <p>
+	 * Unlike {@link #getMessagesToSend(Transaction, ContactId, long, long)}
+	 * this method may return messages that have already been sent and are
+	 * not yet due for retransmission.
 	 * <p/>
 	 * Read-only.
 	 */
-	Map<MessageId, Integer> getUnackedMessagesToSend(Transaction txn,
+	Collection<MessageId> getUnackedMessagesToSend(Transaction txn,
 			ContactId c) throws DbException;
 
 	/**
@@ -498,15 +541,18 @@ public interface DatabaseComponent extends TransactionManager {
 	 */
 	long getNextCleanupDeadline(Transaction txn) throws DbException;
 
-	/*
+	/**
 	 * Returns the next time (in milliseconds since the Unix epoch) when a
-	 * message is due to be sent to the given contact. The returned value may
-	 * be zero if a message is due to be sent immediately, or Long.MAX_VALUE if
-	 * no messages are scheduled to be sent.
+	 * message is due to be sent to the given contact over a transport with
+	 * the given latency.
+	 * <p>
+	 * The returned value may be zero if a message is due to be sent
+	 * immediately, or Long.MAX_VALUE if no messages are scheduled to be sent.
 	 * <p/>
 	 * Read-only.
 	 */
-	long getNextSendTime(Transaction txn, ContactId c) throws DbException;
+	long getNextSendTime(Transaction txn, ContactId c, long maxLatency)
+			throws DbException;
 
 	/**
 	 * Returns the pending contact with the given ID.
@@ -648,6 +694,13 @@ public interface DatabaseComponent extends TransactionManager {
 	void removeTransportKeys(Transaction txn, TransportId t, KeySetId k)
 			throws DbException;
 
+	/**
+	 * Records an ack for the given messages as having been sent to the given
+	 * contact.
+	 */
+	void setAckSent(Transaction txn, ContactId c, Collection<MessageId> acked)
+			throws DbException;
+
 	/**
 	 * Sets the cleanup timer duration for the given message. This does not
 	 * start the message's cleanup timer.
@@ -694,6 +747,13 @@ public interface DatabaseComponent extends TransactionManager {
 	void setMessageState(Transaction txn, MessageId m, MessageState state)
 			throws DbException;
 
+	/**
+	 * Records the given messages as having been sent to the given contact
+	 * over a transport with the given maximum latency.
+	 */
+	void setMessagesSent(Transaction txn, ContactId c,
+			Collection<MessageId> sent, long maxLatency) throws DbException;
+
 	/**
 	 * Adds dependencies for a message
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/db/DatabaseExecutor.java

@@ -18,6 +18,10 @@ import static java.lang.annotation.RetentionPolicy.RUNTIME;
  * submitted, tasks are not run concurrently, and submitting a task will never
  * block. Tasks must not run indefinitely. Tasks submitted during shutdown are
  * discarded.
+ * <p>
+ * It is not mandatory to use this executor for database tasks. The database
+ * can be accessed from any thread, but this executor's guarantee that tasks
+ * are run in the order they're submitted may be useful in some cases.
  */
 @Qualifier
 @Target({FIELD, METHOD, PARAMETER})

bramble-api/src/main/java/org/briarproject/bramble/api/db/Transaction.java

@@ -45,6 +45,9 @@ public class Transaction {
 	/**
 	 * Attaches an event to be broadcast when the transaction has been
 	 * committed. The event will be broadcast on the {@link EventExecutor}.
+	 * Events and {@link #attach(Runnable) tasks} are submitted to the
+	 * {@link EventExecutor} in the order they were attached to the
+	 * transaction.
 	 */
 	public void attach(Event e) {
 		if (actions == null) actions = new ArrayList<>();
@@ -54,6 +57,9 @@ public class Transaction {
 	/**
 	 * Attaches a task to be executed when the transaction has been
 	 * committed. The task will be run on the {@link EventExecutor}.
+	 * {@link #attach(Event) Events} and tasks are submitted to the
+	 * {@link EventExecutor} in the order they were attached to the
+	 * transaction.
 	 */
 	public void attach(Runnable r) {
 		if (actions == null) actions = new ArrayList<>();

bramble-api/src/main/java/org/briarproject/bramble/api/db/TransactionManager.java

@@ -1,51 +1,95 @@
 package org.briarproject.bramble.api.db;
 
+import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import javax.annotation.Nullable;
+import javax.annotation.concurrent.ThreadSafe;
 
+/**
+ * An interface for managing database transactions.
+ * <p>
+ * Read-only transactions may access the database concurrently. Read-write
+ * transactions access the database exclusively, so starting a read-only or
+ * read-write transaction will block until there are no read-write
+ * transactions in progress.
+ * <p>
+ * Failing to {@link #endTransaction(Transaction) end} a transaction will
+ * prevent other callers from accessing the database, so it is recommended to
+ * use the {@link #transaction(boolean, DbRunnable)},
+ * {@link #transactionWithResult(boolean, DbCallable)} and
+ * {@link #transactionWithNullableResult(boolean, NullableDbCallable)} methods
+ * where possible, which handle committing or aborting the transaction on the
+ * caller's behalf.
+ * <p>
+ * Transactions are not reentrant, i.e. it is not permitted to start a
+ * transaction on a thread that already has a transaction in progress.
+ */
+@ThreadSafe
 @NotNullByDefault
 public interface TransactionManager {
 
 	/**
-	 * Starts a new transaction and returns an object representing it.
-	 * <p/>
-	 * This method acquires locks, so it must not be called while holding a
-	 * lock.
+	 * Starts a new transaction and returns an object representing it. This
+	 * method acquires the database lock, which is held until
+	 * {@link #endTransaction(Transaction)} is called.
 	 *
-	 * @param readOnly true if the transaction will only be used for reading.
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	Transaction startTransaction(boolean readOnly) throws DbException;
 
 	/**
 	 * Commits a transaction to the database.
+	 * {@link #endTransaction(Transaction)} must be called to release the
+	 * database lock.
 	 */
 	void commitTransaction(Transaction txn) throws DbException;
 
 	/**
-	 * Ends a transaction. If the transaction has not been committed,
-	 * it will be aborted. If the transaction has been committed,
-	 * any events attached to the transaction are broadcast.
-	 * The database lock will be released in either case.
+	 * Ends a transaction. If the transaction has not been committed by
+	 * calling {@link #commitTransaction(Transaction)}, it is aborted and the
+	 * database lock is released.
+	 * <p>
+	 * If the transaction has been committed, any
+	 * {@link Transaction#attach events} attached to the transaction are
+	 * broadcast and any {@link Transaction#attach(Runnable) tasks} attached
+	 * to the transaction are submitted to the {@link EventExecutor}. The
+	 * database lock is then released.
 	 */
 	void endTransaction(Transaction txn);
 
 	/**
-	 * Runs the given task within a transaction.
+	 * Runs the given task within a transaction. The database lock is held
+	 * while running the task.
+	 *
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	<E extends Exception> void transaction(boolean readOnly,
 			DbRunnable<E> task) throws DbException, E;
 
 	/**
 	 * Runs the given task within a transaction and returns the result of the
-	 * task.
+	 * task. The database lock is held while running the task.
+	 *
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	<R, E extends Exception> R transactionWithResult(boolean readOnly,
 			DbCallable<R, E> task) throws DbException, E;
 
 	/**
 	 * Runs the given task within a transaction and returns the result of the
-	 * task, which may be null.
+	 * task, which may be null. The database lock is held while running the
+	 * task.
+	 *
+	 * @param readOnly True if the transaction will only be used for reading,
+	 * in which case the database lock can be shared with other read-only
+	 * transactions.
 	 */
 	@Nullable
 	<R, E extends Exception> R transactionWithNullableResult(boolean readOnly,

bramble-api/src/main/java/org/briarproject/bramble/api/lifecycle/LifecycleManager.java

@@ -37,8 +37,14 @@ public interface LifecycleManager {
 	 */
 	enum LifecycleState {
 
-		STARTING, MIGRATING_DATABASE, COMPACTING_DATABASE, STARTING_SERVICES,
-		RUNNING, STOPPING;
+		CREATED,
+		STARTING,
+		MIGRATING_DATABASE,
+		COMPACTING_DATABASE,
+		STARTING_SERVICES,
+		RUNNING,
+		STOPPING,
+		STOPPED;
 
 		public boolean isAfter(LifecycleState state) {
 			return ordinal() > state.ordinal();

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxConstants.java

@@ -0,0 +1,73 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.plugin.TransportId;
+
+import java.util.List;
+
+import static java.util.Collections.singletonList;
+import static java.util.concurrent.TimeUnit.DAYS;
+import static java.util.concurrent.TimeUnit.HOURS;
+import static org.briarproject.bramble.api.transport.TransportConstants.MAX_FRAME_LENGTH;
+import static org.briarproject.bramble.api.transport.TransportConstants.MAX_PAYLOAD_LENGTH;
+import static org.briarproject.bramble.api.transport.TransportConstants.STREAM_HEADER_LENGTH;
+import static org.briarproject.bramble.api.transport.TransportConstants.TAG_LENGTH;
+
+public interface MailboxConstants {
+
+	/**
+	 * The transport ID of the mailbox plugin.
+	 */
+	TransportId ID = new TransportId("org.briarproject.bramble.mailbox");
+
+	/**
+	 * Mailbox API versions that we support as a client. This is reported to our
+	 * contacts by {@link MailboxUpdateManager}.
+	 */
+	List<MailboxVersion> CLIENT_SUPPORTS = singletonList(
+			new MailboxVersion(1, 0));
+
+	/**
+	 * The constant returned by
+	 * {@link MailboxHelper#getHighestCommonMajorVersion(List, List)}
+	 * when the server is too old to support our major version.
+	 */
+	int API_SERVER_TOO_OLD = -1;
+
+	/**
+	 * The constant returned by
+	 * {@link MailboxHelper#getHighestCommonMajorVersion(List, List)}
+	 * when we as a client are too old to support the server's major version.
+	 */
+	int API_CLIENT_TOO_OLD = -2;
+
+	/**
+	 * The maximum length of a file that can be uploaded to or downloaded from
+	 * a mailbox.
+	 */
+	int MAX_FILE_BYTES = 1024 * 1024;
+
+	/**
+	 * The maximum length of the plaintext payload of a file, such that the
+	 * ciphertext is no more than {@link #MAX_FILE_BYTES}.
+	 */
+	int MAX_FILE_PAYLOAD_BYTES =
+			(MAX_FILE_BYTES - TAG_LENGTH - STREAM_HEADER_LENGTH)
+					/ MAX_FRAME_LENGTH * MAX_PAYLOAD_LENGTH;
+
+	/**
+	 * The number of connection failures
+	 * that indicate a problem with the mailbox.
+	 */
+	int PROBLEM_NUM_CONNECTION_FAILURES = 5;
+
+	/**
+	 * The time in milliseconds since the last connection success
+	 * that need to pass to indicates a problem with the mailbox.
+	 */
+	long PROBLEM_MS_SINCE_LAST_SUCCESS = HOURS.toMillis(1);
+
+	/**
+	 * The maximum latency of the mailbox transport in milliseconds.
+	 */
+	long MAX_LATENCY = DAYS.toMillis(14);
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxDirectory.java

@@ -0,0 +1,22 @@
+package org.briarproject.bramble.api.mailbox;
+
+import java.io.File;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import javax.inject.Qualifier;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.ElementType.PARAMETER;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+/**
+ * Annotation for injecting the {@link File directory} where the Mailbox plugin
+ * should store its state.
+ */
+@Qualifier
+@Target({FIELD, METHOD, PARAMETER})
+@Retention(RUNTIME)
+public @interface MailboxDirectory {
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxHelper.java

@@ -0,0 +1,35 @@
+package org.briarproject.bramble.api.mailbox;
+
+import java.util.List;
+import java.util.TreeSet;
+
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.API_CLIENT_TOO_OLD;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.API_SERVER_TOO_OLD;
+
+class MailboxHelper {
+
+	/**
+	 * Returns the highest major version that both client and server support
+	 * or {@link MailboxConstants#API_SERVER_TOO_OLD} if the server is too old
+	 * or {@link MailboxConstants#API_CLIENT_TOO_OLD} if the client is too old.
+	 */
+	static int getHighestCommonMajorVersion(
+			List<MailboxVersion> client, List<MailboxVersion> server) {
+		TreeSet<Integer> clientVersions = new TreeSet<>();
+		for (MailboxVersion version : client) {
+			clientVersions.add(version.getMajor());
+		}
+		TreeSet<Integer> serverVersions = new TreeSet<>();
+		for (MailboxVersion version : server) {
+			serverVersions.add(version.getMajor());
+		}
+		for (int clientVersion : clientVersions.descendingSet()) {
+			if (serverVersions.contains(clientVersion)) return clientVersion;
+		}
+		if (clientVersions.last() < serverVersions.last()) {
+			return API_CLIENT_TOO_OLD;
+		}
+		return API_SERVER_TOO_OLD;
+	}
+
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxId.java

@@ -2,6 +2,7 @@ package org.briarproject.bramble.api.mailbox;
 
 import com.fasterxml.jackson.annotation.JsonValue;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.UniqueId;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
@@ -32,7 +33,7 @@ public abstract class MailboxId extends UniqueId {
 		}
 		try {
 			return fromHexString(token);
-		} catch (IllegalArgumentException e) {
+		} catch (FormatException e) {
 			throw new InvalidMailboxIdException();
 		}
 	}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxManager.java

@@ -2,6 +2,8 @@ package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.event.OwnMailboxConnectionStatusEvent;
 
 import javax.annotation.Nullable;
 
@@ -32,4 +34,23 @@ public interface MailboxManager {
 	 */
 	MailboxPairingTask startPairingTask(String qrCodePayload);
 
+	/**
+	 * Can be used by the UI to test the mailbox connection.
+	 *
+	 * @return true (success) or false (error).
+	 * A {@link OwnMailboxConnectionStatusEvent} might be broadcast with a new
+	 * {@link MailboxStatus}.
+	 */
+	boolean checkConnection();
+
+	/**
+	 * Unpairs the owner's mailbox and tries to wipe it.
+	 * As this makes a network call, it should be run on the {@link IoExecutor}.
+	 *
+	 * @return true if we could wipe the mailbox, false if we couldn't.
+	 * It is advised to inform the user to wipe the mailbox themselves,
+	 * if we failed to wipe it.
+	 */
+	@IoExecutor
+	boolean unPair() throws DbException;
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxProperties.java

@@ -2,30 +2,56 @@ package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
+import java.util.List;
+
+import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 
 @Immutable
 @NotNullByDefault
 public class MailboxProperties {
 
-	private final String baseUrl;
+	private final String onion;
 	private final MailboxAuthToken authToken;
 	private final boolean owner;
+	private final List<MailboxVersion> serverSupports;
+	@Nullable
+	private final MailboxFolderId inboxId; // Null for own mailbox
+	@Nullable
+	private final MailboxFolderId outboxId; // Null for own mailbox
 
-	public MailboxProperties(String baseUrl, MailboxAuthToken authToken,
-			boolean owner) {
-		this.baseUrl = baseUrl;
+	/**
+	 * Constructor for properties used by the mailbox's owner.
+	 */
+	public MailboxProperties(String onion, MailboxAuthToken authToken,
+			List<MailboxVersion> serverSupports) {
+		this.onion = onion;
 		this.authToken = authToken;
-		this.owner = owner;
+		this.owner = true;
+		this.serverSupports = serverSupports;
+		this.inboxId = null;
+		this.outboxId = null;
 	}
 
-	public String getBaseUrl() {
-		return baseUrl;
+	/**
+	 * Constructor for properties used by a contact of the mailbox's owner.
+	 */
+	public MailboxProperties(String onion, MailboxAuthToken authToken,
+			List<MailboxVersion> serverSupports, MailboxFolderId inboxId,
+			MailboxFolderId outboxId) {
+		this.onion = onion;
+		this.authToken = authToken;
+		this.owner = false;
+		this.serverSupports = serverSupports;
+		this.inboxId = inboxId;
+		this.outboxId = outboxId;
 	}
 
+	/**
+	 * Returns the onion address of the mailbox, excluding the .onion suffix.
+	 */
 	public String getOnion() {
-		return baseUrl.replaceFirst("^http://", "")
-				.replaceFirst("\\.onion$", "");
+		return onion;
 	}
 
 	public MailboxAuthToken getAuthToken() {
@@ -35,4 +61,18 @@ public class MailboxProperties {
 	public boolean isOwner() {
 		return owner;
 	}
+
+	public List<MailboxVersion> getServerSupports() {
+		return serverSupports;
+	}
+
+	@Nullable
+	public MailboxFolderId getInboxId() {
+		return inboxId;
+	}
+
+	@Nullable
+	public MailboxFolderId getOutboxId() {
+		return outboxId;
+	}
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxPropertiesUpdate.java

@@ -1,41 +0,0 @@
-package org.briarproject.bramble.api.mailbox;
-
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-
-import javax.annotation.concurrent.Immutable;
-
-@Immutable
-@NotNullByDefault
-public class MailboxPropertiesUpdate {
-
-	private final String onion;
-	private final MailboxAuthToken authToken;
-	private final MailboxFolderId inboxId;
-	private final MailboxFolderId outboxId;
-
-	public MailboxPropertiesUpdate(String onion,
-			MailboxAuthToken authToken, MailboxFolderId inboxId,
-			MailboxFolderId outboxId) {
-		this.onion = onion;
-		this.authToken = authToken;
-		this.inboxId = inboxId;
-		this.outboxId = outboxId;
-	}
-
-	public String getOnion() {
-		return onion;
-	}
-
-	public MailboxAuthToken getAuthToken() {
-		return authToken;
-	}
-
-	public MailboxFolderId getInboxId() {
-		return inboxId;
-	}
-
-	public MailboxFolderId getOutboxId() {
-		return outboxId;
-	}
-
-}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxPropertyManager.java

@@ -1,67 +0,0 @@
-package org.briarproject.bramble.api.mailbox;
-
-import org.briarproject.bramble.api.contact.ContactId;
-import org.briarproject.bramble.api.db.DbException;
-import org.briarproject.bramble.api.db.Transaction;
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.sync.ClientId;
-
-import javax.annotation.Nullable;
-
-@NotNullByDefault
-public interface MailboxPropertyManager {
-
-	/**
-	 * The unique ID of the mailbox property client.
-	 */
-	ClientId CLIENT_ID =
-			new ClientId("org.briarproject.bramble.mailbox.properties");
-
-	/**
-	 * The current major version of the mailbox property client.
-	 */
-	int MAJOR_VERSION = 0;
-
-	/**
-	 * The current minor version of the mailbox property client.
-	 */
-	int MINOR_VERSION = 0;
-
-	/**
-	 * The number of properties required for a (non-empty) update message.
-	 */
-	int PROP_COUNT = 4;
-
-	/**
-	 * The required properties of a non-empty update message.
-	 */
-	String PROP_KEY_ONION = "onion";
-	String PROP_KEY_AUTHTOKEN = "authToken";
-	String PROP_KEY_INBOXID = "inboxId";
-	String PROP_KEY_OUTBOXID = "outboxId";
-
-	/**
-	 * Length of the Onion property.
-	 */
-	int PROP_ONION_LENGTH = 56;
-
-	/**
-	 * Message metadata key for the version number of a local or remote update,
-	 * as a BDF long.
-	 */
-	String MSG_KEY_VERSION = "version";
-
-	/**
-	 * Message metadata key for whether an update is local or remote, as a BDF
-	 * boolean.
-	 */
-	String MSG_KEY_LOCAL = "local";
-
-	@Nullable
-	MailboxPropertiesUpdate getLocalProperties(Transaction txn, ContactId c)
-			throws DbException;
-
-	@Nullable
-	MailboxPropertiesUpdate getRemoteProperties(Transaction txn, ContactId c)
-			throws DbException;
-}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxSettingsManager.java

@@ -7,6 +7,8 @@ import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.lifecycle.LifecycleManager;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
+import java.util.List;
+
 import javax.annotation.Nullable;
 
 @NotNullByDefault
@@ -26,11 +28,16 @@ public interface MailboxSettingsManager {
 	void setOwnMailboxProperties(Transaction txn, MailboxProperties p)
 			throws DbException;
 
+	void removeOwnMailboxProperties(Transaction txn) throws DbException;
+
 	MailboxStatus getOwnMailboxStatus(Transaction txn) throws DbException;
 
 	void recordSuccessfulConnection(Transaction txn, long now)
 			throws DbException;
 
+	void recordSuccessfulConnection(Transaction txn, long now,
+			List<MailboxVersion> versions) throws DbException;
+
 	void recordFailedConnectionAttempt(Transaction txn, long now)
 			throws DbException;
 
@@ -47,7 +54,8 @@ public interface MailboxSettingsManager {
 		 * @param txn A read-write transaction
 		 * @param ownOnion Our new mailbox's onion (56 base32 chars)
 		 */
-		void mailboxPaired(Transaction txn, String ownOnion)
+		void mailboxPaired(Transaction txn, String ownOnion,
+				List<MailboxVersion> serverSupports)
 				throws DbException;
 
 		/**

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxStatus.java

@@ -2,20 +2,30 @@ package org.briarproject.bramble.api.mailbox;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
+import java.util.List;
+
 import javax.annotation.concurrent.Immutable;
 
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.CLIENT_SUPPORTS;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.PROBLEM_MS_SINCE_LAST_SUCCESS;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.PROBLEM_NUM_CONNECTION_FAILURES;
+import static org.briarproject.bramble.api.mailbox.MailboxHelper.getHighestCommonMajorVersion;
+
 @Immutable
 @NotNullByDefault
 public class MailboxStatus {
 
 	private final long lastAttempt, lastSuccess;
 	private final int attemptsSinceSuccess;
+	private final List<MailboxVersion> serverSupports;
 
 	public MailboxStatus(long lastAttempt, long lastSuccess,
-			int attemptsSinceSuccess) {
+			int attemptsSinceSuccess,
+			List<MailboxVersion> serverSupports) {
 		this.lastAttempt = lastAttempt;
 		this.lastSuccess = lastSuccess;
 		this.attemptsSinceSuccess = attemptsSinceSuccess;
+		this.serverSupports = serverSupports;
 	}
 
 	/**
@@ -56,4 +66,21 @@ public class MailboxStatus {
 	public int getAttemptsSinceSuccess() {
 		return attemptsSinceSuccess;
 	}
+
+	/**
+	 * @return true if this status indicates a problem with the mailbox.
+	 */
+	public boolean hasProblem(long now) {
+		return attemptsSinceSuccess >= PROBLEM_NUM_CONNECTION_FAILURES &&
+				(now - lastSuccess) >= PROBLEM_MS_SINCE_LAST_SUCCESS;
+	}
+
+	/**
+	 * @return a positive integer if the mailbox is compatible. Same result as
+	 * {@link MailboxHelper#getHighestCommonMajorVersion(List, List)}.
+	 */
+	public int getMailboxCompatibility() {
+		return getHighestCommonMajorVersion(CLIENT_SUPPORTS, serverSupports);
+	}
+
 }

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxUpdate.java

@@ -0,0 +1,31 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class MailboxUpdate {
+	private final boolean hasMailbox;
+	private final List<MailboxVersion> clientSupports;
+
+	public MailboxUpdate(List<MailboxVersion> clientSupports) {
+		this(clientSupports, false);
+	}
+
+	MailboxUpdate(List<MailboxVersion> clientSupports, boolean hasMailbox) {
+		this.clientSupports = clientSupports;
+		this.hasMailbox = hasMailbox;
+	}
+
+	public List<MailboxVersion> getClientSupports() {
+		return clientSupports;
+	}
+
+	public boolean hasMailbox() {
+		return hasMailbox;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxUpdateManager.java

@@ -0,0 +1,105 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.ClientId;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
+public interface MailboxUpdateManager {
+
+	/**
+	 * The unique ID of the mailbox update (properties) client.
+	 */
+	ClientId CLIENT_ID =
+			new ClientId("org.briarproject.bramble.mailbox.properties");
+
+	/**
+	 * The current major version of the mailbox update (properties) client.
+	 */
+	int MAJOR_VERSION = 2;
+
+	/**
+	 * The current minor version of the mailbox update (properties) client.
+	 */
+	int MINOR_VERSION = 0;
+
+	/**
+	 * The number of properties required for an update message with a mailbox.
+	 * <p>
+	 * The required properties are {@link #PROP_KEY_ONION},
+	 * {@link #PROP_KEY_AUTHTOKEN}, {@link #PROP_KEY_INBOXID} and
+	 * {@link #PROP_KEY_OUTBOXID}.
+	 */
+	int PROP_COUNT = 4;
+
+	/**
+	 * The onion address of the mailbox, excluding the .onion suffix.
+	 */
+	String PROP_KEY_ONION = "onion";
+
+	/**
+	 * A bearer token for accessing the mailbox (64 hex digits).
+	 */
+	String PROP_KEY_AUTHTOKEN = "authToken";
+
+	/**
+	 * A folder ID for downloading messages (64 hex digits).
+	 */
+	String PROP_KEY_INBOXID = "inboxId";
+
+	/**
+	 * A folder ID for uploading messages (64 hex digits).
+	 */
+	String PROP_KEY_OUTBOXID = "outboxId";
+
+	/**
+	 * Length of the {@link #PROP_KEY_ONION} property.
+	 */
+	int PROP_ONION_LENGTH = 56;
+
+	/**
+	 * Message metadata key for the version number of a local or remote update,
+	 * as a BDF long.
+	 */
+	String MSG_KEY_VERSION = "version";
+
+	/**
+	 * Message metadata key for whether an update is local or remote, as a BDF
+	 * boolean.
+	 */
+	String MSG_KEY_LOCAL = "local";
+
+	/**
+	 * Key in the client's local group for storing the clientSupports list that
+	 * was last sent out.
+	 */
+	String GROUP_KEY_SENT_CLIENT_SUPPORTS = "sentClientSupports";
+
+	/**
+	 * Returns the latest {@link MailboxUpdate} sent to the given contact.
+	 * <p>
+	 * If we have our own mailbox then the update will be a
+	 * {@link MailboxUpdateWithMailbox} containing the
+	 * {@link MailboxProperties} the contact should use for communicating with
+	 * our mailbox.
+	 */
+	MailboxUpdate getLocalUpdate(Transaction txn, ContactId c)
+			throws DbException;
+
+	/**
+	 * Returns the latest {@link MailboxUpdate} received from the given
+	 * contact, or null if no update has been received.
+	 * <p>
+	 * If the contact has a mailbox then the update will be a
+	 * {@link MailboxUpdateWithMailbox} containing the
+	 * {@link MailboxProperties} we should use for communicating with the
+	 * contact's mailbox.
+	 */
+	@Nullable
+	MailboxUpdate getRemoteUpdate(Transaction txn, ContactId c)
+			throws DbException;
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxUpdateWithMailbox.java

@@ -0,0 +1,30 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.List;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class MailboxUpdateWithMailbox extends MailboxUpdate {
+
+	private final MailboxProperties properties;
+
+	public MailboxUpdateWithMailbox(List<MailboxVersion> clientSupports,
+			MailboxProperties properties) {
+		super(clientSupports, true);
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		this.properties = properties;
+	}
+
+	public MailboxUpdateWithMailbox(MailboxUpdateWithMailbox o,
+			List<MailboxVersion> newClientSupports) {
+		this(newClientSupports, o.getMailboxProperties());
+	}
+
+	public MailboxProperties getMailboxProperties() {
+		return properties;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/MailboxVersion.java

@@ -0,0 +1,44 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+@Immutable
+@NotNullByDefault
+public class MailboxVersion implements Comparable<MailboxVersion> {
+
+	private final int major;
+	private final int minor;
+
+	public MailboxVersion(int major, int minor) {
+		this.major = major;
+		this.minor = minor;
+	}
+
+	public int getMajor() {
+		return major;
+	}
+
+	public int getMinor() {
+		return minor;
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (o instanceof MailboxVersion) {
+			MailboxVersion v = (MailboxVersion) o;
+			return major == v.major && minor == v.minor;
+		}
+		return false;
+	}
+
+	@Override
+	public int compareTo(MailboxVersion v) {
+		int c = major - v.major;
+		if (c != 0) {
+			return c;
+		}
+		return minor - v.minor;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/RemoteMailboxPropertiesUpdateEvent.java

@@ -1,36 +0,0 @@
-package org.briarproject.bramble.api.mailbox;
-
-import org.briarproject.bramble.api.contact.ContactId;
-import org.briarproject.bramble.api.event.Event;
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-
-import javax.annotation.Nullable;
-import javax.annotation.concurrent.Immutable;
-
-/**
- * An event that is broadcast when {@link MailboxPropertiesUpdate} are received
- * from a contact.
- */
-@Immutable
-@NotNullByDefault
-public class RemoteMailboxPropertiesUpdateEvent extends Event {
-
-	private final ContactId contactId;
-	@Nullable
-	private final MailboxPropertiesUpdate mailboxPropertiesUpdate;
-
-	public RemoteMailboxPropertiesUpdateEvent(ContactId contactId,
-			@Nullable MailboxPropertiesUpdate mailboxPropertiesUpdate) {
-		this.contactId = contactId;
-		this.mailboxPropertiesUpdate = mailboxPropertiesUpdate;
-	}
-
-	public ContactId getContact() {
-		return contactId;
-	}
-
-	@Nullable
-	public MailboxPropertiesUpdate getMailboxPropertiesUpdate() {
-		return mailboxPropertiesUpdate;
-	}
-}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/event/MailboxProblemEvent.java

@@ -0,0 +1,19 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast by {@link MailboxSettingsManager} when
+ * recording a connection failure for own Mailbox
+ * that has persistent for long enough for the mailbox owner to become active
+ * and fix the problem with the mailbox.
+ */
+@Immutable
+@NotNullByDefault
+public class MailboxProblemEvent extends Event {
+
+}

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/event/OwnMailboxConnectionStatusEvent.java

@@ -1,6 +1,8 @@
-package org.briarproject.bramble.api.mailbox;
+package org.briarproject.bramble.api.mailbox.event;
 
 import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxStatus;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import javax.annotation.concurrent.Immutable;

bramble-api/src/main/java/org/briarproject/bramble/api/mailbox/event/RemoteMailboxUpdateEvent.java

@@ -0,0 +1,34 @@
+package org.briarproject.bramble.api.mailbox.event;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.Immutable;
+
+/**
+ * An event that is broadcast when {@link MailboxUpdate} are received
+ * from a contact.
+ */
+@Immutable
+@NotNullByDefault
+public class RemoteMailboxUpdateEvent extends Event {
+
+	private final ContactId contactId;
+	private final MailboxUpdate mailboxUpdate;
+
+	public RemoteMailboxUpdateEvent(ContactId contactId,
+			MailboxUpdate mailboxUpdate) {
+		this.contactId = contactId;
+		this.mailboxUpdate = mailboxUpdate;
+	}
+
+	public ContactId getContact() {
+		return contactId;
+	}
+
+	public MailboxUpdate getMailboxUpdate() {
+		return mailboxUpdate;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/plugin/TorConstants.java

@@ -1,5 +1,7 @@
 package org.briarproject.bramble.api.plugin;
 
+import static java.util.concurrent.TimeUnit.SECONDS;
+
 public interface TorConstants {
 
 	TransportId ID = new TransportId("org.briarproject.bramble.tor");
@@ -10,8 +12,9 @@ public interface TorConstants {
 	int DEFAULT_SOCKS_PORT = 59050;
 	int DEFAULT_CONTROL_PORT = 59051;
 
-	int CONNECT_TO_PROXY_TIMEOUT = 5000; // Milliseconds
-	int EXTRA_SOCKET_TIMEOUT = 30000; // Milliseconds
+	int CONNECT_TO_PROXY_TIMEOUT = (int) SECONDS.toMillis(5);
+	int EXTRA_CONNECT_TIMEOUT = (int) SECONDS.toMillis(120);
+	int EXTRA_SOCKET_TIMEOUT = (int) SECONDS.toMillis(30);
 
 	// Local settings (not shared with contacts)
 	String PREF_TOR_NETWORK = "network2";

bramble-api/src/main/java/org/briarproject/bramble/api/record/RecordWriter.java

@@ -12,4 +12,6 @@ public interface RecordWriter {
 	void flush() throws IOException;
 
 	void close() throws IOException;
+
+	long getBytesWritten();
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/OutgoingSessionRecord.java

@@ -0,0 +1,37 @@
+package org.briarproject.bramble.api.sync;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.Collection;
+import java.util.concurrent.CopyOnWriteArrayList;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * A container for holding the IDs of messages sent and acked during an
+ * outgoing {@link SyncSession}, so they can be recorded in the DB as sent
+ * or acked at some later time.
+ */
+@ThreadSafe
+@NotNullByDefault
+public class OutgoingSessionRecord {
+
+	private final Collection<MessageId> ackedIds = new CopyOnWriteArrayList<>();
+	private final Collection<MessageId> sentIds = new CopyOnWriteArrayList<>();
+
+	public void onAckSent(Collection<MessageId> acked) {
+		ackedIds.addAll(acked);
+	}
+
+	public void onMessageSent(MessageId sent) {
+		sentIds.add(sent);
+	}
+
+	public Collection<MessageId> getAckedIds() {
+		return ackedIds;
+	}
+
+	public Collection<MessageId> getSentIds() {
+		return sentIds;
+	}
+}

bramble-api/src/main/java/org/briarproject/bramble/api/sync/SyncRecordWriter.java

@@ -20,4 +20,6 @@ public interface SyncRecordWriter {
 	void writePriority(Priority p) throws IOException;
 
 	void flush() throws IOException;
+
+	long getBytesWritten();
 }

bramble-api/src/main/java/org/briarproject/bramble/api/sync/SyncSessionFactory.java

@@ -12,12 +12,30 @@ import javax.annotation.Nullable;
 @NotNullByDefault
 public interface SyncSessionFactory {
 
+	/**
+	 * Creates a session for receiving data from a contact.
+	 */
 	SyncSession createIncomingSession(ContactId c, InputStream in,
 			PriorityHandler handler);
 
+	/**
+	 * Creates a session for sending data to a contact over a simplex transport.
+	 *
+	 * @param eager True if messages should be sent eagerly, ie regardless of
+	 * whether they're due for retransmission.
+	 */
 	SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
 			long maxLatency, boolean eager, StreamWriter streamWriter);
 
+	/**
+	 * Creates a session for sending data to a contact via a mailbox. The IDs
+	 * of any messages sent or acked will be added to the given
+	 * {@link OutgoingSessionRecord}.
+	 */
+	SyncSession createSimplexOutgoingSession(ContactId c, TransportId t,
+			long maxLatency, StreamWriter streamWriter,
+			OutgoingSessionRecord sessionRecord);
+
 	SyncSession createDuplexOutgoingSession(ContactId c, TransportId t,
 			long maxLatency, int maxIdleTime, StreamWriter streamWriter,
 			@Nullable Priority priority);

bramble-api/src/main/java/org/briarproject/bramble/api/sync/event/GroupVisibilityUpdatedEvent.java

@@ -3,6 +3,7 @@ package org.briarproject.bramble.api.sync.event;
 import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.event.Event;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.Group.Visibility;
 
 import java.util.Collection;
 
@@ -15,12 +16,19 @@ import javax.annotation.concurrent.Immutable;
 @NotNullByDefault
 public class GroupVisibilityUpdatedEvent extends Event {
 
+	private final Visibility visibility;
 	private final Collection<ContactId> affected;
 
-	public GroupVisibilityUpdatedEvent(Collection<ContactId> affected) {
+	public GroupVisibilityUpdatedEvent(Visibility visibility,
+			Collection<ContactId> affected) {
+		this.visibility = visibility;
 		this.affected = affected;
 	}
 
+	public Visibility getVisibility() {
+		return visibility;
+	}
+
 	/**
 	 * Returns the contacts affected by the update.
 	 */

bramble-api/src/main/java/org/briarproject/bramble/api/system/TaskScheduler.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.api.system;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.util.concurrent.Executor;
@@ -16,6 +17,8 @@ public interface TaskScheduler {
 	 * <p>
 	 * If the platform supports wake locks, a wake lock will be held while
 	 * submitting and running the task.
+	 *
+	 * @return A {@link Cancellable} for cancelling the task.
 	 */
 	Cancellable schedule(Runnable task, Executor executor, long delay,
 			TimeUnit unit);
@@ -27,17 +30,11 @@ public interface TaskScheduler {
 	 * <p>
 	 * If the platform supports wake locks, a wake lock will be held while
 	 * submitting and running the task.
+	 *
+	 * @return A {@link Cancellable} for cancelling all future executions of
+	 * the task.
 	 */
 	Cancellable scheduleWithFixedDelay(Runnable task, Executor executor,
 			long delay, long interval, TimeUnit unit);
 
-	interface Cancellable {
-
-		/**
-		 * Cancels the task if it has not already started running. If the task
-		 * is {@link #scheduleWithFixedDelay(Runnable, Executor, long, long, TimeUnit) periodic},
-		 * all future executions of the task are cancelled.
-		 */
-		void cancel();
-	}
 }

bramble-api/src/main/java/org/briarproject/bramble/util/IoUtils.java

@@ -40,7 +40,7 @@ public class IoUtils {
 		}
 	}
 
-	private static void delete(File f) {
+	public static void delete(File f) {
 		if (!f.delete() && LOG.isLoggable(WARNING))
 			LOG.warning("Could not delete " + f.getAbsolutePath());
 	}

bramble-api/src/main/java/org/briarproject/bramble/util/StringUtils.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.util;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.io.UnsupportedEncodingException;
@@ -95,10 +96,10 @@ public class StringUtils {
 	/**
 	 * Converts the given hex string to a byte array.
 	 */
-	public static byte[] fromHexString(String hex) {
+	public static byte[] fromHexString(String hex) throws FormatException {
 		int len = hex.length();
 		if (len % 2 != 0)
-			throw new IllegalArgumentException("Not a hex string");
+			throw new FormatException();
 		byte[] bytes = new byte[len / 2];
 		for (int i = 0, j = 0; i < len; i += 2, j++) {
 			int high = hexDigitToInt(hex.charAt(i));
@@ -108,11 +109,11 @@ public class StringUtils {
 		return bytes;
 	}
 
-	private static int hexDigitToInt(char c) {
+	private static int hexDigitToInt(char c) throws FormatException {
 		if (c >= '0' && c <= '9') return c - '0';
 		if (c >= 'A' && c <= 'F') return c - 'A' + 10;
 		if (c >= 'a' && c <= 'f') return c - 'a' + 10;
-		throw new IllegalArgumentException("Not a hex digit: " + c);
+		throw new FormatException();
 	}
 
 	public static String trim(String s) {
@@ -130,13 +131,13 @@ public class StringUtils {
 		return MAC.matcher(mac).matches();
 	}
 
-	public static byte[] macToBytes(String mac) {
-		if (!MAC.matcher(mac).matches()) throw new IllegalArgumentException();
+	public static byte[] macToBytes(String mac) throws FormatException {
+		if (!MAC.matcher(mac).matches()) throw new FormatException();
 		return fromHexString(mac.replaceAll(":", ""));
 	}
 
-	public static String macToString(byte[] mac) {
-		if (mac.length != 6) throw new IllegalArgumentException();
+	public static String macToString(byte[] mac) throws FormatException {
+		if (mac.length != 6) throw new FormatException();
 		StringBuilder s = new StringBuilder();
 		for (byte b : mac) {
 			if (s.length() > 0) s.append(':');

bramble-api/src/test/java/org/briarproject/bramble/api/mailbox/MailboxHelperTest.java

@@ -0,0 +1,44 @@
+package org.briarproject.bramble.api.mailbox;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.API_CLIENT_TOO_OLD;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.API_SERVER_TOO_OLD;
+import static org.briarproject.bramble.api.mailbox.MailboxHelper.getHighestCommonMajorVersion;
+import static org.junit.Assert.assertEquals;
+
+public class MailboxHelperTest {
+
+	private final Random random = new Random();
+
+	@Test
+	public void testGetHighestCommonMajorVersion() {
+		assertEquals(2, getHighestCommonMajorVersion(v(2), v(2)));
+		assertEquals(2, getHighestCommonMajorVersion(v(1, 2), v(2, 3, 4)));
+		assertEquals(2, getHighestCommonMajorVersion(v(2, 3, 4), v(2)));
+		assertEquals(2, getHighestCommonMajorVersion(v(2), v(2, 3, 4)));
+
+		assertEquals(API_CLIENT_TOO_OLD,
+				getHighestCommonMajorVersion(v(2), v(3, 4)));
+		assertEquals(API_CLIENT_TOO_OLD,
+				getHighestCommonMajorVersion(v(2), v(1, 3)));
+		assertEquals(API_SERVER_TOO_OLD,
+				getHighestCommonMajorVersion(v(3, 4, 5), v(2)));
+		assertEquals(API_SERVER_TOO_OLD,
+				getHighestCommonMajorVersion(v(1, 3), v(2)));
+	}
+
+	private List<MailboxVersion> v(int... ints) {
+		List<MailboxVersion> versions = new ArrayList<>(ints.length);
+		for (int v : ints) {
+			// minor versions should not matter
+			versions.add(new MailboxVersion(v, random.nextInt(42)));
+		}
+		return versions;
+	}
+
+}

bramble-api/src/test/java/org/briarproject/bramble/test/TestUtils.java

@@ -20,7 +20,12 @@ import org.briarproject.bramble.api.identity.Author;
 import org.briarproject.bramble.api.identity.AuthorId;
 import org.briarproject.bramble.api.identity.Identity;
 import org.briarproject.bramble.api.identity.LocalAuthor;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportProperties;
 import org.briarproject.bramble.api.sync.ClientId;
@@ -35,6 +40,7 @@ import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.security.NoSuchAlgorithmException;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -45,6 +51,7 @@ import java.util.Random;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import javax.annotation.Nullable;
+import javax.crypto.Cipher;
 
 import static java.util.Arrays.asList;
 import static org.briarproject.bramble.api.crypto.CryptoConstants.MAX_AGREEMENT_PUBLIC_KEY_BYTES;
@@ -221,6 +228,19 @@ public class TestUtils {
 				getAgreementPublicKey(), verified);
 	}
 
+	public static MailboxProperties getMailboxProperties(boolean owner,
+			List<MailboxVersion> serverSupports) {
+		String onion = getRandomString(56);
+		MailboxAuthToken authToken = new MailboxAuthToken(getRandomId());
+		if (owner) {
+			return new MailboxProperties(onion, authToken, serverSupports);
+		}
+		MailboxFolderId inboxId = new MailboxFolderId(getRandomId());
+		MailboxFolderId outboxId = new MailboxFolderId(getRandomId());
+		return new MailboxProperties(onion, authToken, serverSupports,
+				inboxId, outboxId);
+	}
+
 	public static void writeBytes(File file, byte[] bytes)
 			throws IOException {
 		FileOutputStream outputStream = new FileOutputStream(file);
@@ -273,22 +293,38 @@ public class TestUtils {
 		return Math.sqrt(getVariance(samples));
 	}
 
-	public static boolean isOptionalTestEnabled(Class testClass) {
+	public static boolean isOptionalTestEnabled(Class<?> testClass) {
 		String optionalTests = System.getenv("OPTIONAL_TESTS");
 		return optionalTests != null &&
 				asList(optionalTests.split(",")).contains(testClass.getName());
 	}
 
-	public static boolean mailboxPropertiesUpdateEqual(
-			@Nullable MailboxPropertiesUpdate a,
-			@Nullable MailboxPropertiesUpdate b) {
+	public static boolean mailboxUpdateEqual(@Nullable MailboxUpdate a,
+			@Nullable MailboxUpdate b) {
+		if (a == null || b == null) {
+			return a == b;
+		}
+		if (!a.hasMailbox() && !b.hasMailbox()) {
+			return a.getClientSupports().equals(b.getClientSupports());
+		} else if (a.hasMailbox() && b.hasMailbox()) {
+			MailboxUpdateWithMailbox am = (MailboxUpdateWithMailbox) a;
+			MailboxUpdateWithMailbox bm = (MailboxUpdateWithMailbox) b;
+			return am.getClientSupports().equals(bm.getClientSupports()) &&
+					mailboxPropertiesEqual(am.getMailboxProperties(),
+							bm.getMailboxProperties());
+		}
+		return false;
+	}
+
+	public static boolean mailboxPropertiesEqual(@Nullable MailboxProperties a,
+			@Nullable MailboxProperties b) {
 		if (a == null || b == null) {
 			return a == b;
 		}
 		return a.getOnion().equals(b.getOnion()) &&
 				a.getAuthToken().equals(b.getAuthToken()) &&
-				a.getInboxId().equals(b.getInboxId()) &&
-				a.getOutboxId().equals(b.getOutboxId());
+				a.isOwner() == b.isOwner() &&
+				a.getServerSupports().equals(b.getServerSupports());
 	}
 
 	public static boolean hasEvent(Transaction txn,
@@ -301,4 +337,13 @@ public class TestUtils {
 		}
 		return false;
 	}
+
+	public static boolean isCryptoStrengthUnlimited() {
+		try {
+			return Cipher.getMaxAllowedKeyLength("AES/CBC/PKCS5Padding")
+					== Integer.MAX_VALUE;
+		} catch (NoSuchAlgorithmException e) {
+			throw new AssertionError();
+		}
+	}
 }

bramble-core/build.gradle

@@ -16,7 +16,7 @@ dependencies {
 	implementation 'org.bitlet:weupnp:0.1.4'
 	implementation 'net.i2p.crypto:eddsa:0.2.0'
 	implementation 'org.whispersystems:curve25519-java:0.5.0'
-	implementation 'org.briarproject:jtorctl:0.3'
+	implementation 'org.briarproject:jtorctl:0.5'
 
 	//noinspection GradleDependency
 	implementation "com.squareup.okhttp3:okhttp:$okhttp_version"
@@ -32,6 +32,7 @@ dependencies {
 	testImplementation "org.jmock:jmock-junit4:$jmock_version"
 	testImplementation "org.jmock:jmock-imposters:$jmock_version"
 	testImplementation "com.squareup.okhttp3:mockwebserver:4.9.3"
+	testImplementation "org.briarproject:mailbox-lib:0.1"
 
 	testAnnotationProcessor "com.google.dagger:dagger-compiler:$dagger_version"
 

bramble-core/src/main/java/org/briarproject/bramble/account/AccountManagerImpl.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.account;
 
+import org.briarproject.bramble.api.FormatException;
 import org.briarproject.bramble.api.account.AccountManager;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.crypto.DecryptionException;
@@ -209,7 +210,13 @@ class AccountManagerImpl implements AccountManager {
 			LOG.warning("Failed to load encrypted database key");
 			throw new DecryptionException(INVALID_CIPHERTEXT);
 		}
-		byte[] ciphertext = fromHexString(hex);
+		byte[] ciphertext;
+		try {
+			ciphertext = fromHexString(hex);
+		} catch (FormatException e) {
+			LOG.warning("Encrypted database key has invalid format");
+			throw new DecryptionException(INVALID_CIPHERTEXT);
+		}
 		KeyStrengthener keyStrengthener = databaseConfig.getKeyStrengthener();
 		byte[] plaintext = crypto.decryptWithPassword(ciphertext, password,
 				keyStrengthener);

bramble-core/src/main/java/org/briarproject/bramble/client/ClientHelperImpl.java

@@ -25,7 +25,10 @@ import org.briarproject.bramble.api.identity.Author;
 import org.briarproject.bramble.api.identity.AuthorFactory;
 import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportProperties;
@@ -39,12 +42,13 @@ import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.security.GeneralSecurityException;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 
-import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
@@ -52,12 +56,12 @@ import static org.briarproject.bramble.api.client.ContactGroupConstants.GROUP_KE
 import static org.briarproject.bramble.api.identity.Author.FORMAT_VERSION;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_AUTHOR_NAME_LENGTH;
 import static org.briarproject.bramble.api.identity.AuthorConstants.MAX_PUBLIC_KEY_LENGTH;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_COUNT;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_AUTHTOKEN;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_INBOXID;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_ONION;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_KEY_OUTBOXID;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.PROP_ONION_LENGTH;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_COUNT;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_AUTHTOKEN;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_INBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_ONION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_KEY_OUTBOXID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.PROP_ONION_LENGTH;
 import static org.briarproject.bramble.api.properties.TransportPropertyConstants.MAX_PROPERTIES_PER_TRANSPORT;
 import static org.briarproject.bramble.api.properties.TransportPropertyConstants.MAX_PROPERTY_LENGTH;
 import static org.briarproject.bramble.util.ValidationUtils.checkLength;
@@ -412,11 +416,28 @@ class ClientHelperImpl implements ClientHelper {
 	}
 
 	@Override
-	@Nullable
-	public MailboxPropertiesUpdate parseAndValidateMailboxPropertiesUpdate(
-			BdfDictionary properties) throws FormatException {
+	public MailboxUpdate parseAndValidateMailboxUpdate(BdfList clientSupports,
+			BdfList serverSupports, BdfDictionary properties)
+			throws FormatException {
+		List<MailboxVersion> clientSupportsList =
+				parseMailboxVersionList(clientSupports);
+		List<MailboxVersion> serverSupportsList =
+				parseMailboxVersionList(serverSupports);
+
+		// We must always learn what Mailbox API version(s) the client supports
+		if (clientSupports.isEmpty()) {
+			throw new FormatException();
+		}
 		if (properties.isEmpty()) {
-			return null;
+			// No mailbox -- cannot claim to support any API versions!
+			if (!serverSupports.isEmpty()) {
+				throw new FormatException();
+			}
+			return new MailboxUpdate(clientSupportsList);
+		}
+		// Mailbox must be accompanied by the Mailbox API version(s) it supports
+		if (serverSupports.isEmpty()) {
+			throw new FormatException();
 		}
 		// Accepting more props than we need, for forward compatibility
 		if (properties.size() < PROP_COUNT) {
@@ -435,9 +456,25 @@ class ClientHelperImpl implements ClientHelper {
 		checkLength(inboxId, UniqueId.LENGTH);
 		byte[] outboxId = properties.getRaw(PROP_KEY_OUTBOXID);
 		checkLength(outboxId, UniqueId.LENGTH);
-		return new MailboxPropertiesUpdate(onion,
-				new MailboxAuthToken(authToken), new MailboxFolderId(inboxId),
-				new MailboxFolderId(outboxId));
+		MailboxProperties props = new MailboxProperties(onion,
+				new MailboxAuthToken(authToken), serverSupportsList,
+				new MailboxFolderId(inboxId), new MailboxFolderId(outboxId));
+		return new MailboxUpdateWithMailbox(clientSupportsList, props);
+	}
+
+	@Override
+	public List<MailboxVersion> parseMailboxVersionList(BdfList bdfList)
+			throws FormatException {
+		List<MailboxVersion> list = new ArrayList<>();
+		for (int i = 0; i < bdfList.size(); i++) {
+			BdfList element = bdfList.getList(i);
+			if (element.size() != 2) {
+				throw new FormatException();
+			}
+			list.add(new MailboxVersion(element.getLong(0).intValue(),
+					element.getLong(1).intValue()));
+		}
+		return list;
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/connection/Connection.java

@@ -54,7 +54,7 @@ abstract class Connection {
 		}
 	}
 
-	private byte[] readTag(InputStream in) throws IOException {
+	byte[] readTag(InputStream in) throws IOException {
 		byte[] tag = new byte[TAG_LENGTH];
 		read(in, tag);
 		return tag;

bramble-core/src/main/java/org/briarproject/bramble/connection/ConnectionManagerImpl.java

@@ -13,6 +13,7 @@ import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
 import org.briarproject.bramble.api.properties.TransportPropertyManager;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
 import org.briarproject.bramble.api.sync.SyncSessionFactory;
 import org.briarproject.bramble.api.transport.KeyManager;
 import org.briarproject.bramble.api.transport.StreamReaderFactory;
@@ -67,7 +68,15 @@ class ConnectionManagerImpl implements ConnectionManager {
 			TransportConnectionReader r) {
 		ioExecutor.execute(new IncomingSimplexSyncConnection(keyManager,
 				connectionRegistry, streamReaderFactory, streamWriterFactory,
-				syncSessionFactory, transportPropertyManager, t, r));
+				syncSessionFactory, transportPropertyManager, t, r, null));
+	}
+
+	@Override
+	public void manageIncomingConnection(TransportId t,
+			TransportConnectionReader r, TagController c) {
+		ioExecutor.execute(new IncomingSimplexSyncConnection(keyManager,
+				connectionRegistry, streamReaderFactory, streamWriterFactory,
+				syncSessionFactory, transportPropertyManager, t, r, c));
 	}
 
 	@Override
@@ -92,7 +101,16 @@ class ConnectionManagerImpl implements ConnectionManager {
 			TransportConnectionWriter w) {
 		ioExecutor.execute(new OutgoingSimplexSyncConnection(keyManager,
 				connectionRegistry, streamReaderFactory, streamWriterFactory,
-				syncSessionFactory, transportPropertyManager, c, t, w));
+				syncSessionFactory, transportPropertyManager, c, t, w, null));
+	}
+
+	@Override
+	public void manageOutgoingConnection(ContactId c, TransportId t,
+			TransportConnectionWriter w, OutgoingSessionRecord sessionRecord) {
+		ioExecutor.execute(new OutgoingSimplexSyncConnection(keyManager,
+				connectionRegistry, streamReaderFactory, streamWriterFactory,
+				syncSessionFactory, transportPropertyManager, c, t, w,
+				sessionRecord));
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/connection/IncomingSimplexSyncConnection.java

@@ -1,7 +1,9 @@
 package org.briarproject.bramble.connection;
 
+import org.briarproject.bramble.api.connection.ConnectionManager.TagController;
 import org.briarproject.bramble.api.connection.ConnectionRegistry;
 import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportConnectionReader;
 import org.briarproject.bramble.api.plugin.TransportId;
@@ -15,6 +17,8 @@ import org.briarproject.bramble.api.transport.StreamWriterFactory;
 
 import java.io.IOException;
 
+import javax.annotation.Nullable;
+
 import static java.util.logging.Level.WARNING;
 import static org.briarproject.bramble.util.LogUtils.logException;
 
@@ -23,6 +27,8 @@ class IncomingSimplexSyncConnection extends SyncConnection implements Runnable {
 
 	private final TransportId transportId;
 	private final TransportConnectionReader reader;
+	@Nullable
+	private final TagController tagController;
 
 	IncomingSimplexSyncConnection(KeyManager keyManager,
 			ConnectionRegistry connectionRegistry,
@@ -30,33 +36,50 @@ class IncomingSimplexSyncConnection extends SyncConnection implements Runnable {
 			StreamWriterFactory streamWriterFactory,
 			SyncSessionFactory syncSessionFactory,
 			TransportPropertyManager transportPropertyManager,
-			TransportId transportId, TransportConnectionReader reader) {
+			TransportId transportId,
+			TransportConnectionReader reader,
+			@Nullable TagController tagController) {
 		super(keyManager, connectionRegistry, streamReaderFactory,
 				streamWriterFactory, syncSessionFactory,
 				transportPropertyManager);
 		this.transportId = transportId;
 		this.reader = reader;
+		this.tagController = tagController;
 	}
 
 	@Override
 	public void run() {
 		// Read and recognise the tag
-		StreamContext ctx = recogniseTag(reader, transportId);
+		byte[] tag;
+		StreamContext ctx;
+		try {
+			tag = readTag(reader.getInputStream());
+			// If we have a tag controller, defer marking the tag as recognised
+			if (tagController == null) {
+				ctx = keyManager.getStreamContext(transportId, tag);
+			} else {
+				ctx = keyManager.getStreamContextOnly(transportId, tag);
+			}
+		} catch (IOException | DbException e) {
+			logException(LOG, WARNING, e);
+			onError();
+			return;
+		}
 		if (ctx == null) {
 			LOG.info("Unrecognised tag");
-			onError(false);
+			onError();
 			return;
 		}
 		ContactId contactId = ctx.getContactId();
 		if (contactId == null) {
 			LOG.warning("Received rendezvous stream, expected contact");
-			onError(true);
+			onError(tag);
 			return;
 		}
 		if (ctx.isHandshakeMode()) {
 			// TODO: Support handshake mode for contacts
 			LOG.warning("Received handshake tag, expected rotation mode");
-			onError(true);
+			onError(tag);
 			return;
 		}
 		try {
@@ -65,15 +88,33 @@ class IncomingSimplexSyncConnection extends SyncConnection implements Runnable {
 					LOG.info("Ignoring priority for simplex connection");
 			// Create and run the incoming session
 			createIncomingSession(ctx, reader, handler).run();
+			// Success
+			markTagAsRecognisedIfRequired(false, tag);
 			reader.dispose(false, true);
 		} catch (IOException e) {
 			logException(LOG, WARNING, e);
-			onError(true);
+			onError(tag);
 		}
 	}
 
-	private void onError(boolean recognised) {
-		disposeOnError(reader, recognised);
+	private void onError() {
+		disposeOnError(reader, false);
+	}
+
+	private void onError(byte[] tag) {
+		markTagAsRecognisedIfRequired(true, tag);
+		disposeOnError(reader, true);
+	}
+
+	private void markTagAsRecognisedIfRequired(boolean exception, byte[] tag) {
+		if (tagController != null &&
+				tagController.shouldMarkTagAsRecognised(exception)) {
+			try {
+				keyManager.markTagAsRecognised(transportId, tag);
+			} catch (DbException e) {
+				logException(LOG, WARNING, e);
+			}
+		}
 	}
 }
 

bramble-core/src/main/java/org/briarproject/bramble/connection/OutgoingSimplexSyncConnection.java

@@ -6,6 +6,7 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
 import org.briarproject.bramble.api.plugin.TransportId;
 import org.briarproject.bramble.api.properties.TransportPropertyManager;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
 import org.briarproject.bramble.api.sync.SyncSession;
 import org.briarproject.bramble.api.sync.SyncSessionFactory;
 import org.briarproject.bramble.api.transport.KeyManager;
@@ -16,6 +17,8 @@ import org.briarproject.bramble.api.transport.StreamWriterFactory;
 
 import java.io.IOException;
 
+import javax.annotation.Nullable;
+
 import static java.util.logging.Level.WARNING;
 import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
 import static org.briarproject.bramble.util.LogUtils.logException;
@@ -26,6 +29,8 @@ class OutgoingSimplexSyncConnection extends SyncConnection implements Runnable {
 	private final ContactId contactId;
 	private final TransportId transportId;
 	private final TransportConnectionWriter writer;
+	@Nullable
+	private final OutgoingSessionRecord sessionRecord;
 
 	OutgoingSimplexSyncConnection(KeyManager keyManager,
 			ConnectionRegistry connectionRegistry,
@@ -34,13 +39,15 @@ class OutgoingSimplexSyncConnection extends SyncConnection implements Runnable {
 			SyncSessionFactory syncSessionFactory,
 			TransportPropertyManager transportPropertyManager,
 			ContactId contactId, TransportId transportId,
-			TransportConnectionWriter writer) {
+			TransportConnectionWriter writer,
+			@Nullable OutgoingSessionRecord sessionRecord) {
 		super(keyManager, connectionRegistry, streamReaderFactory,
 				streamWriterFactory, syncSessionFactory,
 				transportPropertyManager);
 		this.contactId = contactId;
 		this.transportId = transportId;
 		this.writer = writer;
+		this.sessionRecord = sessionRecord;
 	}
 
 	@Override
@@ -71,10 +78,16 @@ class OutgoingSimplexSyncConnection extends SyncConnection implements Runnable {
 		StreamWriter streamWriter = streamWriterFactory.createStreamWriter(
 				w.getOutputStream(), ctx);
 		ContactId c = requireNonNull(ctx.getContactId());
-		// Use eager retransmission if the transport is lossy and cheap
-		return syncSessionFactory.createSimplexOutgoingSession(c,
-				ctx.getTransportId(), w.getMaxLatency(), w.isLossyAndCheap(),
-				streamWriter);
+		if (sessionRecord == null) {
+			// Use eager retransmission if the transport is lossy and cheap
+			return syncSessionFactory.createSimplexOutgoingSession(c,
+					ctx.getTransportId(), w.getMaxLatency(),
+					w.isLossyAndCheap(), streamWriter);
+		} else {
+			return syncSessionFactory.createSimplexOutgoingSession(c,
+					ctx.getTransportId(), w.getMaxLatency(), streamWriter,
+					sessionRecord);
+		}
 	}
 }
 

bramble-core/src/main/java/org/briarproject/bramble/db/Database.java

@@ -163,16 +163,11 @@ interface Database<T> {
 			throws DbException;
 
 	/**
-	 * Returns true if there are any acks or messages to send to the given
-	 * contact over a transport with the given maximum latency.
+	 * Returns true if there are any acks to send to the given contact.
 	 * <p/>
 	 * Read-only.
-	 *
-	 * @param eager True if messages that are not yet due for retransmission
-	 * should be included
 	 */
-	boolean containsAnythingToSend(T txn, ContactId c, long maxLatency,
-			boolean eager) throws DbException;
+	boolean containsAcksToSend(T txn, ContactId c) throws DbException;
 
 	/**
 	 * Returns true if the database contains the given contact for the given
@@ -212,6 +207,18 @@ interface Database<T> {
 	 */
 	boolean containsMessage(T txn, MessageId m) throws DbException;
 
+	/**
+	 * Returns true if there are any messages to send to the given
+	 * contact over a transport with the given maximum latency.
+	 * <p/>
+	 * Read-only.
+	 *
+	 * @param eager True if messages that are not yet due for retransmission
+	 * should be included
+	 */
+	boolean containsMessagesToSend(T txn, ContactId c, long maxLatency,
+			boolean eager) throws DbException;
+
 	/**
 	 * Returns true if the database contains the given pending contact.
 	 * <p/>
@@ -406,6 +413,12 @@ interface Database<T> {
 	Collection<MessageId> getMessageIds(T txn, GroupId g, Metadata query)
 			throws DbException;
 
+	/**
+	 * Returns the length of the given message in bytes, including the
+	 * message header.
+	 */
+	int getMessageLength(T txn, MessageId m) throws DbException;
+
 	/**
 	 * Returns the metadata for all delivered messages in the given group.
 	 * <p/>
@@ -496,7 +509,8 @@ interface Database<T> {
 
 	/**
 	 * Returns the IDs of some messages that are eligible to be sent to the
-	 * given contact, up to the given total length.
+	 * given contact. The total length of the messages including record headers
+	 * will be no more than the given capacity.
 	 * <p/>
 	 * Unlike {@link #getUnackedMessagesToSend(Object, ContactId)} this method
 	 * does not return messages that have already been sent unless they are
@@ -504,20 +518,20 @@ interface Database<T> {
 	 * <p/>
 	 * Read-only.
 	 */
-	Collection<MessageId> getMessagesToSend(T txn, ContactId c, int maxLength,
+	Collection<MessageId> getMessagesToSend(T txn, ContactId c, long capacity,
 			long maxLatency) throws DbException;
 
 	/**
 	 * Returns the IDs of all messages that are eligible to be sent to the
-	 * given contact, together with their raw lengths.
+	 * given contact.
 	 * <p/>
-	 * Unlike {@link #getMessagesToSend(Object, ContactId, int, long)} this
+	 * Unlike {@link #getMessagesToSend(Object, ContactId, long, long)} this
 	 * method may return messages that have already been sent and are not yet
 	 * due for retransmission.
 	 * <p/>
 	 * Read-only.
 	 */
-	Map<MessageId, Integer> getUnackedMessagesToSend(T txn, ContactId c)
+	Collection<MessageId> getUnackedMessagesToSend(T txn, ContactId c)
 			throws DbException;
 
 	/**
@@ -573,13 +587,16 @@ interface Database<T> {
 
 	/**
 	 * Returns the next time (in milliseconds since the Unix epoch) when a
-	 * message is due to be sent to the given contact. The returned value may
-	 * be zero if a message is due to be sent immediately, or Long.MAX_VALUE
-	 * if no messages are scheduled to be sent.
+	 * message is due to be sent to the given contact over a transport with
+	 * the given latency.
+	 * <p>
+	 * The returned value may be zero if a message is due to be sent
+	 * immediately, or Long.MAX_VALUE if no messages are scheduled to be sent.
 	 * <p/>
 	 * Read-only.
 	 */
-	long getNextSendTime(T txn, ContactId c) throws DbException;
+	long getNextSendTime(T txn, ContactId c, long maxLatency)
+			throws DbException;
 
 	/**
 	 * Returns the pending contact with the given ID.
@@ -598,13 +615,14 @@ interface Database<T> {
 
 	/**
 	 * Returns the IDs of some messages that are eligible to be sent to the
-	 * given contact and have been requested by the contact, up to the given
-	 * total length.
+	 * given contact and have been requested by the contact. The total length
+	 * of the messages including record headers will be no more than the given
+	 * capacity.
 	 * <p/>
 	 * Read-only.
 	 */
 	Collection<MessageId> getRequestedMessagesToSend(T txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException;
+			long capacity, long maxLatency) throws DbException;
 
 	/**
 	 * Returns all settings in the given namespace.

bramble-core/src/main/java/org/briarproject/bramble/db/DatabaseComponentImpl.java

@@ -75,7 +75,6 @@ import org.briarproject.bramble.api.transport.TransportKeys;
 
 import java.util.ArrayList;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.Executor;
@@ -87,6 +86,7 @@ import javax.annotation.Nullable;
 import javax.annotation.concurrent.ThreadSafe;
 import javax.inject.Inject;
 
+import static java.util.Collections.singletonList;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
@@ -342,12 +342,12 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Override
-	public boolean containsAnythingToSend(Transaction transaction, ContactId c,
-			long maxLatency, boolean eager) throws DbException {
+	public boolean containsAcksToSend(Transaction transaction, ContactId c)
+			throws DbException {
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
-		return db.containsAnythingToSend(txn, c, maxLatency, eager);
+		return db.containsAcksToSend(txn, c);
 	}
 
 	@Override
@@ -373,6 +373,15 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.containsIdentity(txn, a);
 	}
 
+	@Override
+	public boolean containsMessagesToSend(Transaction transaction, ContactId c,
+			long maxLatency, boolean eager) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.containsMessagesToSend(txn, c, maxLatency, eager);
+	}
+
 	@Override
 	public boolean containsPendingContact(Transaction transaction,
 			PendingContactId p) throws DbException {
@@ -424,13 +433,14 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	@Nullable
 	@Override
 	public Collection<Message> generateBatch(Transaction transaction,
-			ContactId c, int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
 		Collection<MessageId> ids =
-				db.getMessagesToSend(txn, c, maxLength, maxLatency);
+				db.getMessagesToSend(txn, c, capacity, maxLatency);
+		if (ids.isEmpty()) return null;
 		long totalLength = 0;
 		List<Message> messages = new ArrayList<>(ids.size());
 		for (MessageId m : ids) {
@@ -439,38 +449,11 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			messages.add(message);
 			db.updateRetransmissionData(txn, c, m, maxLatency);
 		}
-		if (ids.isEmpty()) return null;
 		db.lowerRequestedFlag(txn, c, ids);
 		transaction.attach(new MessagesSentEvent(c, ids, totalLength));
 		return messages;
 	}
 
-	@Override
-	public Collection<Message> generateBatch(Transaction transaction,
-			ContactId c, Collection<MessageId> ids, long maxLatency)
-			throws DbException {
-		if (transaction.isReadOnly()) throw new IllegalArgumentException();
-		T txn = unbox(transaction);
-		if (!db.containsContact(txn, c))
-			throw new NoSuchContactException();
-		long totalLength = 0;
-		List<Message> messages = new ArrayList<>(ids.size());
-		List<MessageId> sentIds = new ArrayList<>(ids.size());
-		for (MessageId m : ids) {
-			if (db.containsVisibleMessage(txn, c, m)) {
-				Message message = db.getMessage(txn, m);
-				totalLength += message.getRawLength();
-				messages.add(message);
-				sentIds.add(m);
-				db.updateRetransmissionData(txn, c, m, maxLatency);
-			}
-		}
-		if (messages.isEmpty()) return messages;
-		db.lowerRequestedFlag(txn, c, sentIds);
-		transaction.attach(new MessagesSentEvent(c, sentIds, totalLength));
-		return messages;
-	}
-
 	@Nullable
 	@Override
 	public Offer generateOffer(Transaction transaction, ContactId c,
@@ -505,13 +488,14 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	@Nullable
 	@Override
 	public Collection<Message> generateRequestedBatch(Transaction transaction,
-			ContactId c, int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		if (transaction.isReadOnly()) throw new IllegalArgumentException();
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
 		Collection<MessageId> ids =
-				db.getRequestedMessagesToSend(txn, c, maxLength, maxLatency);
+				db.getRequestedMessagesToSend(txn, c, capacity, maxLatency);
+		if (ids.isEmpty()) return null;
 		long totalLength = 0;
 		List<Message> messages = new ArrayList<>(ids.size());
 		for (MessageId m : ids) {
@@ -520,7 +504,6 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			messages.add(message);
 			db.updateRetransmissionData(txn, c, m, maxLatency);
 		}
-		if (ids.isEmpty()) return null;
 		db.lowerRequestedFlag(txn, c, ids);
 		transaction.attach(new MessagesSentEvent(c, ids, totalLength));
 		return messages;
@@ -635,6 +618,24 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return db.getMessageIds(txn, g, query);
 	}
 
+	@Override
+	public Collection<MessageId> getMessagesToAck(Transaction transaction,
+			ContactId c, int maxMessages) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.getMessagesToAck(txn, c, maxMessages);
+	}
+
+	@Override
+	public Collection<MessageId> getMessagesToSend(Transaction transaction,
+			ContactId c, long capacity, long maxLatency) throws DbException {
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		return db.getMessagesToSend(txn, c, capacity, maxLatency);
+	}
+
 	@Override
 	public Collection<MessageId> getMessagesToValidate(Transaction transaction)
 			throws DbException {
@@ -740,10 +741,29 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		return status;
 	}
 
+	@Nullable
 	@Override
-	public Map<MessageId, Integer> getUnackedMessagesToSend(
-			Transaction transaction,
-			ContactId c) throws DbException {
+	public Message getMessageToSend(Transaction transaction, ContactId c,
+			MessageId m, long maxLatency, boolean markAsSent)
+			throws DbException {
+		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		if (!db.containsVisibleMessage(txn, c, m)) return null;
+		Message message = db.getMessage(txn, m);
+		if (markAsSent) {
+			db.updateRetransmissionData(txn, c, m, maxLatency);
+			db.lowerRequestedFlag(txn, c, singletonList(m));
+			transaction.attach(new MessagesSentEvent(c, singletonList(m),
+					message.getRawLength()));
+		}
+		return message;
+	}
+
+	@Override
+	public Collection<MessageId> getUnackedMessagesToSend(
+			Transaction transaction, ContactId c) throws DbException {
 		T txn = unbox(transaction);
 		if (!db.containsContact(txn, c))
 			throw new NoSuchContactException();
@@ -794,10 +814,10 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	}
 
 	@Override
-	public long getNextSendTime(Transaction transaction, ContactId c)
-			throws DbException {
+	public long getNextSendTime(Transaction transaction, ContactId c,
+			long maxLatency) throws DbException {
 		T txn = unbox(transaction);
-		return db.getNextSendTime(txn, c);
+		return db.getNextSendTime(txn, c, maxLatency);
 	}
 
 	@Override
@@ -1005,7 +1025,8 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 				db.getGroupVisibility(txn, id).keySet();
 		db.removeGroup(txn, id);
 		transaction.attach(new GroupRemovedEvent(g));
-		transaction.attach(new GroupVisibilityUpdatedEvent(affected));
+		transaction.attach(new GroupVisibilityUpdatedEvent(INVISIBLE,
+				affected));
 	}
 
 	@Override
@@ -1069,6 +1090,20 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		db.removeTransportKeys(txn, t, k);
 	}
 
+	@Override
+	public void setAckSent(Transaction transaction, ContactId c,
+			Collection<MessageId> acked) throws DbException {
+		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		List<MessageId> visible = new ArrayList<>(acked.size());
+		for (MessageId m : acked) {
+			if (db.containsVisibleMessage(txn, c, m)) visible.add(m);
+		}
+		db.lowerAckFlag(txn, c, visible);
+	}
+
 	@Override
 	public void setCleanupTimerDuration(Transaction transaction, MessageId m,
 			long duration) throws DbException {
@@ -1115,8 +1150,8 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		if (old == INVISIBLE) db.addGroupVisibility(txn, c, g, v == SHARED);
 		else if (v == INVISIBLE) db.removeGroupVisibility(txn, c, g);
 		else db.setGroupVisibility(txn, c, g, v == SHARED);
-		List<ContactId> affected = Collections.singletonList(c);
-		transaction.attach(new GroupVisibilityUpdatedEvent(affected));
+		List<ContactId> affected = singletonList(c);
+		transaction.attach(new GroupVisibilityUpdatedEvent(v, affected));
 	}
 
 	@Override
@@ -1163,6 +1198,28 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 		transaction.attach(new MessageStateChangedEvent(m, false, state));
 	}
 
+	@Override
+	public void setMessagesSent(Transaction transaction, ContactId c,
+			Collection<MessageId> sent, long maxLatency) throws DbException {
+		if (transaction.isReadOnly()) throw new IllegalArgumentException();
+		T txn = unbox(transaction);
+		if (!db.containsContact(txn, c))
+			throw new NoSuchContactException();
+		long totalLength = 0;
+		List<MessageId> visible = new ArrayList<>(sent.size());
+		for (MessageId m : sent) {
+			if (db.containsVisibleMessage(txn, c, m)) {
+				visible.add(m);
+				totalLength += db.getMessageLength(txn, m);
+				db.updateRetransmissionData(txn, c, m, maxLatency);
+			}
+		}
+		db.lowerRequestedFlag(txn, c, visible);
+		if (!visible.isEmpty()) {
+			transaction.attach(new MessagesSentEvent(c, visible, totalLength));
+		}
+	}
+
 	@Override
 	public void addMessageDependencies(Transaction transaction,
 			Message dependent, Collection<MessageId> dependencies)

bramble-core/src/main/java/org/briarproject/bramble/db/JdbcDatabase.java

@@ -51,7 +51,6 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
-import java.util.LinkedHashMap;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
@@ -70,12 +69,14 @@ import static java.sql.Types.BOOLEAN;
 import static java.sql.Types.INTEGER;
 import static java.sql.Types.VARCHAR;
 import static java.util.Arrays.asList;
+import static java.util.logging.Level.FINE;
 import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.db.DatabaseComponent.NO_CLEANUP_DEADLINE;
 import static org.briarproject.bramble.api.db.DatabaseComponent.TIMER_NOT_STARTED;
 import static org.briarproject.bramble.api.db.Metadata.REMOVE;
+import static org.briarproject.bramble.api.record.Record.RECORD_HEADER_BYTES;
 import static org.briarproject.bramble.api.sync.Group.Visibility.INVISIBLE;
 import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
 import static org.briarproject.bramble.api.sync.Group.Visibility.VISIBLE;
@@ -102,6 +103,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 	// Package access for testing
 	static final int CODE_SCHEMA_VERSION = 50;
 
+	/**
+	 * The maximum number of idle connections to keep open.
+	 */
+	private static final int MAX_CONNECTION_POOL_SIZE = 1;
+
 	// Time period offsets for incoming transport keys
 	private static final int OFFSET_PREV = -1;
 	private static final int OFFSET_CURR = 0;
@@ -363,7 +369,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 	private final Condition connectionsChanged = connectionsLock.newCondition();
 
 	@GuardedBy("connectionsLock")
-	private final LinkedList<Connection> connections = new LinkedList<>();
+	private final LinkedList<Connection> connectionPool = new LinkedList<>();
 
 	@GuardedBy("connectionsLock")
 	private int openConnections = 0;
@@ -571,7 +577,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 		connectionsLock.lock();
 		try {
 			if (closed) throw new DbClosedException();
-			txn = connections.poll();
+			txn = connectionPool.poll();
+			logConnectionCounts();
 		} finally {
 			connectionsLock.unlock();
 		}
@@ -582,7 +589,14 @@ abstract class JdbcDatabase implements Database<Connection> {
 				txn.setAutoCommit(false);
 				connectionsLock.lock();
 				try {
+					// The DB may have been closed since the check above
+					if (closed) {
+						tryToClose(txn, LOG, WARNING);
+						throw new DbClosedException();
+					}
 					openConnections++;
+					logConnectionCounts();
+					connectionsChanged.signalAll();
 				} finally {
 					connectionsLock.unlock();
 				}
@@ -593,67 +607,91 @@ abstract class JdbcDatabase implements Database<Connection> {
 		return txn;
 	}
 
-	@Override
-	public void abortTransaction(Connection txn) {
-		try {
-			txn.rollback();
-			connectionsLock.lock();
-			try {
-				connections.add(txn);
-				connectionsChanged.signalAll();
-			} finally {
-				connectionsLock.unlock();
-			}
-		} catch (SQLException e) {
-			// Try to close the connection
-			logException(LOG, WARNING, e);
-			tryToClose(txn, LOG, WARNING);
-			// Whatever happens, allow the database to close
-			connectionsLock.lock();
-			try {
-				openConnections--;
-				connectionsChanged.signalAll();
-			} finally {
-				connectionsLock.unlock();
-			}
+	@GuardedBy("connectionsLock")
+	private void logConnectionCounts() {
+		if (LOG.isLoggable(FINE)) {
+			LOG.fine(openConnections + " connections open, "
+					+ connectionPool.size() + " in pool");
 		}
 	}
 
 	@Override
-	public void commitTransaction(Connection txn) throws DbException {
+	public void abortTransaction(Connection txn) {
+		// The transaction may have been aborted due to an earlier exception,
+		// so close the connection rather than returning it to the pool
 		try {
-			txn.commit();
+			txn.rollback();
 		} catch (SQLException e) {
-			throw new DbException(e);
+			logException(LOG, WARNING, e);
 		}
+		closeConnection(txn);
+	}
+
+	private void closeConnection(Connection txn) {
+		tryToClose(txn, LOG, WARNING);
 		connectionsLock.lock();
 		try {
-			connections.add(txn);
+			openConnections--;
+			logConnectionCounts();
 			connectionsChanged.signalAll();
 		} finally {
 			connectionsLock.unlock();
 		}
 	}
 
-	void closeAllConnections() throws SQLException {
+	@Override
+	public void commitTransaction(Connection txn) throws DbException {
+		// If the transaction commits successfully then return the connection
+		// to the pool, otherwise close it
+		try {
+			txn.commit();
+			returnConnectionToPool(txn);
+		} catch (SQLException e) {
+			logException(LOG, WARNING, e);
+			closeConnection(txn);
+			throw new DbException(e);
+		}
+	}
+
+	private void returnConnectionToPool(Connection txn) {
+		boolean shouldClose;
+		connectionsLock.lock();
+		try {
+			shouldClose = connectionPool.size() >= MAX_CONNECTION_POOL_SIZE;
+			if (shouldClose) openConnections--;
+			else connectionPool.add(txn);
+			logConnectionCounts();
+			connectionsChanged.signalAll();
+		} finally {
+			connectionsLock.unlock();
+		}
+		if (shouldClose) tryToClose(txn, LOG, WARNING);
+	}
+
+	void closeAllConnections() {
 		boolean interrupted = false;
 		connectionsLock.lock();
 		try {
 			closed = true;
-			for (Connection c : connections) c.close();
-			openConnections -= connections.size();
-			connections.clear();
+			for (Connection c : connectionPool) tryToClose(c, LOG, WARNING);
+			openConnections -= connectionPool.size();
+			connectionPool.clear();
 			while (openConnections > 0) {
+				if (LOG.isLoggable(INFO)) {
+					LOG.info("Waiting for " + openConnections
+							+ " connections to be closed");
+				}
 				try {
 					connectionsChanged.await();
 				} catch (InterruptedException e) {
 					LOG.warning("Interrupted while closing connections");
 					interrupted = true;
 				}
-				for (Connection c : connections) c.close();
-				openConnections -= connections.size();
-				connections.clear();
+				for (Connection c : connectionPool) tryToClose(c, LOG, WARNING);
+				openConnections -= connectionPool.size();
+				connectionPool.clear();
 			}
+			LOG.info("All connections closed");
 		} finally {
 			connectionsLock.unlock();
 		}
@@ -1109,8 +1147,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public boolean containsAnythingToSend(Connection txn, ContactId c,
-			long maxLatency, boolean eager) throws DbException {
+	public boolean containsAcksToSend(Connection txn, ContactId c)
+			throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
@@ -1122,34 +1160,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 			boolean acksToSend = rs.next();
 			rs.close();
 			ps.close();
-			if (acksToSend) return true;
-			if (eager) {
-				sql = "SELECT NULL from statuses"
-						+ " WHERE contactId = ? AND state = ?"
-						+ " AND groupShared = TRUE AND messageShared = TRUE"
-						+ " AND deleted = FALSE AND seen = FALSE";
-				ps = txn.prepareStatement(sql);
-				ps.setInt(1, c.getInt());
-				ps.setInt(2, DELIVERED.getValue());
-			} else {
-				long now = clock.currentTimeMillis();
-				sql = "SELECT NULL FROM statuses"
-						+ " WHERE contactId = ? AND state = ?"
-						+ " AND groupShared = TRUE AND messageShared = TRUE"
-						+ " AND deleted = FALSE AND seen = FALSE"
-						+ " AND (expiry <= ? OR maxLatency IS NULL"
-						+ " OR ? < maxLatency)";
-				ps = txn.prepareStatement(sql);
-				ps.setInt(1, c.getInt());
-				ps.setInt(2, DELIVERED.getValue());
-				ps.setLong(3, now);
-				ps.setLong(4, maxLatency);
-			}
-			rs = ps.executeQuery();
-			boolean messagesToSend = rs.next();
-			rs.close();
-			ps.close();
-			return messagesToSend;
+			return acksToSend;
 		} catch (SQLException e) {
 			tryToClose(rs, LOG, WARNING);
 			tryToClose(ps, LOG, WARNING);
@@ -1269,6 +1280,46 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public boolean containsMessagesToSend(Connection txn, ContactId c,
+			long maxLatency, boolean eager) throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			if (eager) {
+				String sql = "SELECT NULL from statuses"
+						+ " WHERE contactId = ? AND state = ?"
+						+ " AND groupShared = TRUE AND messageShared = TRUE"
+						+ " AND deleted = FALSE AND seen = FALSE";
+				ps = txn.prepareStatement(sql);
+				ps.setInt(1, c.getInt());
+				ps.setInt(2, DELIVERED.getValue());
+			} else {
+				long now = clock.currentTimeMillis();
+				String sql = "SELECT NULL FROM statuses"
+						+ " WHERE contactId = ? AND state = ?"
+						+ " AND groupShared = TRUE AND messageShared = TRUE"
+						+ " AND deleted = FALSE AND seen = FALSE"
+						+ " AND (expiry <= ? OR maxLatency IS NULL"
+						+ " OR ? < maxLatency)";
+				ps = txn.prepareStatement(sql);
+				ps.setInt(1, c.getInt());
+				ps.setInt(2, DELIVERED.getValue());
+				ps.setLong(3, now);
+				ps.setLong(4, maxLatency);
+			}
+			rs = ps.executeQuery();
+			boolean messagesToSend = rs.next();
+			rs.close();
+			ps.close();
+			return messagesToSend;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public boolean containsPendingContact(Connection txn, PendingContactId p)
 			throws DbException {
@@ -1879,6 +1930,31 @@ abstract class JdbcDatabase implements Database<Connection> {
 		}
 	}
 
+	@Override
+	public int getMessageLength(Connection txn, MessageId m)
+			throws DbException {
+		PreparedStatement ps = null;
+		ResultSet rs = null;
+		try {
+			String sql = "SELECT length from messages"
+					+ " WHERE messageId = ? AND state = ?";
+			ps = txn.prepareStatement(sql);
+			ps.setBytes(1, m.getBytes());
+			ps.setInt(2, DELIVERED.getValue());
+			rs = ps.executeQuery();
+			if (!rs.next()) throw new DbStateException();
+			int length = rs.getInt(1);
+			if (rs.next()) throw new DbStateException();
+			rs.close();
+			ps.close();
+			return length;
+		} catch (SQLException e) {
+			tryToClose(rs, LOG, WARNING);
+			tryToClose(ps, LOG, WARNING);
+			throw new DbException(e);
+		}
+	}
+
 	@Override
 	public Map<MessageId, Metadata> getMessageMetadata(Connection txn,
 			GroupId g) throws DbException {
@@ -2227,8 +2303,8 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public Collection<MessageId> getMessagesToSend(Connection txn, ContactId c,
-			int maxLength, long maxLatency) throws DbException {
+	public Collection<MessageId> getMessagesToSend(Connection txn,
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		long now = clock.currentTimeMillis();
 		PreparedStatement ps = null;
 		ResultSet rs = null;
@@ -2248,12 +2324,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 			ps.setLong(4, maxLatency);
 			rs = ps.executeQuery();
 			List<MessageId> ids = new ArrayList<>();
-			int total = 0;
 			while (rs.next()) {
 				int length = rs.getInt(1);
-				if (total + length > maxLength) break;
+				if (capacity < RECORD_HEADER_BYTES + length) break;
 				ids.add(new MessageId(rs.getBytes(2)));
-				total += length;
+				capacity -= RECORD_HEADER_BYTES + length;
 			}
 			rs.close();
 			ps.close();
@@ -2266,12 +2341,12 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public Map<MessageId, Integer> getUnackedMessagesToSend(Connection txn,
+	public Collection<MessageId> getUnackedMessagesToSend(Connection txn,
 			ContactId c) throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
-			String sql = "SELECT length, messageId FROM statuses"
+			String sql = "SELECT messageId FROM statuses"
 					+ " WHERE contactId = ? AND state = ?"
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE AND seen = FALSE"
@@ -2280,15 +2355,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 			ps.setInt(1, c.getInt());
 			ps.setInt(2, DELIVERED.getValue());
 			rs = ps.executeQuery();
-			Map<MessageId, Integer> results = new LinkedHashMap<>();
-			while (rs.next()) {
-				int length = rs.getInt(1);
-				MessageId id = new MessageId(rs.getBytes(2));
-				results.put(id, length);
-			}
+			List<MessageId> ids = new ArrayList<>();
+			while (rs.next()) ids.add(new MessageId(rs.getBytes(1)));
 			rs.close();
 			ps.close();
-			return results;
+			return ids;
 		} catch (SQLException e) {
 			tryToClose(rs, LOG, WARNING);
 			tryToClose(ps, LOG, WARNING);
@@ -2401,6 +2472,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 				MessageId m = new MessageId(rs.getBytes(1));
 				GroupId g = new GroupId(rs.getBytes(2));
 				Collection<MessageId> messageIds = ids.get(g);
+				//noinspection Java8MapApi
 				if (messageIds == null) {
 					messageIds = new ArrayList<>();
 					ids.put(g, messageIds);
@@ -2418,12 +2490,28 @@ abstract class JdbcDatabase implements Database<Connection> {
 	}
 
 	@Override
-	public long getNextSendTime(Connection txn, ContactId c)
+	public long getNextSendTime(Connection txn, ContactId c, long maxLatency)
 			throws DbException {
 		PreparedStatement ps = null;
 		ResultSet rs = null;
 		try {
-			String sql = "SELECT expiry FROM statuses"
+			// Are any messages sendable immediately?
+			String sql = "SELECT NULL FROM statuses"
+					+ " WHERE contactId = ? AND state = ?"
+					+ " AND groupShared = TRUE AND messageShared = TRUE"
+					+ " AND deleted = FALSE AND seen = FALSE"
+					+ " AND (maxLatency IS NULL OR ? < maxLatency)";
+			ps = txn.prepareStatement(sql);
+			ps.setInt(1, c.getInt());
+			ps.setInt(2, DELIVERED.getValue());
+			ps.setLong(3, maxLatency);
+			rs = ps.executeQuery();
+			boolean found = rs.next();
+			rs.close();
+			ps.close();
+			if (found) return 0;
+			// When is the earliest expiry time (could be in the past)?
+			sql = "SELECT expiry FROM statuses"
 					+ " WHERE contactId = ? AND state = ?"
 					+ " AND groupShared = TRUE AND messageShared = TRUE"
 					+ " AND deleted = FALSE AND seen = FALSE"
@@ -2527,7 +2615,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 
 	@Override
 	public Collection<MessageId> getRequestedMessagesToSend(Connection txn,
-			ContactId c, int maxLength, long maxLatency) throws DbException {
+			ContactId c, long capacity, long maxLatency) throws DbException {
 		long now = clock.currentTimeMillis();
 		PreparedStatement ps = null;
 		ResultSet rs = null;
@@ -2547,12 +2635,11 @@ abstract class JdbcDatabase implements Database<Connection> {
 			ps.setLong(4, maxLatency);
 			rs = ps.executeQuery();
 			List<MessageId> ids = new ArrayList<>();
-			int total = 0;
 			while (rs.next()) {
 				int length = rs.getInt(1);
-				if (total + length > maxLength) break;
+				if (capacity < RECORD_HEADER_BYTES + length) break;
 				ids.add(new MessageId(rs.getBytes(2)));
-				total += length;
+				capacity -= RECORD_HEADER_BYTES + length;
 			}
 			rs.close();
 			ps.close();
@@ -2706,6 +2793,7 @@ abstract class JdbcDatabase implements Database<Connection> {
 				ContactId c = new ContactId(rs.getInt(1));
 				TransportId t = new TransportId(rs.getString(2));
 				Collection<TransportId> transportIds = ids.get(c);
+				//noinspection Java8MapApi
 				if (transportIds == null) {
 					transportIds = new ArrayList<>();
 					ids.put(c, transportIds);

bramble-core/src/main/java/org/briarproject/bramble/io/TimeoutMonitorImpl.java

@@ -1,10 +1,10 @@
 package org.briarproject.bramble.io;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.io.TimeoutMonitor;
 import org.briarproject.bramble.api.lifecycle.IoExecutor;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 import org.briarproject.bramble.api.system.Wakeful;
 
 import java.io.IOException;

bramble-core/src/main/java/org/briarproject/bramble/lifecycle/LifecycleManagerImpl.java

@@ -18,7 +18,7 @@ import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Semaphore;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.logging.Logger;
 
 import javax.annotation.concurrent.ThreadSafe;
@@ -29,10 +29,12 @@ import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.COMPACTING_DATABASE;
+import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.CREATED;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.MIGRATING_DATABASE;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.RUNNING;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STARTING;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STARTING_SERVICES;
+import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STOPPED;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.STOPPING;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.StartResult.ALREADY_RUNNING;
 import static org.briarproject.bramble.api.lifecycle.LifecycleManager.StartResult.CLOCK_ERROR;
@@ -60,12 +62,11 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 	private final List<Service> services;
 	private final List<OpenDatabaseHook> openDatabaseHooks;
 	private final List<ExecutorService> executors;
-	private final Semaphore startStopSemaphore = new Semaphore(1);
 	private final CountDownLatch dbLatch = new CountDownLatch(1);
 	private final CountDownLatch startupLatch = new CountDownLatch(1);
 	private final CountDownLatch shutdownLatch = new CountDownLatch(1);
-
-	private volatile LifecycleState state = STARTING;
+	private final AtomicReference<LifecycleState> state =
+			new AtomicReference<>(CREATED);
 
 	@Inject
 	LifecycleManagerImpl(DatabaseComponent db, EventBus eventBus,
@@ -102,8 +103,8 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 
 	@Override
 	public StartResult startServices(SecretKey dbKey) {
-		if (!startStopSemaphore.tryAcquire()) {
-			LOG.info("Already starting or stopping");
+		if (!state.compareAndSet(CREATED, STARTING)) {
+			LOG.warning("Already running");
 			return ALREADY_RUNNING;
 		}
 		long now = clock.currentTimeMillis();
@@ -135,7 +136,7 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 			});
 
 			LOG.info("Starting services");
-			state = STARTING_SERVICES;
+			state.set(STARTING_SERVICES);
 			dbLatch.countDown();
 			eventBus.broadcast(new LifecycleEvent(STARTING_SERVICES));
 
@@ -148,7 +149,7 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 				}
 			}
 
-			state = RUNNING;
+			state.set(RUNNING);
 			startupLatch.countDown();
 			eventBus.broadcast(new LifecycleEvent(RUNNING));
 			return SUCCESS;
@@ -164,59 +165,58 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 		} catch (ServiceException e) {
 			logException(LOG, WARNING, e);
 			return SERVICE_ERROR;
-		} finally {
-			startStopSemaphore.release();
 		}
 	}
 
 	@Override
 	public void onDatabaseMigration() {
-		state = MIGRATING_DATABASE;
+		state.set(MIGRATING_DATABASE);
 		eventBus.broadcast(new LifecycleEvent(MIGRATING_DATABASE));
 	}
 
 	@Override
 	public void onDatabaseCompaction() {
-		state = COMPACTING_DATABASE;
+		state.set(COMPACTING_DATABASE);
 		eventBus.broadcast(new LifecycleEvent(COMPACTING_DATABASE));
 	}
 
 	@Override
 	public void stopServices() {
-		try {
-			startStopSemaphore.acquire();
-		} catch (InterruptedException e) {
-			LOG.warning("Interrupted while waiting to stop services");
+		if (!state.compareAndSet(RUNNING, STOPPING)) {
+			LOG.warning("Not running");
 			return;
 		}
-		try {
-			LOG.info("Stopping services");
-			state = STOPPING;
-			eventBus.broadcast(new LifecycleEvent(STOPPING));
-			for (Service s : services) {
+		LOG.info("Stopping services");
+		eventBus.broadcast(new LifecycleEvent(STOPPING));
+		for (Service s : services) {
+			try {
 				long start = now();
 				s.stopService();
 				if (LOG.isLoggable(FINE)) {
 					logDuration(LOG, "Stopping service "
 							+ s.getClass().getSimpleName(), start);
 				}
+			} catch (ServiceException e) {
+				logException(LOG, WARNING, e);
 			}
-			for (ExecutorService e : executors) {
-				if (LOG.isLoggable(FINE)) {
-					LOG.fine("Stopping executor "
-							+ e.getClass().getSimpleName());
-				}
-				e.shutdownNow();
+		}
+		for (ExecutorService e : executors) {
+			if (LOG.isLoggable(FINE)) {
+				LOG.fine("Stopping executor "
+						+ e.getClass().getSimpleName());
 			}
+			e.shutdownNow();
+		}
+		try {
 			long start = now();
 			db.close();
 			logDuration(LOG, "Closing database", start);
-			shutdownLatch.countDown();
-		} catch (DbException | ServiceException e) {
+		} catch (DbException e) {
 			logException(LOG, WARNING, e);
-		} finally {
-			startStopSemaphore.release();
 		}
+		state.set(STOPPED);
+		shutdownLatch.countDown();
+		eventBus.broadcast(new LifecycleEvent(STOPPED));
 	}
 
 	@Override
@@ -236,6 +236,6 @@ class LifecycleManagerImpl implements LifecycleManager, MigrationListener {
 
 	@Override
 	public LifecycleState getLifecycleState() {
-		return state;
+		return state.get();
 	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ApiCall.java

@@ -0,0 +1,21 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+/**
+ * An interface for calling an API endpoint with the option to retry the call.
+ */
+interface ApiCall {
+
+	/**
+	 * This method makes a synchronous call to an API endpoint and returns
+	 * true if the call should be retried, in which case the method may be
+	 * called again on the same {@link ApiCall} instance after a delay.
+	 *
+	 * @return True if the API call needs to be retried, or false if the API
+	 * call succeeded or {@link TolerableFailureException failed tolerably}.
+	 */
+	@IoExecutor
+	boolean callApi();
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ConnectivityChecker.java

@@ -0,0 +1,43 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * An interface for checking whether a mailbox is reachable.
+ */
+@ThreadSafe
+@NotNullByDefault
+interface ConnectivityChecker {
+
+	/**
+	 * Destroys the checker. Any current connectivity check is cancelled.
+	 */
+	void destroy();
+
+	/**
+	 * Starts a connectivity check if needed and calls the given observer when
+	 * the check succeeds. If a check is already running then the observer is
+	 * called when the check succeeds. If a connectivity check has recently
+	 * succeeded then the observer is called immediately.
+	 * <p>
+	 * Observers are removed after being called, or when the checker is
+	 * {@link #destroy() destroyed}.
+	 */
+	void checkConnectivity(MailboxProperties properties,
+			ConnectivityObserver o);
+
+	/**
+	 * Removes an observer that was added via
+	 * {@link #checkConnectivity(MailboxProperties, ConnectivityObserver)}. If
+	 * there are no remaining observers and a connectivity check is running
+	 * then the check will be cancelled.
+	 */
+	void removeObserver(ConnectivityObserver o);
+
+	interface ConnectivityObserver {
+		void onConnectivityCheckSucceeded();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ConnectivityCheckerImpl.java

@@ -0,0 +1,122 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+abstract class ConnectivityCheckerImpl implements ConnectivityChecker {
+
+	/**
+	 * If no more than this much time has elapsed since the last connectivity
+	 * check succeeded, consider the result to be fresh and don't check again.
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final long CONNECTIVITY_CHECK_FRESHNESS_MS = 10_000;
+
+	private final Object lock = new Object();
+
+	protected final Clock clock;
+	private final MailboxApiCaller mailboxApiCaller;
+
+	@GuardedBy("lock")
+	private boolean destroyed = false;
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable connectivityCheck = null;
+
+	@GuardedBy("lock")
+	private long lastConnectivityCheckSucceeded = 0;
+
+	@GuardedBy("lock")
+	private final List<ConnectivityObserver> connectivityObservers =
+			new ArrayList<>();
+
+	/**
+	 * Creates an {@link ApiCall} for checking whether the mailbox is
+	 * reachable. The {@link ApiCall} should call
+	 * {@link #onConnectivityCheckSucceeded(long)} if the check succeeds.
+	 */
+	abstract ApiCall createConnectivityCheckTask(MailboxProperties properties);
+
+	ConnectivityCheckerImpl(Clock clock, MailboxApiCaller mailboxApiCaller) {
+		this.clock = clock;
+		this.mailboxApiCaller = mailboxApiCaller;
+	}
+
+	@Override
+	public void destroy() {
+		synchronized (lock) {
+			destroyed = true;
+			connectivityObservers.clear();
+			if (connectivityCheck != null) {
+				connectivityCheck.cancel();
+				connectivityCheck = null;
+			}
+		}
+	}
+
+	@Override
+	public void checkConnectivity(MailboxProperties properties,
+			ConnectivityObserver o) {
+		boolean callNow = false;
+		synchronized (lock) {
+			if (destroyed) return;
+			if (connectivityCheck == null) {
+				// No connectivity check is running
+				long now = clock.currentTimeMillis();
+				if (now - lastConnectivityCheckSucceeded
+						> CONNECTIVITY_CHECK_FRESHNESS_MS) {
+					// The last connectivity check is stale, start a new one
+					connectivityObservers.add(o);
+					ApiCall task = createConnectivityCheckTask(properties);
+					connectivityCheck = mailboxApiCaller.retryWithBackoff(task);
+				} else {
+					// The last connectivity check is fresh
+					callNow = true;
+				}
+			} else {
+				// A connectivity check is running, wait for it to succeed
+				connectivityObservers.add(o);
+			}
+		}
+		if (callNow) o.onConnectivityCheckSucceeded();
+	}
+
+	protected void onConnectivityCheckSucceeded(long now) {
+		List<ConnectivityObserver> observers;
+		synchronized (lock) {
+			if (destroyed) return;
+			connectivityCheck = null;
+			lastConnectivityCheckSucceeded = now;
+			observers = new ArrayList<>(connectivityObservers);
+			connectivityObservers.clear();
+		}
+		for (ConnectivityObserver o : observers) {
+			o.onConnectivityCheckSucceeded();
+		}
+	}
+
+	@Override
+	public void removeObserver(ConnectivityObserver o) {
+		synchronized (lock) {
+			if (destroyed) return;
+			connectivityObservers.remove(o);
+			if (connectivityObservers.isEmpty() && connectivityCheck != null) {
+				connectivityCheck.cancel();
+				connectivityCheck = null;
+			}
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxClient.java

@@ -0,0 +1,120 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Logger.getLogger;
+
+@ThreadSafe
+@NotNullByDefault
+class ContactMailboxClient implements MailboxClient {
+
+	private static final Logger LOG =
+			getLogger(ContactMailboxClient.class.getName());
+
+	private final MailboxWorkerFactory workerFactory;
+	private final ConnectivityChecker connectivityChecker;
+	private final TorReachabilityMonitor reachabilityMonitor;
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	@Nullable
+	private MailboxWorker uploadWorker = null, downloadWorker = null;
+
+	ContactMailboxClient(MailboxWorkerFactory workerFactory,
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor) {
+		this.workerFactory = workerFactory;
+		this.connectivityChecker = connectivityChecker;
+		this.reachabilityMonitor = reachabilityMonitor;
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		// Nothing to do until contact is assigned
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		MailboxWorker uploadWorker, downloadWorker;
+		synchronized (lock) {
+			uploadWorker = this.uploadWorker;
+			this.uploadWorker = null;
+			downloadWorker = this.downloadWorker;
+			this.downloadWorker = null;
+		}
+		if (uploadWorker != null) uploadWorker.destroy();
+		if (downloadWorker != null) downloadWorker.destroy();
+	}
+
+	@Override
+	public void assignContactForUpload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		LOG.info("Contact assigned for upload");
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		// For a contact's mailbox we should always be uploading to the outbox
+		// assigned to us by the contact
+		if (!folderId.equals(properties.getOutboxId())) {
+			throw new IllegalArgumentException();
+		}
+		MailboxWorker uploadWorker = workerFactory.createUploadWorker(
+				connectivityChecker, properties, folderId, contactId);
+		synchronized (lock) {
+			if (this.uploadWorker != null) throw new IllegalStateException();
+			this.uploadWorker = uploadWorker;
+		}
+		uploadWorker.start();
+	}
+
+	@Override
+	public void deassignContactForUpload(ContactId contactId) {
+		LOG.info("Contact deassigned for upload");
+		MailboxWorker uploadWorker;
+		synchronized (lock) {
+			uploadWorker = this.uploadWorker;
+			this.uploadWorker = null;
+		}
+		if (uploadWorker != null) uploadWorker.destroy();
+	}
+
+	@Override
+	public void assignContactForDownload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		LOG.info("Contact assigned for download");
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		// For a contact's mailbox we should always be downloading from the
+		// inbox assigned to us by the contact
+		if (!folderId.equals(properties.getInboxId())) {
+			throw new IllegalArgumentException();
+		}
+		MailboxWorker downloadWorker =
+				workerFactory.createDownloadWorkerForContactMailbox(
+						connectivityChecker, reachabilityMonitor, properties);
+		synchronized (lock) {
+			if (this.downloadWorker != null) throw new IllegalStateException();
+			this.downloadWorker = downloadWorker;
+		}
+		downloadWorker.start();
+	}
+
+	@Override
+	public void deassignContactForDownload(ContactId contactId) {
+		LOG.info("Contact deassigned for download");
+		MailboxWorker downloadWorker;
+		synchronized (lock) {
+			downloadWorker = this.downloadWorker;
+			this.downloadWorker = null;
+		}
+		if (downloadWorker != null) downloadWorker.destroy();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxConnectivityChecker.java

@@ -0,0 +1,32 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+class ContactMailboxConnectivityChecker extends ConnectivityCheckerImpl {
+
+	private final MailboxApi mailboxApi;
+
+	ContactMailboxConnectivityChecker(Clock clock,
+			MailboxApiCaller mailboxApiCaller, MailboxApi mailboxApi) {
+		super(clock, mailboxApiCaller);
+		this.mailboxApi = mailboxApi;
+	}
+
+	@Override
+	ApiCall createConnectivityCheckTask(MailboxProperties properties) {
+		if (properties.isOwner()) throw new IllegalArgumentException();
+		return new SimpleApiCall(() -> {
+			if (!mailboxApi.checkStatus(properties)) throw new ApiException();
+			// Call the observers and cache the result
+			onConnectivityCheckSucceeded(clock.currentTimeMillis());
+		});
+	}
+
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/ContactMailboxDownloadWorker.java

@@ -0,0 +1,65 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.MailboxFile;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+import java.io.IOException;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Queue;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.Collections.emptyList;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+
+@ThreadSafe
+@NotNullByDefault
+class ContactMailboxDownloadWorker extends MailboxDownloadWorker {
+
+	ContactMailboxDownloadWorker(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor torReachabilityMonitor,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxProperties mailboxProperties) {
+		super(connectivityChecker, torReachabilityMonitor, mailboxApiCaller,
+				mailboxApi, mailboxFileManager, mailboxProperties);
+		if (mailboxProperties.isOwner()) throw new IllegalArgumentException();
+	}
+
+	@Override
+	protected ApiCall createApiCallForDownloadCycle() {
+		return new SimpleApiCall(this::apiCallListInbox);
+	}
+
+	private void apiCallListInbox() throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		LOG.info("Listing inbox");
+		MailboxFolderId folderId =
+				requireNonNull(mailboxProperties.getInboxId());
+		List<MailboxFile> files;
+		try {
+			files = mailboxApi.getFiles(mailboxProperties, folderId);
+		} catch (TolerableFailureException e) {
+			LOG.warning("Inbox folder does not exist");
+			files = emptyList();
+		}
+		if (files.isEmpty()) {
+			onDownloadCycleFinished();
+		} else {
+			Queue<FolderFile> queue = new LinkedList<>();
+			for (MailboxFile file : files) {
+				queue.add(new FolderFile(folderId, file.name));
+			}
+			downloadNextFile(queue);
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApi.java

@@ -7,6 +7,8 @@ import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxFileId;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.io.File;
 import java.io.IOException;
@@ -16,8 +18,12 @@ import java.util.List;
 import javax.annotation.Nonnull;
 import javax.annotation.concurrent.Immutable;
 
+@NotNullByDefault
 interface MailboxApi {
 
+	List<MailboxVersion> getServerSupports(MailboxProperties properties)
+			throws IOException, ApiException;
+
 	/**
 	 * Sets up the mailbox with the setup token.
 	 *
@@ -25,7 +31,7 @@ interface MailboxApi {
 	 * @return the owner token
 	 * @throws ApiException for 401 response.
 	 */
-	MailboxAuthToken setup(MailboxProperties properties)
+	MailboxProperties setup(MailboxProperties properties)
 			throws IOException, ApiException;
 
 	/**
@@ -85,9 +91,13 @@ interface MailboxApi {
 	 * Used by owner and contacts to list their files to retrieve.
 	 * <p>
 	 * Returns 200 OK with the list of files in JSON.
+	 *
+	 * @throws TolerableFailureException if response code is 404 (folder does
+	 * not exist or client is not authorised to download from it)
 	 */
 	List<MailboxFile> getFiles(MailboxProperties properties,
-			MailboxFolderId folderId) throws IOException, ApiException;
+			MailboxFolderId folderId)
+			throws IOException, ApiException, TolerableFailureException;
 
 	/**
 	 * Used by owner and contacts to retrieve a file.
@@ -96,17 +106,22 @@ interface MailboxApi {
 	 * in the response body.
 	 *
 	 * @param file the empty file the response bytes will be written into.
+	 * @throws TolerableFailureException if response code is 404 (folder does
+	 * not exist, client is not authorised to download from folder, or file
+	 * does not exist)
 	 */
 	void getFile(MailboxProperties properties, MailboxFolderId folderId,
-			MailboxFileId fileId, File file) throws IOException, ApiException;
+			MailboxFileId fileId, File file)
+			throws IOException, ApiException, TolerableFailureException;
 
 	/**
 	 * Used by owner and contacts to delete files.
 	 * <p>
 	 * Returns 200 OK (no exception) if deletion was successful.
 	 *
-	 * @throws TolerableFailureException on 404 response,
-	 * because file was most likely deleted already.
+	 * @throws TolerableFailureException if response code is 404 (folder does
+	 * not exist, client is not authorised to download from folder, or file
+	 * does not exist)
 	 */
 	void deleteFile(MailboxProperties properties, MailboxFolderId folderId,
 			MailboxFileId fileId)

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApiCaller.java

@@ -0,0 +1,34 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import static java.util.concurrent.TimeUnit.DAYS;
+import static java.util.concurrent.TimeUnit.MINUTES;
+
+@NotNullByDefault
+interface MailboxApiCaller {
+
+	/**
+	 * The minimum interval between retries in milliseconds.
+	 */
+	long MIN_RETRY_INTERVAL_MS = MINUTES.toMillis(1);
+
+	/**
+	 * The maximum interval between retries in milliseconds.
+	 */
+	long MAX_RETRY_INTERVAL_MS = DAYS.toMillis(1);
+
+	/**
+	 * Asynchronously calls the given API call on the {@link IoExecutor},
+	 * automatically retrying at increasing intervals until the API call
+	 * returns false or retries are cancelled.
+	 * <p>
+	 * This method is safe to call while holding a lock.
+	 *
+	 * @return A {@link Cancellable} that can be used to cancel any future
+	 * retries.
+	 */
+	Cancellable retryWithBackoff(ApiCall apiCall);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApiCallerImpl.java

@@ -0,0 +1,98 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.TaskScheduler;
+
+import java.util.concurrent.Executor;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+import static java.lang.Math.min;
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+
+@Immutable
+@NotNullByDefault
+class MailboxApiCallerImpl implements MailboxApiCaller {
+
+	private final TaskScheduler taskScheduler;
+	private final Executor ioExecutor;
+
+	@Inject
+	MailboxApiCallerImpl(TaskScheduler taskScheduler,
+			@IoExecutor Executor ioExecutor) {
+		this.taskScheduler = taskScheduler;
+		this.ioExecutor = ioExecutor;
+	}
+
+	@Override
+	public Cancellable retryWithBackoff(ApiCall apiCall) {
+		Task task = new Task(apiCall);
+		task.start();
+		return task;
+	}
+
+	private class Task implements Cancellable {
+
+		private final ApiCall apiCall;
+		private final Object lock = new Object();
+
+		@GuardedBy("lock")
+		@Nullable
+		private Cancellable scheduledTask = null;
+
+		@GuardedBy("lock")
+		private boolean cancelled = false;
+
+		@GuardedBy("lock")
+		private long retryIntervalMs = MIN_RETRY_INTERVAL_MS;
+
+		private Task(ApiCall apiCall) {
+			this.apiCall = apiCall;
+		}
+
+		private void start() {
+			synchronized (lock) {
+				if (cancelled) throw new AssertionError();
+				ioExecutor.execute(this::callApi);
+			}
+		}
+
+		@IoExecutor
+		private void callApi() {
+			synchronized (lock) {
+				if (cancelled) return;
+			}
+			// The call returns true if we should retry
+			if (apiCall.callApi()) {
+				synchronized (lock) {
+					if (cancelled) return;
+					scheduledTask = taskScheduler.schedule(this::callApi,
+							ioExecutor, retryIntervalMs, MILLISECONDS);
+					// Increase the retry interval each time we retry
+					retryIntervalMs =
+							min(MAX_RETRY_INTERVAL_MS, retryIntervalMs * 2);
+				}
+			} else {
+				synchronized (lock) {
+					scheduledTask = null;
+				}
+			}
+		}
+
+		@Override
+		public void cancel() {
+			Cancellable scheduledTask;
+			synchronized (lock) {
+				cancelled = true;
+				scheduledTask = this.scheduledTask;
+				this.scheduledTask = null;
+			}
+			if (scheduledTask != null) scheduledTask.cancel();
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxApiImpl.java

@@ -14,6 +14,7 @@ import org.briarproject.bramble.api.mailbox.MailboxFileId;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
 import org.briarproject.bramble.api.mailbox.MailboxId;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.io.File;
@@ -41,26 +42,47 @@ import static org.briarproject.bramble.util.IoUtils.copyAndClose;
 @NotNullByDefault
 class MailboxApiImpl implements MailboxApi {
 
-	private final WeakSingletonProvider<OkHttpClient> httpClientProvider;
-	private final JsonMapper mapper = JsonMapper.builder()
-			.enable(BLOCK_UNSAFE_POLYMORPHIC_BASE_TYPES)
-			.build();
 	private static final MediaType JSON =
 			requireNonNull(MediaType.parse("application/json; charset=utf-8"));
 	private static final MediaType FILE =
 			requireNonNull(MediaType.parse("application/octet-stream"));
 
+	private final WeakSingletonProvider<OkHttpClient> httpClientProvider;
+	private final JsonMapper mapper = JsonMapper.builder()
+			.enable(BLOCK_UNSAFE_POLYMORPHIC_BASE_TYPES)
+			.build();
+	private final UrlConverter urlConverter;
+
 	@Inject
-	MailboxApiImpl(WeakSingletonProvider<OkHttpClient> httpClientProvider) {
+	MailboxApiImpl(WeakSingletonProvider<OkHttpClient> httpClientProvider,
+			UrlConverter urlConverter) {
 		this.httpClientProvider = httpClientProvider;
+		this.urlConverter = urlConverter;
 	}
 
 	@Override
-	public MailboxAuthToken setup(MailboxProperties properties)
+	public List<MailboxVersion> getServerSupports(MailboxProperties properties)
+			throws IOException, ApiException {
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		Response response = sendGetRequest(properties, "/versions");
+		if (response.code() != 200) throw new ApiException();
+
+		ResponseBody body = response.body();
+		if (body == null) throw new ApiException();
+		try {
+			JsonNode node = mapper.readTree(body.string());
+			return parseServerSupports(node);
+		} catch (JacksonException e) {
+			throw new ApiException();
+		}
+	}
+
+	@Override
+	public MailboxProperties setup(MailboxProperties properties)
 			throws IOException, ApiException {
 		if (!properties.isOwner()) throw new IllegalArgumentException();
 		Request request = getRequestBuilder(properties.getAuthToken())
-				.url(properties.getBaseUrl() + "/setup")
+				.url(getBaseUrl(properties) + "/setup")
 				.put(EMPTY_REQUEST)
 				.build();
 		OkHttpClient client = httpClientProvider.get();
@@ -75,17 +97,40 @@ class MailboxApiImpl implements MailboxApi {
 			if (tokenNode == null) {
 				throw new ApiException();
 			}
-			String ownerToken = tokenNode.textValue();
-			return MailboxAuthToken.fromString(ownerToken);
+			return new MailboxProperties(properties.getOnion(),
+					MailboxAuthToken.fromString(tokenNode.textValue()),
+					parseServerSupports(node));
 		} catch (JacksonException | InvalidMailboxIdException e) {
 			throw new ApiException();
 		}
 	}
 
+	private List<MailboxVersion> parseServerSupports(JsonNode node)
+			throws ApiException {
+		List<MailboxVersion> serverSupports = new ArrayList<>();
+		ArrayNode serverSupportsNode = getArray(node, "serverSupports");
+		for (JsonNode versionNode : serverSupportsNode) {
+			if (!versionNode.isObject()) throw new ApiException();
+			ObjectNode objectNode = (ObjectNode) versionNode;
+			JsonNode majorNode = objectNode.get("major");
+			JsonNode minorNode = objectNode.get("minor");
+			if (majorNode == null || !majorNode.isNumber()) {
+				throw new ApiException();
+			}
+			if (minorNode == null || !minorNode.isNumber()) {
+				throw new ApiException();
+			}
+			int major = majorNode.asInt();
+			int minor = minorNode.asInt();
+			if (major < 0 || minor < 0) throw new ApiException();
+			serverSupports.add(new MailboxVersion(major, minor));
+		}
+		return serverSupports;
+	}
+
 	@Override
 	public boolean checkStatus(MailboxProperties properties)
 			throws IOException, ApiException {
-		if (!properties.isOwner()) throw new IllegalArgumentException();
 		Response response = sendGetRequest(properties, "/status");
 		if (response.code() == 401) throw new ApiException();
 		return response.isSuccessful();
@@ -96,7 +141,7 @@ class MailboxApiImpl implements MailboxApi {
 			throws IOException, ApiException {
 		if (!properties.isOwner()) throw new IllegalArgumentException();
 		Request request = getRequestBuilder(properties.getAuthToken())
-				.url(properties.getBaseUrl() + "/")
+				.url(getBaseUrl(properties) + "/")
 				.delete()
 				.build();
 		OkHttpClient client = httpClientProvider.get();
@@ -121,7 +166,7 @@ class MailboxApiImpl implements MailboxApi {
 	public void deleteContact(MailboxProperties properties, ContactId contactId)
 			throws IOException, ApiException, TolerableFailureException {
 		if (!properties.isOwner()) throw new IllegalArgumentException();
-		String url = properties.getBaseUrl() + "/contacts/" +
+		String url = getBaseUrl(properties) + "/contacts/" +
 				contactId.getInt();
 		Request request = getRequestBuilder(properties.getAuthToken())
 				.delete()
@@ -171,9 +216,11 @@ class MailboxApiImpl implements MailboxApi {
 
 	@Override
 	public List<MailboxFile> getFiles(MailboxProperties properties,
-			MailboxFolderId folderId) throws IOException, ApiException {
+			MailboxFolderId folderId)
+			throws IOException, ApiException, TolerableFailureException {
 		String path = "/files/" + folderId;
 		Response response = sendGetRequest(properties, path);
+		if (response.code() == 404) throw new TolerableFailureException();
 		if (response.code() != 200) throw new ApiException();
 
 		ResponseBody body = response.body();
@@ -207,9 +254,11 @@ class MailboxApiImpl implements MailboxApi {
 
 	@Override
 	public void getFile(MailboxProperties properties, MailboxFolderId folderId,
-			MailboxFileId fileId, File file) throws IOException, ApiException {
+			MailboxFileId fileId, File file)
+			throws IOException, ApiException, TolerableFailureException {
 		String path = "/files/" + folderId + "/" + fileId;
 		Response response = sendGetRequest(properties, path);
+		if (response.code() == 404) throw new TolerableFailureException();
 		if (response.code() != 200) throw new ApiException();
 
 		ResponseBody body = response.body();
@@ -225,7 +274,7 @@ class MailboxApiImpl implements MailboxApi {
 		String path = "/files/" + folderId + "/" + fileId;
 		Request request = getRequestBuilder(properties.getAuthToken())
 				.delete()
-				.url(properties.getBaseUrl() + path)
+				.url(getBaseUrl(properties) + path)
 				.build();
 		OkHttpClient client = httpClientProvider.get();
 		Response response = client.newCall(request).execute();
@@ -267,7 +316,7 @@ class MailboxApiImpl implements MailboxApi {
 	private Response sendGetRequest(MailboxProperties properties, String path)
 			throws IOException {
 		Request request = getRequestBuilder(properties.getAuthToken())
-				.url(properties.getBaseUrl() + path)
+				.url(getBaseUrl(properties) + path)
 				.build();
 		OkHttpClient client = httpClientProvider.get();
 		return client.newCall(request).execute();
@@ -276,7 +325,7 @@ class MailboxApiImpl implements MailboxApi {
 	private Response sendPostRequest(MailboxProperties properties, String path,
 			RequestBody body) throws IOException {
 		Request request = getRequestBuilder(properties.getAuthToken())
-				.url(properties.getBaseUrl() + path)
+				.url(getBaseUrl(properties) + path)
 				.post(body)
 				.build();
 		OkHttpClient client = httpClientProvider.get();
@@ -298,4 +347,7 @@ class MailboxApiImpl implements MailboxApi {
 		return (ArrayNode) arrayNode;
 	}
 
+	private String getBaseUrl(MailboxProperties properties) {
+		return urlConverter.convertOnionToBaseUrl(properties.getOnion());
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxClient.java

@@ -0,0 +1,55 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+interface MailboxClient {
+
+	/**
+	 * Asynchronously starts the client.
+	 */
+	void start();
+
+	/**
+	 * Destroys the client and its workers, cancelling any pending tasks or
+	 * retries.
+	 */
+	void destroy();
+
+	/**
+	 * Assigns a contact to the client for upload.
+	 *
+	 * @param properties Properties for communicating with the mailbox
+	 * managed by this client.
+	 * @param folderId The ID of the folder to which files will be uploaded.
+	 */
+	void assignContactForUpload(ContactId c, MailboxProperties properties,
+			MailboxFolderId folderId);
+
+	/**
+	 * Deassigns a contact from the client for upload.
+	 */
+	void deassignContactForUpload(ContactId c);
+
+	/**
+	 * Assigns a contact to the client for download.
+	 *
+	 * @param properties Properties for communicating with the mailbox
+	 * managed by this client.
+	 * @param folderId The ID of the folder from which files will be
+	 * downloaded.
+	 */
+	void assignContactForDownload(ContactId c, MailboxProperties properties,
+			MailboxFolderId folderId);
+
+	/**
+	 * Deassigns a contact from the client for download.
+	 */
+	void deassignContactForDownload(ContactId c);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxDownloadWorker.java

@@ -0,0 +1,250 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.mailbox.MailboxFileId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+import org.briarproject.bramble.mailbox.TorReachabilityMonitor.TorReachabilityObserver;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.Queue;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Logger.getLogger;
+
+@ThreadSafe
+@NotNullByDefault
+abstract class MailboxDownloadWorker implements MailboxWorker,
+		ConnectivityObserver, TorReachabilityObserver {
+
+	/**
+	 * When the worker is started it waits for a connectivity check, then
+	 * starts its first download cycle: checking for files to download,
+	 * downloading and deleting the files, and checking again until all files
+	 * have been downloaded and deleted.
+	 * <p>
+	 * The worker then waits for our Tor hidden service to be reachable before
+	 * starting its second download cycle. This ensures that if a contact
+	 * tried and failed to connect to our hidden service before it was
+	 * reachable, and therefore uploaded a file to the mailbox instead, we'll
+	 * find the file in the second download cycle.
+	 */
+	protected enum State {
+		CREATED,
+		CONNECTIVITY_CHECK,
+		DOWNLOAD_CYCLE_1,
+		WAITING_FOR_TOR,
+		DOWNLOAD_CYCLE_2,
+		FINISHED,
+		DESTROYED
+	}
+
+	protected static final Logger LOG =
+			getLogger(MailboxDownloadWorker.class.getName());
+
+	private final ConnectivityChecker connectivityChecker;
+	private final TorReachabilityMonitor torReachabilityMonitor;
+	protected final MailboxApiCaller mailboxApiCaller;
+	protected final MailboxApi mailboxApi;
+	private final MailboxFileManager mailboxFileManager;
+	protected final MailboxProperties mailboxProperties;
+	protected final Object lock = new Object();
+
+	@GuardedBy("lock")
+	protected State state = State.CREATED;
+
+	@GuardedBy("lock")
+	@Nullable
+	protected Cancellable apiCall = null;
+
+	/**
+	 * Creates the API call that starts the worker's download cycle.
+	 */
+	protected abstract ApiCall createApiCallForDownloadCycle();
+
+	MailboxDownloadWorker(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor torReachabilityMonitor,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxProperties mailboxProperties) {
+		this.connectivityChecker = connectivityChecker;
+		this.torReachabilityMonitor = torReachabilityMonitor;
+		this.mailboxApiCaller = mailboxApiCaller;
+		this.mailboxApi = mailboxApi;
+		this.mailboxFileManager = mailboxFileManager;
+		this.mailboxProperties = mailboxProperties;
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		synchronized (lock) {
+			// Don't allow the worker to be reused
+			if (state != State.CREATED) return;
+			state = State.CONNECTIVITY_CHECK;
+		}
+		// Avoid leaking observer in case destroy() is called concurrently
+		// before observer is added
+		connectivityChecker.checkConnectivity(mailboxProperties, this);
+		boolean destroyed;
+		synchronized (lock) {
+			destroyed = state == State.DESTROYED;
+		}
+		if (destroyed) connectivityChecker.removeObserver(this);
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		Cancellable apiCall;
+		synchronized (lock) {
+			state = State.DESTROYED;
+			apiCall = this.apiCall;
+			this.apiCall = null;
+		}
+		if (apiCall != null) apiCall.cancel();
+		connectivityChecker.removeObserver(this);
+		torReachabilityMonitor.removeObserver(this);
+	}
+
+	@Override
+	public void onConnectivityCheckSucceeded() {
+		LOG.info("Connectivity check succeeded");
+		synchronized (lock) {
+			if (state != State.CONNECTIVITY_CHECK) return;
+			state = State.DOWNLOAD_CYCLE_1;
+			// Start first download cycle
+			apiCall = mailboxApiCaller.retryWithBackoff(
+					createApiCallForDownloadCycle());
+		}
+	}
+
+	void onDownloadCycleFinished() {
+		boolean addObserver = false;
+		synchronized (lock) {
+			if (state == State.DOWNLOAD_CYCLE_1) {
+				LOG.info("First download cycle finished");
+				state = State.WAITING_FOR_TOR;
+				apiCall = null;
+				addObserver = true;
+			} else if (state == State.DOWNLOAD_CYCLE_2) {
+				LOG.info("Second download cycle finished");
+				state = State.FINISHED;
+				apiCall = null;
+			}
+		}
+		if (addObserver) {
+			// Avoid leaking observer in case destroy() is called concurrently
+			// before observer is added
+			torReachabilityMonitor.addOneShotObserver(this);
+			boolean destroyed;
+			synchronized (lock) {
+				destroyed = state == State.DESTROYED;
+			}
+			if (destroyed) torReachabilityMonitor.removeObserver(this);
+		}
+	}
+
+	void downloadNextFile(Queue<FolderFile> queue) {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+			if (queue.isEmpty()) {
+				// Check for files again, as new files may have arrived while
+				// we were downloading
+				apiCall = mailboxApiCaller.retryWithBackoff(
+						createApiCallForDownloadCycle());
+			} else {
+				FolderFile file = queue.remove();
+				apiCall = mailboxApiCaller.retryWithBackoff(
+						new SimpleApiCall(() ->
+								apiCallDownloadFile(file, queue)));
+			}
+		}
+	}
+
+	private void apiCallDownloadFile(FolderFile file, Queue<FolderFile> queue)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		LOG.info("Downloading file");
+		File tempFile = mailboxFileManager.createTempFileForDownload();
+		try {
+			mailboxApi.getFile(mailboxProperties, file.folderId, file.fileId,
+					tempFile);
+		} catch (IOException | ApiException e) {
+			if (!tempFile.delete()) {
+				LOG.warning("Failed to delete temporary file");
+			}
+			throw e;
+		} catch (TolerableFailureException e) {
+			// File not found - continue to the next file
+			LOG.warning("File does not exist");
+			if (!tempFile.delete()) {
+				LOG.warning("Failed to delete temporary file");
+			}
+			downloadNextFile(queue);
+			return;
+		}
+		mailboxFileManager.handleDownloadedFile(tempFile);
+		deleteFile(file, queue);
+	}
+
+	private void deleteFile(FolderFile file, Queue<FolderFile> queue) {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+			apiCall = mailboxApiCaller.retryWithBackoff(
+					new SimpleApiCall(() -> apiCallDeleteFile(file, queue)));
+		}
+	}
+
+	private void apiCallDeleteFile(FolderFile file, Queue<FolderFile> queue)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		try {
+			mailboxApi.deleteFile(mailboxProperties, file.folderId,
+					file.fileId);
+		} catch (TolerableFailureException e) {
+			// File not found - continue to the next file
+			LOG.warning("File does not exist");
+		}
+		downloadNextFile(queue);
+	}
+
+	@Override
+	public void onTorReachable() {
+		LOG.info("Our Tor hidden service is reachable");
+		synchronized (lock) {
+			if (state != State.WAITING_FOR_TOR) return;
+			state = State.DOWNLOAD_CYCLE_2;
+			// Start second download cycle
+			apiCall = mailboxApiCaller.retryWithBackoff(
+					createApiCallForDownloadCycle());
+		}
+	}
+
+	// Package access for testing
+	static class FolderFile {
+
+		final MailboxFolderId folderId;
+		final MailboxFileId fileId;
+
+		FolderFile(MailboxFolderId folderId, MailboxFileId fileId) {
+			this.folderId = folderId;
+			this.fileId = fileId;
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxFileManager.java

@@ -0,0 +1,34 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
+
+import java.io.File;
+import java.io.IOException;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+interface MailboxFileManager {
+
+	/**
+	 * Creates an empty file for storing a download.
+	 */
+	File createTempFileForDownload() throws IOException;
+
+	/**
+	 * Creates a file to be uploaded to the given contact and writes any
+	 * waiting data to the file. The IDs of any messages sent or acked will
+	 * be added to the given {@link OutgoingSessionRecord}.
+	 */
+	File createAndWriteTempFileForUpload(ContactId contactId,
+			OutgoingSessionRecord sessionRecord) throws IOException;
+
+	/**
+	 * Handles a file that has been downloaded. The file should be created
+	 * with {@link #createTempFileForDownload()}.
+	 */
+	void handleDownloadedFile(File f);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxFileManagerImpl.java

@@ -0,0 +1,271 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.connection.ConnectionManager;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.lifecycle.LifecycleManager;
+import org.briarproject.bramble.api.mailbox.MailboxDirectory;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.PluginManager;
+import org.briarproject.bramble.api.plugin.TransportConnectionReader;
+import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
+import org.briarproject.bramble.api.plugin.event.TransportActiveEvent;
+import org.briarproject.bramble.api.plugin.simplex.SimplexPlugin;
+import org.briarproject.bramble.api.properties.TransportProperties;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.lifecycle.LifecycleManager.LifecycleState.RUNNING;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.ID;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.bramble.api.plugin.file.FileConstants.PROP_PATH;
+import static org.briarproject.bramble.util.IoUtils.delete;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@ThreadSafe
+@NotNullByDefault
+class MailboxFileManagerImpl implements MailboxFileManager, EventListener {
+
+	private static final Logger LOG =
+			getLogger(MailboxFileManagerImpl.class.getName());
+
+	// Package access for testing
+	static final String DOWNLOAD_DIR_NAME = "downloads";
+	static final String UPLOAD_DIR_NAME = "uploads";
+
+	private final Executor ioExecutor;
+	private final PluginManager pluginManager;
+	private final ConnectionManager connectionManager;
+	private final LifecycleManager lifecycleManager;
+	private final File mailboxDir;
+	private final EventBus eventBus;
+	private final CountDownLatch orphanLatch = new CountDownLatch(1);
+
+	@Inject
+	MailboxFileManagerImpl(@IoExecutor Executor ioExecutor,
+			PluginManager pluginManager,
+			ConnectionManager connectionManager,
+			LifecycleManager lifecycleManager,
+			@MailboxDirectory File mailboxDir,
+			EventBus eventBus) {
+		this.ioExecutor = ioExecutor;
+		this.pluginManager = pluginManager;
+		this.connectionManager = connectionManager;
+		this.lifecycleManager = lifecycleManager;
+		this.mailboxDir = mailboxDir;
+		this.eventBus = eventBus;
+	}
+
+	@Override
+	public File createTempFileForDownload() throws IOException {
+		return createTempFile(DOWNLOAD_DIR_NAME);
+	}
+
+	@Override
+	public File createAndWriteTempFileForUpload(ContactId contactId,
+			OutgoingSessionRecord sessionRecord) throws IOException {
+		File f = createTempFile(UPLOAD_DIR_NAME);
+		// We shouldn't reach this point until the plugin has been started
+		SimplexPlugin plugin =
+				(SimplexPlugin) requireNonNull(pluginManager.getPlugin(ID));
+		TransportProperties p = new TransportProperties();
+		p.put(PROP_PATH, f.getAbsolutePath());
+		TransportConnectionWriter writer = plugin.createWriter(p);
+		if (writer == null) {
+			delete(f);
+			throw new IOException();
+		}
+		MailboxFileWriter decorated = new MailboxFileWriter(writer);
+		LOG.info("Writing file for upload");
+		connectionManager.manageOutgoingConnection(contactId, ID, decorated,
+				sessionRecord);
+		if (decorated.awaitDisposal()) {
+			// An exception was thrown during the session - delete the file
+			delete(f);
+			throw new IOException();
+		}
+		return f;
+	}
+
+	private File createTempFile(String dirName) throws IOException {
+		// Wait for orphaned files to be handled before creating new files
+		try {
+			orphanLatch.await();
+		} catch (InterruptedException e) {
+			throw new IOException(e);
+		}
+		File dir = createDirectoryIfNeeded(dirName);
+		return File.createTempFile("mailbox", ".tmp", dir);
+	}
+
+	private File createDirectoryIfNeeded(String name) throws IOException {
+		File dir = new File(mailboxDir, name);
+		//noinspection ResultOfMethodCallIgnored
+		dir.mkdirs();
+		if (!dir.isDirectory()) {
+			throw new IOException("Failed to create directory '" + name + "'");
+		}
+		return dir;
+	}
+
+	@Override
+	public void handleDownloadedFile(File f) {
+		// We shouldn't reach this point until the plugin has been started
+		SimplexPlugin plugin =
+				(SimplexPlugin) requireNonNull(pluginManager.getPlugin(ID));
+		TransportProperties p = new TransportProperties();
+		p.put(PROP_PATH, f.getAbsolutePath());
+		TransportConnectionReader reader = plugin.createReader(p);
+		if (reader == null) {
+			LOG.warning("Failed to create reader for downloaded file");
+			return;
+		}
+		TransportConnectionReader decorated = new MailboxFileReader(reader, f);
+		LOG.info("Reading downloaded file");
+		connectionManager.manageIncomingConnection(ID, decorated,
+				exception -> isHandlingComplete(exception, true));
+	}
+
+	private boolean isHandlingComplete(boolean exception, boolean recognised) {
+		// If we've successfully read the file then we're done
+		if (!exception && recognised) return true;
+		// If the app is shutting down we may get spurious IO exceptions
+		// due to executors being shut down. Leave the file in the download
+		// directory and we'll try to read it again at the next startup
+		return !lifecycleManager.getLifecycleState().isAfter(RUNNING);
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		// Wait for the transport to become active before handling orphaned
+		// files so that we can get the plugin from the plugin manager
+		if (e instanceof TransportActiveEvent) {
+			TransportActiveEvent t = (TransportActiveEvent) e;
+			if (t.getTransportId().equals(ID)) {
+				ioExecutor.execute(this::handleOrphanedFiles);
+				eventBus.removeListener(this);
+			}
+		}
+	}
+
+	/**
+	 * This method is called at startup, as soon as the plugin is started, to
+	 * delete any files that were left in the upload directory at the last
+	 * shutdown and handle any files that were left in the download directory.
+	 */
+	@IoExecutor
+	private void handleOrphanedFiles() {
+		try {
+			File uploadDir = createDirectoryIfNeeded(UPLOAD_DIR_NAME);
+			File[] orphanedUploads = uploadDir.listFiles();
+			if (orphanedUploads != null) {
+				for (File f : orphanedUploads) delete(f);
+			}
+			File downloadDir = createDirectoryIfNeeded(DOWNLOAD_DIR_NAME);
+			File[] orphanedDownloads = downloadDir.listFiles();
+			// Now that we've got the list of orphaned downloads, new files
+			// can be created in the download directory
+			orphanLatch.countDown();
+			if (orphanedDownloads != null) {
+				for (File f : orphanedDownloads) handleDownloadedFile(f);
+			}
+		} catch (IOException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	private class MailboxFileReader implements TransportConnectionReader {
+
+		private final TransportConnectionReader delegate;
+		private final File file;
+
+		private MailboxFileReader(TransportConnectionReader delegate,
+				File file) {
+			this.delegate = delegate;
+			this.file = file;
+		}
+
+		@Override
+		public InputStream getInputStream() throws IOException {
+			return delegate.getInputStream();
+		}
+
+		@Override
+		public void dispose(boolean exception, boolean recognised)
+				throws IOException {
+			delegate.dispose(exception, recognised);
+			if (isHandlingComplete(exception, recognised)) {
+				LOG.info("Deleting downloaded file");
+				delete(file);
+			}
+		}
+	}
+
+	private static class MailboxFileWriter
+			implements TransportConnectionWriter {
+
+		private final TransportConnectionWriter delegate;
+		private final BlockingQueue<Boolean> disposalResult =
+				new ArrayBlockingQueue<>(1);
+
+		private MailboxFileWriter(TransportConnectionWriter delegate) {
+			this.delegate = delegate;
+		}
+
+		@Override
+		public long getMaxLatency() {
+			return delegate.getMaxLatency();
+		}
+
+		@Override
+		public int getMaxIdleTime() {
+			return delegate.getMaxIdleTime();
+		}
+
+		@Override
+		public boolean isLossyAndCheap() {
+			return delegate.isLossyAndCheap();
+		}
+
+		@Override
+		public OutputStream getOutputStream() throws IOException {
+			return delegate.getOutputStream();
+		}
+
+		@Override
+		public void dispose(boolean exception) throws IOException {
+			delegate.dispose(exception);
+			disposalResult.add(exception);
+		}
+
+		/**
+		 * Waits for the delegate to be disposed and returns true if an
+		 * exception occurred.
+		 */
+		private boolean awaitDisposal() {
+			try {
+				return disposalResult.take();
+			} catch (InterruptedException e) {
+				LOG.info("Interrupted while waiting for disposal");
+				return true;
+			}
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxManagerImpl.java

@@ -2,27 +2,44 @@ package org.briarproject.bramble.mailbox;
 
 import org.briarproject.bramble.api.db.DbException;
 import org.briarproject.bramble.api.db.Transaction;
+import org.briarproject.bramble.api.db.TransactionManager;
 import org.briarproject.bramble.api.lifecycle.IoExecutor;
 import org.briarproject.bramble.api.mailbox.MailboxManager;
 import org.briarproject.bramble.api.mailbox.MailboxPairingTask;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
 import org.briarproject.bramble.api.mailbox.MailboxStatus;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
 
+import java.io.IOException;
+import java.util.List;
 import java.util.concurrent.Executor;
+import java.util.logging.Logger;
 
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.GuardedBy;
 import javax.annotation.concurrent.Immutable;
 import javax.inject.Inject;
 
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
 @Immutable
 @NotNullByDefault
 class MailboxManagerImpl implements MailboxManager {
 
+	private static final String TAG = MailboxManagerImpl.class.getName();
+	private final static Logger LOG = getLogger(TAG);
+
 	private final Executor ioExecutor;
+	private final MailboxApi api;
+	private final TransactionManager db;
 	private final MailboxSettingsManager mailboxSettingsManager;
 	private final MailboxPairingTaskFactory pairingTaskFactory;
+	private final Clock clock;
 	private final Object lock = new Object();
 
 	@Nullable
@@ -32,11 +49,17 @@ class MailboxManagerImpl implements MailboxManager {
 	@Inject
 	MailboxManagerImpl(
 			@IoExecutor Executor ioExecutor,
+			MailboxApi api,
+			TransactionManager db,
 			MailboxSettingsManager mailboxSettingsManager,
-			MailboxPairingTaskFactory pairingTaskFactory) {
+			MailboxPairingTaskFactory pairingTaskFactory,
+			Clock clock) {
 		this.ioExecutor = ioExecutor;
+		this.api = api;
+		this.db = db;
 		this.mailboxSettingsManager = mailboxSettingsManager;
 		this.pairingTaskFactory = pairingTaskFactory;
+		this.clock = clock;
 	}
 
 	@Override
@@ -75,4 +98,62 @@ class MailboxManagerImpl implements MailboxManager {
 		return created;
 	}
 
+	@Override
+	public boolean checkConnection() {
+		List<MailboxVersion> versions = null;
+		try {
+			MailboxProperties props = db.transactionWithNullableResult(true,
+					mailboxSettingsManager::getOwnMailboxProperties);
+			if (props == null) throw new DbException();
+			versions = api.getServerSupports(props);
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+			// we don't treat this is a failure to record
+			return false;
+		} catch (IOException | MailboxApi.ApiException e) {
+			// we record this as a failure
+			logException(LOG, WARNING, e);
+		}
+		try {
+			recordCheckResult(versions);
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+		return versions != null;
+	}
+
+	private void recordCheckResult(@Nullable List<MailboxVersion> versions)
+			throws DbException {
+		long now = clock.currentTimeMillis();
+		db.transaction(false, txn -> {
+			if (versions != null) {
+				mailboxSettingsManager
+						.recordSuccessfulConnection(txn, now, versions);
+			} else {
+				mailboxSettingsManager.recordFailedConnectionAttempt(txn, now);
+			}
+		});
+	}
+
+	@Override
+	public boolean unPair() throws DbException {
+		MailboxProperties properties = db.transactionWithNullableResult(true,
+				mailboxSettingsManager::getOwnMailboxProperties);
+		if (properties == null) {
+			// no more mailbox, that's strange but possible if called in quick
+			// succession, so let's return true this time
+			return true;
+		}
+		boolean wasWiped;
+		try {
+			api.wipeMailbox(properties);
+			wasWiped = true;
+		} catch (IOException | MailboxApi.ApiException e) {
+			logException(LOG, WARNING, e);
+			wasWiped = false;
+		}
+		db.transaction(false,
+				mailboxSettingsManager::removeOwnMailboxProperties);
+		return wasWiped;
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxModule.java

@@ -1,34 +1,42 @@
 package org.briarproject.bramble.mailbox;
 
+import org.briarproject.bramble.api.FeatureFlags;
 import org.briarproject.bramble.api.client.ClientHelper;
 import org.briarproject.bramble.api.contact.ContactManager;
 import org.briarproject.bramble.api.data.MetadataEncoder;
+import org.briarproject.bramble.api.event.EventBus;
 import org.briarproject.bramble.api.lifecycle.LifecycleManager;
 import org.briarproject.bramble.api.mailbox.MailboxManager;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
 import org.briarproject.bramble.api.sync.validation.ValidationManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.versioning.ClientVersioningManager;
 
+import java.util.List;
+
 import javax.inject.Inject;
 import javax.inject.Singleton;
 
 import dagger.Module;
 import dagger.Provides;
 
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.CLIENT_ID;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MAJOR_VERSION;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MINOR_VERSION;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.CLIENT_SUPPORTS;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.CLIENT_ID;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MAJOR_VERSION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MINOR_VERSION;
 
 @Module
 public class MailboxModule {
 
 	public static class EagerSingletons {
 		@Inject
-		MailboxPropertyValidator mailboxPropertyValidator;
+		MailboxUpdateValidator mailboxUpdateValidator;
 		@Inject
-		MailboxPropertyManager mailboxPropertyManager;
+		MailboxUpdateManager mailboxUpdateManager;
+		@Inject
+		MailboxFileManager mailboxFileManager;
 	}
 
 	@Provides
@@ -44,43 +52,78 @@ public class MailboxModule {
 	}
 
 	@Provides
+	@Singleton
 	MailboxSettingsManager provideMailboxSettingsManager(
 			MailboxSettingsManagerImpl mailboxSettingsManager) {
 		return mailboxSettingsManager;
 	}
 
 	@Provides
-	MailboxApi providesMailboxApi(MailboxApiImpl mailboxApi) {
+	UrlConverter provideUrlConverter(UrlConverterImpl urlConverter) {
+		return urlConverter;
+	}
+
+	@Provides
+	MailboxApi provideMailboxApi(MailboxApiImpl mailboxApi) {
 		return mailboxApi;
 	}
 
 	@Provides
 	@Singleton
-	MailboxPropertyValidator provideMailboxPropertyValidator(
-			ValidationManager validationManager, ClientHelper clientHelper,
-			MetadataEncoder metadataEncoder, Clock clock) {
-		MailboxPropertyValidator validator = new MailboxPropertyValidator(
+	MailboxUpdateValidator provideMailboxUpdateValidator(
+			ValidationManager validationManager,
+			ClientHelper clientHelper,
+			MetadataEncoder metadataEncoder,
+			Clock clock,
+			FeatureFlags featureFlags) {
+		MailboxUpdateValidator validator = new MailboxUpdateValidator(
 				clientHelper, metadataEncoder, clock);
-		validationManager.registerMessageValidator(CLIENT_ID, MAJOR_VERSION,
-				validator);
+		if (featureFlags.shouldEnableMailbox()) {
+			validationManager.registerMessageValidator(CLIENT_ID,
+					MAJOR_VERSION, validator);
+		}
 		return validator;
 	}
 
+	@Provides
+	List<MailboxVersion> provideClientSupports() {
+		return CLIENT_SUPPORTS;
+	}
+
 	@Provides
 	@Singleton
-	MailboxPropertyManager provideMailboxPropertyManager(
+	MailboxUpdateManager provideMailboxUpdateManager(
+			FeatureFlags featureFlags,
 			LifecycleManager lifecycleManager,
 			ValidationManager validationManager, ContactManager contactManager,
 			ClientVersioningManager clientVersioningManager,
 			MailboxSettingsManager mailboxSettingsManager,
-			MailboxPropertyManagerImpl mailboxPropertyManager) {
-		lifecycleManager.registerOpenDatabaseHook(mailboxPropertyManager);
-		validationManager.registerIncomingMessageHook(CLIENT_ID, MAJOR_VERSION,
-				mailboxPropertyManager);
-		contactManager.registerContactHook(mailboxPropertyManager);
-		clientVersioningManager.registerClient(CLIENT_ID, MAJOR_VERSION,
-				MINOR_VERSION, mailboxPropertyManager);
-		mailboxSettingsManager.registerMailboxHook(mailboxPropertyManager);
-		return mailboxPropertyManager;
+			MailboxUpdateManagerImpl mailboxUpdateManager) {
+		if (featureFlags.shouldEnableMailbox()) {
+			lifecycleManager.registerOpenDatabaseHook(mailboxUpdateManager);
+			validationManager.registerIncomingMessageHook(CLIENT_ID,
+					MAJOR_VERSION, mailboxUpdateManager);
+			contactManager.registerContactHook(mailboxUpdateManager);
+			clientVersioningManager.registerClient(CLIENT_ID, MAJOR_VERSION,
+					MINOR_VERSION, mailboxUpdateManager);
+			mailboxSettingsManager.registerMailboxHook(mailboxUpdateManager);
+		}
+		return mailboxUpdateManager;
+	}
+
+	@Provides
+	@Singleton
+	MailboxFileManager provideMailboxFileManager(FeatureFlags featureFlags,
+			EventBus eventBus, MailboxFileManagerImpl mailboxFileManager) {
+		if (featureFlags.shouldEnableMailbox()) {
+			eventBus.addListener(mailboxFileManager);
+		}
+		return mailboxFileManager;
+	}
+
+	@Provides
+	MailboxWorkerFactory provideMailboxWorkerFactory(
+			MailboxWorkerFactoryImpl mailboxWorkerFactory) {
+		return mailboxWorkerFactory;
 	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxPairingTaskFactoryImpl.java

@@ -4,8 +4,8 @@ import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.db.DatabaseComponent;
 import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.mailbox.MailboxPairingTask;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.Clock;
 
@@ -24,7 +24,7 @@ class MailboxPairingTaskFactoryImpl implements MailboxPairingTaskFactory {
 	private final Clock clock;
 	private final MailboxApi api;
 	private final MailboxSettingsManager mailboxSettingsManager;
-	private final MailboxPropertyManager mailboxPropertyManager;
+	private final MailboxUpdateManager mailboxUpdateManager;
 
 	@Inject
 	MailboxPairingTaskFactoryImpl(
@@ -34,20 +34,20 @@ class MailboxPairingTaskFactoryImpl implements MailboxPairingTaskFactory {
 			Clock clock,
 			MailboxApi api,
 			MailboxSettingsManager mailboxSettingsManager,
-			MailboxPropertyManager mailboxPropertyManager) {
+			MailboxUpdateManager mailboxUpdateManager) {
 		this.eventExecutor = eventExecutor;
 		this.db = db;
 		this.crypto = crypto;
 		this.clock = clock;
 		this.api = api;
 		this.mailboxSettingsManager = mailboxSettingsManager;
-		this.mailboxPropertyManager = mailboxPropertyManager;
+		this.mailboxUpdateManager = mailboxUpdateManager;
 	}
 
 	@Override
 	public MailboxPairingTask createPairingTask(String qrCodePayload) {
 		return new MailboxPairingTaskImpl(qrCodePayload, eventExecutor, db,
 				crypto, clock, api, mailboxSettingsManager,
-				mailboxPropertyManager);
+				mailboxUpdateManager);
 	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxPairingTaskImpl.java

@@ -11,9 +11,9 @@ import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxPairingState;
 import org.briarproject.bramble.api.mailbox.MailboxPairingTask;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
@@ -51,7 +51,7 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 	private final Clock clock;
 	private final MailboxApi api;
 	private final MailboxSettingsManager mailboxSettingsManager;
-	private final MailboxPropertyManager mailboxPropertyManager;
+	private final MailboxUpdateManager mailboxUpdateManager;
 
 	private final Object lock = new Object();
 	@GuardedBy("lock")
@@ -68,7 +68,7 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 			Clock clock,
 			MailboxApi api,
 			MailboxSettingsManager mailboxSettingsManager,
-			MailboxPropertyManager mailboxPropertyManager) {
+			MailboxUpdateManager mailboxUpdateManager) {
 		this.payload = payload;
 		this.eventExecutor = eventExecutor;
 		this.db = db;
@@ -76,7 +76,7 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 		this.clock = clock;
 		this.api = api;
 		this.mailboxSettingsManager = mailboxSettingsManager;
-		this.mailboxPropertyManager = mailboxPropertyManager;
+		this.mailboxUpdateManager = mailboxUpdateManager;
 		state = new MailboxPairingState.QrCodeReceived();
 	}
 
@@ -115,9 +115,7 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 	private void pairMailbox() throws IOException, ApiException, DbException {
 		MailboxProperties mailboxProperties = decodeQrCodePayload(payload);
 		setState(new MailboxPairingState.Pairing());
-		MailboxAuthToken ownerToken = api.setup(mailboxProperties);
-		MailboxProperties ownerProperties = new MailboxProperties(
-				mailboxProperties.getBaseUrl(), ownerToken, true);
+		MailboxProperties ownerProperties = api.setup(mailboxProperties);
 		long time = clock.currentTimeMillis();
 		db.transaction(false, txn -> {
 			mailboxSettingsManager
@@ -127,9 +125,9 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 			// timers for contacts who doesn't have their own mailbox. This way,
 			// data stranded on our old mailbox will be re-uploaded to our new.
 			for (Contact c : db.getContacts(txn)) {
-				MailboxPropertiesUpdate remoteProps = mailboxPropertyManager
-						.getRemoteProperties(txn, c.getId());
-				if (remoteProps == null) {
+				MailboxUpdate update = mailboxUpdateManager.getRemoteUpdate(
+						txn, c.getId());
+				if (update == null || !update.hasMailbox()) {
 					db.resetUnackedMessagesToSend(txn, c.getId());
 				}
 			}
@@ -179,10 +177,9 @@ class MailboxPairingTaskImpl implements MailboxPairingTask {
 		LOG.info("QR code is valid");
 		byte[] onionPubKey = Arrays.copyOfRange(bytes, 1, 33);
 		String onion = crypto.encodeOnion(onionPubKey);
-		String baseUrl = "http://" + onion + ".onion";
 		byte[] tokenBytes = Arrays.copyOfRange(bytes, 33, 65);
 		MailboxAuthToken setupToken = new MailboxAuthToken(tokenBytes);
-		return new MailboxProperties(baseUrl, setupToken, true);
+		return new MailboxProperties(onion, setupToken, new ArrayList<>());
 	}
 
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxSettingsManagerImpl.java

@@ -8,28 +8,34 @@ import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
 import org.briarproject.bramble.api.mailbox.MailboxStatus;
-import org.briarproject.bramble.api.mailbox.OwnMailboxConnectionStatusEvent;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.mailbox.event.MailboxProblemEvent;
+import org.briarproject.bramble.api.mailbox.event.OwnMailboxConnectionStatusEvent;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.settings.Settings;
 import org.briarproject.bramble.api.settings.SettingsManager;
 
+import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
 
 import javax.annotation.Nullable;
-import javax.annotation.concurrent.Immutable;
+import javax.annotation.concurrent.ThreadSafe;
 import javax.inject.Inject;
 
+import static java.util.Collections.emptyList;
 import static org.briarproject.bramble.util.StringUtils.isNullOrEmpty;
 
-@Immutable
+@ThreadSafe
 @NotNullByDefault
 class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 
 	// Package access for testing
 	static final String SETTINGS_NAMESPACE = "mailbox";
+	// TODO: This currently stores the base URL, not the 56-char onion address
 	static final String SETTINGS_KEY_ONION = "onion";
 	static final String SETTINGS_KEY_TOKEN = "token";
+	static final String SETTINGS_KEY_SERVER_SUPPORTS = "serverSupports";
 	static final String SETTINGS_KEY_LAST_ATTEMPT = "lastAttempt";
 	static final String SETTINGS_KEY_LAST_SUCCESS = "lastSuccess";
 	static final String SETTINGS_KEY_ATTEMPTS = "attempts";
@@ -55,9 +61,10 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		String onion = s.get(SETTINGS_KEY_ONION);
 		String token = s.get(SETTINGS_KEY_TOKEN);
 		if (isNullOrEmpty(onion) || isNullOrEmpty(token)) return null;
+		List<MailboxVersion> serverSupports = parseServerSupports(s);
 		try {
 			MailboxAuthToken tokenId = MailboxAuthToken.fromString(token);
-			return new MailboxProperties(onion, tokenId, true);
+			return new MailboxProperties(onion, tokenId, serverSupports);
 		} catch (InvalidMailboxIdException e) {
 			throw new DbException(e);
 		}
@@ -67,11 +74,24 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 	public void setOwnMailboxProperties(Transaction txn, MailboxProperties p)
 			throws DbException {
 		Settings s = new Settings();
-		s.put(SETTINGS_KEY_ONION, p.getBaseUrl());
+		s.put(SETTINGS_KEY_ONION, p.getOnion());
 		s.put(SETTINGS_KEY_TOKEN, p.getAuthToken().toString());
+		List<MailboxVersion> serverSupports = p.getServerSupports();
+		encodeServerSupports(serverSupports, s);
 		settingsManager.mergeSettings(txn, s, SETTINGS_NAMESPACE);
 		for (MailboxHook hook : hooks) {
-			hook.mailboxPaired(txn, p.getOnion());
+			hook.mailboxPaired(txn, p.getOnion(), p.getServerSupports());
+		}
+	}
+
+	@Override
+	public void removeOwnMailboxProperties(Transaction txn) throws DbException {
+		Settings s = new Settings();
+		s.put(SETTINGS_KEY_ONION, "");
+		s.put(SETTINGS_KEY_TOKEN, "");
+		settingsManager.mergeSettings(txn, s, SETTINGS_NAMESPACE);
+		for (MailboxHook hook : hooks) {
+			hook.mailboxUnpaired(txn);
 		}
 	}
 
@@ -82,18 +102,44 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		long lastAttempt = s.getLong(SETTINGS_KEY_LAST_ATTEMPT, -1);
 		long lastSuccess = s.getLong(SETTINGS_KEY_LAST_SUCCESS, -1);
 		int attempts = s.getInt(SETTINGS_KEY_ATTEMPTS, 0);
-		return new MailboxStatus(lastAttempt, lastSuccess, attempts);
+		List<MailboxVersion> serverSupports;
+		try {
+			serverSupports = parseServerSupports(s);
+		} catch (DbException e) {
+			serverSupports = emptyList();
+		}
+		return new MailboxStatus(lastAttempt, lastSuccess, attempts,
+				serverSupports);
 	}
 
 	@Override
 	public void recordSuccessfulConnection(Transaction txn, long now)
 			throws DbException {
+		recordSuccessfulConnection(txn, now, null);
+	}
+
+	@Override
+	public void recordSuccessfulConnection(Transaction txn, long now,
+			@Nullable List<MailboxVersion> versions) throws DbException {
 		Settings s = new Settings();
+		// fetch version that the server supports first
+		List<MailboxVersion> serverSupports;
+		if (versions == null) {
+			Settings oldSettings =
+					settingsManager.getSettings(txn, SETTINGS_NAMESPACE);
+			serverSupports = parseServerSupports(oldSettings);
+		} else {
+			serverSupports = versions;
+			// store new versions
+			encodeServerSupports(serverSupports, s);
+		}
+		// now record the successful connection
 		s.putLong(SETTINGS_KEY_LAST_ATTEMPT, now);
 		s.putLong(SETTINGS_KEY_LAST_SUCCESS, now);
 		s.putInt(SETTINGS_KEY_ATTEMPTS, 0);
 		settingsManager.mergeSettings(txn, s, SETTINGS_NAMESPACE);
-		MailboxStatus status = new MailboxStatus(now, now, 0);
+		// broadcast status event
+		MailboxStatus status = new MailboxStatus(now, now, 0, serverSupports);
 		txn.attach(new OwnMailboxConnectionStatusEvent(status));
 	}
 
@@ -108,8 +154,11 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		newSettings.putLong(SETTINGS_KEY_LAST_ATTEMPT, now);
 		newSettings.putInt(SETTINGS_KEY_ATTEMPTS, newAttempts);
 		settingsManager.mergeSettings(txn, newSettings, SETTINGS_NAMESPACE);
-		MailboxStatus status = new MailboxStatus(now, lastSuccess, newAttempts);
+		List<MailboxVersion> serverSupports = parseServerSupports(oldSettings);
+		MailboxStatus status = new MailboxStatus(now, lastSuccess, newAttempts,
+				serverSupports);
 		txn.attach(new OwnMailboxConnectionStatusEvent(status));
+		if (status.hasProblem(now)) txn.attach(new MailboxProblemEvent());
 	}
 
 	@Override
@@ -131,4 +180,30 @@ class MailboxSettingsManagerImpl implements MailboxSettingsManager {
 		if (isNullOrEmpty(filename)) return null;
 		return filename;
 	}
+
+	private void encodeServerSupports(List<MailboxVersion> serverSupports,
+			Settings s) {
+		int[] ints = new int[serverSupports.size() * 2];
+		int i = 0;
+		for (MailboxVersion v : serverSupports) {
+			ints[i++] = v.getMajor();
+			ints[i++] = v.getMinor();
+		}
+		s.putIntArray(SETTINGS_KEY_SERVER_SUPPORTS, ints);
+	}
+
+	private List<MailboxVersion> parseServerSupports(Settings s)
+			throws DbException {
+		if (!s.containsKey(SETTINGS_KEY_SERVER_SUPPORTS)) return emptyList();
+		int[] ints = s.getIntArray(SETTINGS_KEY_SERVER_SUPPORTS);
+		// We know we were paired, so we must have proper serverSupports
+		if (ints == null || ints.length == 0 || ints.length % 2 != 0) {
+			throw new DbException();
+		}
+		List<MailboxVersion> serverSupports = new ArrayList<>();
+		for (int i = 0; i < ints.length - 1; i += 2) {
+			serverSupports.add(new MailboxVersion(ints[i], ints[i + 1]));
+		}
+		return serverSupports;
+	}
 }

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxUpdateManagerImpl.java

@@ -8,6 +8,7 @@ import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.contact.ContactManager.ContactHook;
 import org.briarproject.bramble.api.crypto.CryptoComponent;
 import org.briarproject.bramble.api.data.BdfDictionary;
+import org.briarproject.bramble.api.data.BdfEntry;
 import org.briarproject.bramble.api.data.BdfList;
 import org.briarproject.bramble.api.data.MetadataParser;
 import org.briarproject.bramble.api.db.DatabaseComponent;
@@ -18,11 +19,13 @@ import org.briarproject.bramble.api.lifecycle.LifecycleManager.OpenDatabaseHook;
 import org.briarproject.bramble.api.mailbox.MailboxAuthToken;
 import org.briarproject.bramble.api.mailbox.MailboxFolderId;
 import org.briarproject.bramble.api.mailbox.MailboxProperties;
-import org.briarproject.bramble.api.mailbox.MailboxPropertiesUpdate;
-import org.briarproject.bramble.api.mailbox.MailboxPropertyManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
 import org.briarproject.bramble.api.mailbox.MailboxSettingsManager.MailboxHook;
-import org.briarproject.bramble.api.mailbox.RemoteMailboxPropertiesUpdateEvent;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.mailbox.event.RemoteMailboxUpdateEvent;
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 import org.briarproject.bramble.api.sync.Group;
 import org.briarproject.bramble.api.sync.Group.Visibility;
@@ -35,6 +38,7 @@ import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.versioning.ClientVersioningManager;
 import org.briarproject.bramble.api.versioning.ClientVersioningManager.ClientVersioningHook;
 
+import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 
@@ -44,10 +48,11 @@ import javax.inject.Inject;
 import static org.briarproject.bramble.api.sync.validation.IncomingMessageHook.DeliveryAction.ACCEPT_DO_NOT_SHARE;
 
 @NotNullByDefault
-class MailboxPropertyManagerImpl implements MailboxPropertyManager,
+class MailboxUpdateManagerImpl implements MailboxUpdateManager,
 		OpenDatabaseHook, ContactHook, ClientVersioningHook,
 		IncomingMessageHook, MailboxHook {
 
+	private final List<MailboxVersion> clientSupports;
 	private final DatabaseComponent db;
 	private final ClientHelper clientHelper;
 	private final ClientVersioningManager clientVersioningManager;
@@ -59,12 +64,14 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 	private final Group localGroup;
 
 	@Inject
-	MailboxPropertyManagerImpl(DatabaseComponent db, ClientHelper clientHelper,
+	MailboxUpdateManagerImpl(List<MailboxVersion> clientSupports,
+			DatabaseComponent db, ClientHelper clientHelper,
 			ClientVersioningManager clientVersioningManager,
 			MetadataParser metadataParser,
 			ContactGroupFactory contactGroupFactory, Clock clock,
 			MailboxSettingsManager mailboxSettingsManager,
 			CryptoComponent crypto) {
+		this.clientSupports = clientSupports;
 		this.db = db;
 		this.clientHelper = clientHelper;
 		this.clientVersioningManager = clientVersioningManager;
@@ -80,12 +87,46 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 	@Override
 	public void onDatabaseOpened(Transaction txn) throws DbException {
 		if (db.containsGroup(txn, localGroup.getId())) {
-			return;
+			try {
+				BdfDictionary meta = clientHelper.getGroupMetadataAsDictionary(
+						txn, localGroup.getId());
+				BdfList sent = meta.getList(GROUP_KEY_SENT_CLIENT_SUPPORTS);
+				if (clientHelper.parseMailboxVersionList(sent)
+						.equals(clientSupports)) {
+					return;
+				}
+			} catch (FormatException e) {
+				throw new DbException();
+			}
+			// Our current clientSupports list has changed compared to what we
+			// last sent out.
+			for (Contact c : db.getContacts(txn)) {
+				MailboxUpdate latest = getLocalUpdate(txn, c.getId());
+				MailboxUpdate updated;
+				if (latest.hasMailbox()) {
+					updated = new MailboxUpdateWithMailbox(
+							(MailboxUpdateWithMailbox) latest, clientSupports);
+				} else {
+					updated = new MailboxUpdate(clientSupports);
+				}
+				Group g = getContactGroup(c);
+				storeMessageReplaceLatest(txn, g.getId(), updated);
+			}
+		} else {
+			db.addGroup(txn, localGroup);
+			// Set things up for any pre-existing contacts
+			for (Contact c : db.getContacts(txn)) {
+				addingContact(txn, c);
+			}
 		}
-		db.addGroup(txn, localGroup);
-		// Set things up for any pre-existing contacts
-		for (Contact c : db.getContacts(txn)) {
-			addingContact(txn, c);
+
+		try {
+			BdfDictionary meta = BdfDictionary.of(new BdfEntry(
+					GROUP_KEY_SENT_CLIENT_SUPPORTS,
+					encodeSupportsList(clientSupports)));
+			clientHelper.mergeGroupMetadata(txn, localGroup.getId(), meta);
+		} catch (FormatException e) {
+			throw new DbException();
 		}
 	}
 
@@ -100,11 +141,15 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 		db.setGroupVisibility(txn, c.getId(), g.getId(), client);
 		// Attach the contact ID to the group
 		clientHelper.setContactId(txn, g.getId(), c.getId());
-		// If we are paired, create and send props to the newly added contact
 		MailboxProperties ownProps =
 				mailboxSettingsManager.getOwnMailboxProperties(txn);
 		if (ownProps != null) {
-			createAndSendProperties(txn, c, ownProps.getOnion());
+			// We are paired, create and send props to the newly added contact
+			createAndSendUpdateWithMailbox(txn, c, ownProps.getServerSupports(),
+					ownProps.getOnion());
+		} else {
+			// Not paired, but we still want to get our clientSupports sent
+			sendUpdateNoMailbox(txn, c);
 		}
 	}
 
@@ -114,17 +159,17 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 	}
 
 	@Override
-	public void mailboxPaired(Transaction txn, String ownOnion)
-			throws DbException {
+	public void mailboxPaired(Transaction txn, String ownOnion,
+			List<MailboxVersion> serverSupports) throws DbException {
 		for (Contact c : db.getContacts(txn)) {
-			createAndSendProperties(txn, c, ownOnion);
+			createAndSendUpdateWithMailbox(txn, c, serverSupports, ownOnion);
 		}
 	}
 
 	@Override
 	public void mailboxUnpaired(Transaction txn) throws DbException {
 		for (Contact c : db.getContacts(txn)) {
-			sendEmptyProperties(txn, c);
+			sendUpdateNoMailbox(txn, c);
 		}
 	}
 
@@ -156,8 +201,14 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 			}
 			ContactId c = clientHelper.getContactId(txn, m.getGroupId());
 			BdfList body = clientHelper.getMessageAsList(txn, m.getId());
-			MailboxPropertiesUpdate p = parseProperties(body);
-			txn.attach(new RemoteMailboxPropertiesUpdateEvent(c, p));
+			MailboxUpdate u = parseUpdate(body);
+			txn.attach(new RemoteMailboxUpdateEvent(c, u));
+			// Reset message retransmission timers for the contact. Avoiding
+			// messages getting stranded:
+			// - on our mailbox, if they now have a mailbox but didn't before
+			// - on the contact's old mailbox, if they removed their mailbox
+			// - on the contact's old mailbox, if they replaced their mailbox
+			db.resetUnackedMessagesToSend(txn, c);
 		} catch (FormatException e) {
 			throw new InvalidMessageException(e);
 		}
@@ -165,70 +216,81 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 	}
 
 	@Override
-	@Nullable
-	public MailboxPropertiesUpdate getLocalProperties(Transaction txn,
-			ContactId c) throws DbException {
-		return getProperties(txn, db.getContact(txn, c), true);
+	public MailboxUpdate getLocalUpdate(Transaction txn, ContactId c)
+			throws DbException {
+		MailboxUpdate local = getUpdate(txn, db.getContact(txn, c), true);
+		// An update (with or without mailbox) is created when contact is added
+		if (local == null) {
+			throw new DbException();
+		}
+		return local;
 	}
 
 	@Override
 	@Nullable
-	public MailboxPropertiesUpdate getRemoteProperties(Transaction txn,
-			ContactId c) throws DbException {
-		return getProperties(txn, db.getContact(txn, c), false);
+	public MailboxUpdate getRemoteUpdate(Transaction txn, ContactId c)
+			throws DbException {
+		return getUpdate(txn, db.getContact(txn, c), false);
 	}
 
 	/**
 	 * Creates and sends an update message to the given contact. The message
-	 * holds our own mailbox's onion, and generated unique properties. All of
-	 * which the contact needs to communicate with our Mailbox.
+	 * holds our own mailbox's onion, generated unique properties, and lists of
+	 * supported Mailbox API version(s). All of which the contact needs to
+	 * communicate with our Mailbox.
 	 */
-	private void createAndSendProperties(Transaction txn,
-			Contact c, String ownOnion) throws DbException {
-		MailboxPropertiesUpdate p = new MailboxPropertiesUpdate(ownOnion,
+	private void createAndSendUpdateWithMailbox(Transaction txn, Contact c,
+			List<MailboxVersion> serverSupports, String ownOnion)
+			throws DbException {
+		MailboxProperties properties = new MailboxProperties(ownOnion,
 				new MailboxAuthToken(crypto.generateUniqueId().getBytes()),
+				serverSupports,
 				new MailboxFolderId(crypto.generateUniqueId().getBytes()),
 				new MailboxFolderId(crypto.generateUniqueId().getBytes()));
+		MailboxUpdate u =
+				new MailboxUpdateWithMailbox(clientSupports, properties);
 		Group g = getContactGroup(c);
-		storeMessageReplaceLatest(txn, g.getId(), p);
+		storeMessageReplaceLatest(txn, g.getId(), u);
 	}
 
 	/**
-	 * Sends an empty update message to the given contact. The empty update
-	 * indicates for the receiving contact that we no longer have a Mailbox that
-	 * they can use.
+	 * Sends an update message with empty properties to the given contact. The
+	 * empty update indicates for the receiving contact that we don't have any
+	 * Mailbox that they can use. It still includes the list of Mailbox API
+	 * version(s) that we support as a client.
 	 */
-	private void sendEmptyProperties(Transaction txn, Contact c)
+	private void sendUpdateNoMailbox(Transaction txn, Contact c)
 			throws DbException {
 		Group g = getContactGroup(c);
-		storeMessageReplaceLatest(txn, g.getId(), null);
+		MailboxUpdate u = new MailboxUpdate(clientSupports);
+		storeMessageReplaceLatest(txn, g.getId(), u);
 	}
 
 	@Nullable
-	private MailboxPropertiesUpdate getProperties(Transaction txn,
-			Contact c, boolean local) throws DbException {
-		MailboxPropertiesUpdate p = null;
+	private MailboxUpdate getUpdate(Transaction txn, Contact c, boolean local)
+			throws DbException {
+		MailboxUpdate u = null;
 		Group g = getContactGroup(c);
 		try {
 			LatestUpdate latest = findLatest(txn, g.getId(), local);
 			if (latest != null) {
 				BdfList body =
 						clientHelper.getMessageAsList(txn, latest.messageId);
-				p = parseProperties(body);
+				u = parseUpdate(body);
 			}
 		} catch (FormatException e) {
 			throw new DbException(e);
 		}
-		return p;
+		return u;
 	}
 
 	private void storeMessageReplaceLatest(Transaction txn, GroupId g,
-			@Nullable MailboxPropertiesUpdate p) throws DbException {
+			MailboxUpdate u) throws DbException {
 		try {
 			LatestUpdate latest = findLatest(txn, g, true);
 			long version = latest == null ? 1 : latest.version + 1;
 			Message m = clientHelper.createMessage(g, clock.currentTimeMillis(),
-					encodeProperties(version, p));
+					encodeProperties(version, u));
 			BdfDictionary meta = new BdfDictionary();
 			meta.put(MSG_KEY_VERSION, version);
 			meta.put(MSG_KEY_LOCAL, true);
@@ -260,23 +322,36 @@ class MailboxPropertyManagerImpl implements MailboxPropertyManager,
 		return null;
 	}
 
-	private BdfList encodeProperties(long version,
-			@Nullable MailboxPropertiesUpdate p) {
+	private BdfList encodeProperties(long version, MailboxUpdate u) {
 		BdfDictionary dict = new BdfDictionary();
-		if (p != null) {
-			dict.put(PROP_KEY_ONION, p.getOnion());
-			dict.put(PROP_KEY_AUTHTOKEN, p.getAuthToken().getBytes());
-			dict.put(PROP_KEY_INBOXID, p.getInboxId().getBytes());
-			dict.put(PROP_KEY_OUTBOXID, p.getOutboxId().getBytes());
+		BdfList serverSupports = new BdfList();
+		if (u.hasMailbox()) {
+			MailboxUpdateWithMailbox um = (MailboxUpdateWithMailbox) u;
+			MailboxProperties properties = um.getMailboxProperties();
+			serverSupports = encodeSupportsList(properties.getServerSupports());
+			dict.put(PROP_KEY_ONION, properties.getOnion());
+			dict.put(PROP_KEY_AUTHTOKEN, properties.getAuthToken());
+			dict.put(PROP_KEY_INBOXID, properties.getInboxId());
+			dict.put(PROP_KEY_OUTBOXID, properties.getOutboxId());
 		}
-		return BdfList.of(version, dict);
+		return BdfList.of(version, encodeSupportsList(u.getClientSupports()),
+				serverSupports, dict);
 	}
 
-	@Nullable
-	private MailboxPropertiesUpdate parseProperties(BdfList body)
-			throws FormatException {
-		BdfDictionary dict = body.getDictionary(1);
-		return clientHelper.parseAndValidateMailboxPropertiesUpdate(dict);
+	private BdfList encodeSupportsList(List<MailboxVersion> supportsList) {
+		BdfList supports = new BdfList();
+		for (MailboxVersion version : supportsList) {
+			supports.add(BdfList.of(version.getMajor(), version.getMinor()));
+		}
+		return supports;
+	}
+
+	private MailboxUpdate parseUpdate(BdfList body) throws FormatException {
+		BdfList clientSupports = body.getList(1);
+		BdfList serverSupports = body.getList(2);
+		BdfDictionary dict = body.getDictionary(3);
+		return clientHelper.parseAndValidateMailboxUpdate(clientSupports,
+				serverSupports, dict);
 	}
 
 	private Group getContactGroup(Contact c) {

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxUpdateValidator.java

@@ -15,15 +15,15 @@ import org.briarproject.bramble.api.system.Clock;
 
 import javax.annotation.concurrent.Immutable;
 
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MSG_KEY_LOCAL;
-import static org.briarproject.bramble.api.mailbox.MailboxPropertyManager.MSG_KEY_VERSION;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MSG_KEY_LOCAL;
+import static org.briarproject.bramble.api.mailbox.MailboxUpdateManager.MSG_KEY_VERSION;
 import static org.briarproject.bramble.util.ValidationUtils.checkSize;
 
 @Immutable
 @NotNullByDefault
-class MailboxPropertyValidator extends BdfMessageValidator {
+class MailboxUpdateValidator extends BdfMessageValidator {
 
-	MailboxPropertyValidator(ClientHelper clientHelper,
+	MailboxUpdateValidator(ClientHelper clientHelper,
 			MetadataEncoder metadataEncoder, Clock clock) {
 		super(clientHelper, metadataEncoder, clock);
 	}
@@ -31,14 +31,19 @@ class MailboxPropertyValidator extends BdfMessageValidator {
 	@Override
 	protected BdfMessageContext validateMessage(Message m, Group g,
 			BdfList body) throws InvalidMessageException, FormatException {
-		// Version, properties
-		checkSize(body, 2);
+		// Version, Properties, clientSupports, serverSupports
+		checkSize(body, 4);
 		// Version
 		long version = body.getLong(0);
 		if (version < 0) throw new FormatException();
+		// clientSupports
+		BdfList clientSupports = body.getList(1);
+		// serverSupports
+		BdfList serverSupports = body.getList(2);
 		// Properties
-		BdfDictionary dictionary = body.getDictionary(1);
-		clientHelper.parseAndValidateMailboxPropertiesUpdate(dictionary);
+		BdfDictionary dictionary = body.getDictionary(3);
+		clientHelper.parseAndValidateMailboxUpdate(clientSupports,
+				serverSupports, dictionary);
 		// Return the metadata
 		BdfDictionary meta = new BdfDictionary();
 		meta.put(MSG_KEY_VERSION, version);

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxUploadWorker.java

@@ -0,0 +1,394 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventExecutor;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.sync.MessageId;
+import org.briarproject.bramble.api.sync.OutgoingSessionRecord;
+import org.briarproject.bramble.api.sync.event.GroupVisibilityUpdatedEvent;
+import org.briarproject.bramble.api.sync.event.MessageSharedEvent;
+import org.briarproject.bramble.api.sync.event.MessageToAckEvent;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.api.system.TaskScheduler;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.Collection;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static java.util.concurrent.TimeUnit.MINUTES;
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.MAX_LATENCY;
+import static org.briarproject.bramble.api.sync.Group.Visibility.SHARED;
+import static org.briarproject.bramble.util.IoUtils.delete;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@ThreadSafe
+@NotNullByDefault
+class MailboxUploadWorker implements MailboxWorker, ConnectivityObserver,
+		EventListener {
+
+	/**
+	 * When the worker is started it checks for data to send. If data is ready
+	 * to send, the worker waits for a connectivity check, then writes and
+	 * uploads a file and checks again for data to send.
+	 * <p>
+	 * If data is due to be sent at some time in the future, the worker
+	 * schedules a wakeup for that time and also listens for events indicating
+	 * that new data may be ready to send.
+	 * <p>
+	 * If there's no data to send, the worker listens for events indicating
+	 * that new data may be ready to send.
+	 */
+	private enum State {
+		CREATED,
+		CHECKING_FOR_DATA,
+		WAITING_FOR_DATA,
+		CONNECTIVITY_CHECK,
+		WRITING_UPLOADING,
+		DESTROYED
+	}
+
+	private static final Logger LOG =
+			getLogger(MailboxUploadWorker.class.getName());
+
+	/**
+	 * When we're waiting for data to send and an event indicates that new data
+	 * may have become available, wait this long before checking the DB. This
+	 * should help to avoid creating lots of small files when several acks or
+	 * messages become available to send in a short period (eg when reading a
+	 * file downloaded from a mailbox).
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final long CHECK_DELAY_MS = 5_000;
+
+	/**
+	 * How long to wait before retrying when an exception occurs while writing
+	 * a file.
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final long RETRY_DELAY_MS = MINUTES.toMillis(1);
+
+	private final Executor ioExecutor;
+	private final DatabaseComponent db;
+	private final Clock clock;
+	private final TaskScheduler taskScheduler;
+	private final EventBus eventBus;
+	private final ConnectivityChecker connectivityChecker;
+	private final MailboxApiCaller mailboxApiCaller;
+	private final MailboxApi mailboxApi;
+	private final MailboxFileManager mailboxFileManager;
+	private final MailboxProperties mailboxProperties;
+	private final MailboxFolderId folderId;
+	private final ContactId contactId;
+
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	private State state = State.CREATED;
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable wakeupTask = null, checkTask = null, apiCall = null;
+
+	@GuardedBy("lock")
+	@Nullable
+	private File file = null;
+
+	MailboxUploadWorker(@IoExecutor Executor ioExecutor,
+			DatabaseComponent db,
+			Clock clock,
+			TaskScheduler taskScheduler,
+			EventBus eventBus,
+			ConnectivityChecker connectivityChecker,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxProperties mailboxProperties,
+			MailboxFolderId folderId,
+			ContactId contactId) {
+		this.ioExecutor = ioExecutor;
+		this.db = db;
+		this.clock = clock;
+		this.taskScheduler = taskScheduler;
+		this.eventBus = eventBus;
+		this.connectivityChecker = connectivityChecker;
+		this.mailboxApiCaller = mailboxApiCaller;
+		this.mailboxApi = mailboxApi;
+		this.mailboxFileManager = mailboxFileManager;
+		this.mailboxProperties = mailboxProperties;
+		this.folderId = folderId;
+		this.contactId = contactId;
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		synchronized (lock) {
+			// Don't allow the worker to be reused
+			if (state != State.CREATED) return;
+			state = State.CHECKING_FOR_DATA;
+		}
+		ioExecutor.execute(this::checkForDataToSend);
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		Cancellable wakeupTask, checkTask, apiCall;
+		File file;
+		synchronized (lock) {
+			state = State.DESTROYED;
+			wakeupTask = this.wakeupTask;
+			this.wakeupTask = null;
+			checkTask = this.checkTask;
+			this.checkTask = null;
+			apiCall = this.apiCall;
+			this.apiCall = null;
+			file = this.file;
+			this.file = null;
+		}
+		if (wakeupTask != null) wakeupTask.cancel();
+		if (checkTask != null) checkTask.cancel();
+		if (apiCall != null) apiCall.cancel();
+		if (file != null) delete(file);
+		connectivityChecker.removeObserver(this);
+		eventBus.removeListener(this);
+	}
+
+	@IoExecutor
+	private void checkForDataToSend() {
+		synchronized (lock) {
+			checkTask = null;
+			if (state != State.CHECKING_FOR_DATA) return;
+		}
+		LOG.info("Checking for data to send");
+		try {
+			db.transaction(true, txn -> {
+				long nextSendTime;
+				if (db.containsAcksToSend(txn, contactId)) {
+					nextSendTime = 0L;
+				} else {
+					nextSendTime = db.getNextSendTime(txn, contactId,
+							MAX_LATENCY);
+				}
+				// Handle the result on the event executor to avoid races with
+				// incoming events
+				txn.attach(() -> handleNextSendTime(nextSendTime));
+			});
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@EventExecutor
+	private void handleNextSendTime(long nextSendTime) {
+		if (nextSendTime == Long.MAX_VALUE) {
+			// Nothing is sendable now or due to be sent in the future. Wait
+			// for an event indicating that new data may be ready to send
+			waitForDataToSend();
+		} else {
+			// Work out the delay until data's ready to send (may be negative)
+			long delay = nextSendTime - clock.currentTimeMillis();
+			if (delay > 0) {
+				// Schedule a wakeup when data will be ready to send. If an
+				// event is received in the meantime indicating that new data
+				// may be ready to send, we'll cancel the wakeup
+				scheduleWakeup(delay);
+			} else {
+				// Data is ready to send now
+				checkConnectivity();
+			}
+		}
+	}
+
+	@EventExecutor
+	private void waitForDataToSend() {
+		synchronized (lock) {
+			if (state != State.CHECKING_FOR_DATA) return;
+			state = State.WAITING_FOR_DATA;
+			LOG.info("Waiting for data to send");
+		}
+	}
+
+	@EventExecutor
+	private void scheduleWakeup(long delay) {
+		synchronized (lock) {
+			if (state != State.CHECKING_FOR_DATA) return;
+			state = State.WAITING_FOR_DATA;
+			if (LOG.isLoggable(INFO)) {
+				LOG.info("Scheduling wakeup in " + delay + " ms");
+			}
+			wakeupTask = taskScheduler.schedule(this::wakeUp, ioExecutor,
+					delay, MILLISECONDS);
+		}
+	}
+
+	@IoExecutor
+	private void wakeUp() {
+		LOG.info("Woke up");
+		synchronized (lock) {
+			wakeupTask = null;
+			if (state != State.WAITING_FOR_DATA) return;
+			state = State.CHECKING_FOR_DATA;
+		}
+		checkForDataToSend();
+	}
+
+	@EventExecutor
+	private void checkConnectivity() {
+		synchronized (lock) {
+			if (state != State.CHECKING_FOR_DATA) return;
+			state = State.CONNECTIVITY_CHECK;
+		}
+		LOG.info("Checking connectivity");
+		// Avoid leaking observer in case destroy() is called concurrently
+		// before observer is added
+		connectivityChecker.checkConnectivity(mailboxProperties, this);
+		boolean destroyed;
+		synchronized (lock) {
+			destroyed = state == State.DESTROYED;
+		}
+		if (destroyed) connectivityChecker.removeObserver(this);
+	}
+
+	@Override
+	public void onConnectivityCheckSucceeded() {
+		LOG.info("Connectivity check succeeded");
+		synchronized (lock) {
+			if (state != State.CONNECTIVITY_CHECK) return;
+			state = State.WRITING_UPLOADING;
+		}
+		ioExecutor.execute(this::writeAndUploadFile);
+	}
+
+	@IoExecutor
+	private void writeAndUploadFile() {
+		synchronized (lock) {
+			if (state != State.WRITING_UPLOADING) return;
+		}
+		OutgoingSessionRecord sessionRecord = new OutgoingSessionRecord();
+		File file;
+		try {
+			file = mailboxFileManager.createAndWriteTempFileForUpload(
+					contactId, sessionRecord);
+		} catch (IOException e) {
+			logException(LOG, WARNING, e);
+			// Try again after a delay
+			synchronized (lock) {
+				if (state != State.WRITING_UPLOADING) return;
+				state = State.CHECKING_FOR_DATA;
+				checkTask = taskScheduler.schedule(this::checkForDataToSend,
+						ioExecutor, RETRY_DELAY_MS, MILLISECONDS);
+			}
+			return;
+		}
+		boolean deleteFile = false;
+		synchronized (lock) {
+			if (state == State.WRITING_UPLOADING) {
+				this.file = file;
+				apiCall = mailboxApiCaller.retryWithBackoff(
+						new SimpleApiCall(() -> apiCallUploadFile(file,
+								sessionRecord)));
+			} else {
+				deleteFile = true;
+			}
+		}
+		if (deleteFile) delete(file);
+	}
+
+	@IoExecutor
+	private void apiCallUploadFile(File file,
+			OutgoingSessionRecord sessionRecord)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state != State.WRITING_UPLOADING) return;
+		}
+		LOG.info("Uploading file");
+		mailboxApi.addFile(mailboxProperties, folderId, file);
+		markMessagesSentOrAcked(sessionRecord);
+		synchronized (lock) {
+			if (state != State.WRITING_UPLOADING) return;
+			state = State.CHECKING_FOR_DATA;
+			apiCall = null;
+			this.file = null;
+		}
+		delete(file);
+		checkForDataToSend();
+	}
+
+	private void markMessagesSentOrAcked(OutgoingSessionRecord sessionRecord) {
+		Collection<MessageId> acked = sessionRecord.getAckedIds();
+		Collection<MessageId> sent = sessionRecord.getSentIds();
+		try {
+			db.transaction(false, txn -> {
+				if (!acked.isEmpty()) {
+					db.setAckSent(txn, contactId, acked);
+				}
+				if (!sent.isEmpty()) {
+					db.setMessagesSent(txn, contactId, sent, MAX_LATENCY);
+				}
+			});
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof MessageToAckEvent) {
+			MessageToAckEvent m = (MessageToAckEvent) e;
+			if (m.getContactId().equals(contactId)) {
+				LOG.info("Message to ack");
+				onDataToSend();
+			}
+		} else if (e instanceof MessageSharedEvent) {
+			LOG.info("Message shared");
+			onDataToSend();
+		} else if (e instanceof GroupVisibilityUpdatedEvent) {
+			GroupVisibilityUpdatedEvent g = (GroupVisibilityUpdatedEvent) e;
+			if (g.getVisibility() == SHARED &&
+					g.getAffectedContacts().contains(contactId)) {
+				LOG.info("Group shared");
+				onDataToSend();
+			}
+		}
+	}
+
+	@EventExecutor
+	private void onDataToSend() {
+		Cancellable wakeupTask;
+		synchronized (lock) {
+			if (state != State.WAITING_FOR_DATA) return;
+			state = State.CHECKING_FOR_DATA;
+			wakeupTask = this.wakeupTask;
+			this.wakeupTask = null;
+			// Delay the check to avoid creating lots of small files
+			checkTask = taskScheduler.schedule(this::checkForDataToSend,
+					ioExecutor, CHECK_DELAY_MS, MILLISECONDS);
+		}
+		// If we had scheduled a wakeup when data was due to be sent, cancel it
+		if (wakeupTask != null) wakeupTask.cancel();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxWorker.java

@@ -0,0 +1,23 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+/**
+ * A worker that downloads files from a contact's mailbox.
+ */
+@ThreadSafe
+@NotNullByDefault
+interface MailboxWorker {
+
+	/**
+	 * Asynchronously starts the worker.
+	 */
+	void start();
+
+	/**
+	 * Destroys the worker and cancels any pending tasks or retries.
+	 */
+	void destroy();
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxWorkerFactory.java

@@ -0,0 +1,31 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+@ThreadSafe
+@NotNullByDefault
+interface MailboxWorkerFactory {
+
+	MailboxWorker createUploadWorker(ConnectivityChecker connectivityChecker,
+			MailboxProperties properties, MailboxFolderId folderId,
+			ContactId contactId);
+
+	MailboxWorker createDownloadWorkerForContactMailbox(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties);
+
+	MailboxWorker createDownloadWorkerForOwnMailbox(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties);
+
+	MailboxWorker createContactListWorkerForOwnMailbox(
+			ConnectivityChecker connectivityChecker,
+			MailboxProperties properties);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxWorkerFactoryImpl.java

@@ -0,0 +1,95 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.api.system.TaskScheduler;
+
+import java.util.concurrent.Executor;
+
+import javax.annotation.concurrent.Immutable;
+import javax.inject.Inject;
+
+@Immutable
+@NotNullByDefault
+class MailboxWorkerFactoryImpl implements MailboxWorkerFactory {
+
+	private final Executor ioExecutor;
+	private final DatabaseComponent db;
+	private final Clock clock;
+	private final TaskScheduler taskScheduler;
+	private final EventBus eventBus;
+	private final MailboxApiCaller mailboxApiCaller;
+	private final MailboxApi mailboxApi;
+	private final MailboxFileManager mailboxFileManager;
+	private final MailboxUpdateManager mailboxUpdateManager;
+
+	@Inject
+	MailboxWorkerFactoryImpl(@IoExecutor Executor ioExecutor,
+			DatabaseComponent db,
+			Clock clock,
+			TaskScheduler taskScheduler,
+			EventBus eventBus,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxUpdateManager mailboxUpdateManager) {
+		this.ioExecutor = ioExecutor;
+		this.db = db;
+		this.clock = clock;
+		this.taskScheduler = taskScheduler;
+		this.eventBus = eventBus;
+		this.mailboxApiCaller = mailboxApiCaller;
+		this.mailboxApi = mailboxApi;
+		this.mailboxFileManager = mailboxFileManager;
+		this.mailboxUpdateManager = mailboxUpdateManager;
+	}
+
+	@Override
+	public MailboxWorker createUploadWorker(
+			ConnectivityChecker connectivityChecker,
+			MailboxProperties properties, MailboxFolderId folderId,
+			ContactId contactId) {
+		MailboxUploadWorker worker = new MailboxUploadWorker(ioExecutor, db,
+				clock, taskScheduler, eventBus, connectivityChecker,
+				mailboxApiCaller, mailboxApi, mailboxFileManager,
+				properties, folderId, contactId);
+		eventBus.addListener(worker);
+		return worker;
+	}
+
+	@Override
+	public MailboxWorker createDownloadWorkerForContactMailbox(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties) {
+		return new ContactMailboxDownloadWorker(connectivityChecker,
+				reachabilityMonitor, mailboxApiCaller, mailboxApi,
+				mailboxFileManager, properties);
+	}
+
+	@Override
+	public MailboxWorker createDownloadWorkerForOwnMailbox(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties) {
+		return new OwnMailboxDownloadWorker(connectivityChecker,
+				reachabilityMonitor, mailboxApiCaller, mailboxApi,
+				mailboxFileManager, properties);
+	}
+
+	@Override
+	public MailboxWorker createContactListWorkerForOwnMailbox(
+			ConnectivityChecker connectivityChecker,
+			MailboxProperties properties) {
+		return new OwnMailboxContactListWorker(ioExecutor, db, eventBus,
+				connectivityChecker, mailboxApiCaller, mailboxApi,
+				mailboxUpdateManager, properties);
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/OwnMailboxClient.java

@@ -0,0 +1,154 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Logger.getLogger;
+
+@ThreadSafe
+@NotNullByDefault
+class OwnMailboxClient implements MailboxClient {
+
+	private static final Logger LOG =
+			getLogger(OwnMailboxClient.class.getName());
+
+	private final MailboxWorkerFactory workerFactory;
+	private final ConnectivityChecker connectivityChecker;
+	private final TorReachabilityMonitor reachabilityMonitor;
+	private final MailboxWorker contactListWorker;
+	private final Object lock = new Object();
+
+	/**
+	 * Upload workers: one worker per contact assigned for upload.
+	 */
+	@GuardedBy("lock")
+	private final Map<ContactId, MailboxWorker> uploadWorkers = new HashMap<>();
+
+	/**
+	 * Download worker: shared between all contacts assigned for download.
+	 * Null if no contacts are assigned for download.
+	 */
+	@GuardedBy("lock")
+	@Nullable
+	private MailboxWorker downloadWorker = null;
+
+	/**
+	 * IDs of contacts assigned for download, so that we know when to
+	 * create/destroy the download worker.
+	 */
+	@GuardedBy("lock")
+	private final Set<ContactId> assignedForDownload = new HashSet<>();
+
+	OwnMailboxClient(MailboxWorkerFactory workerFactory,
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor reachabilityMonitor,
+			MailboxProperties properties) {
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		this.workerFactory = workerFactory;
+		this.connectivityChecker = connectivityChecker;
+		this.reachabilityMonitor = reachabilityMonitor;
+		contactListWorker = workerFactory.createContactListWorkerForOwnMailbox(
+				connectivityChecker, properties);
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		contactListWorker.start();
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		List<MailboxWorker> uploadWorkers;
+		MailboxWorker downloadWorker;
+		synchronized (lock) {
+			uploadWorkers = new ArrayList<>(this.uploadWorkers.values());
+			this.uploadWorkers.clear();
+			downloadWorker = this.downloadWorker;
+			this.downloadWorker = null;
+		}
+		// Destroy the workers (with apologies to Mr Marx and Mr Engels)
+		for (MailboxWorker worker : uploadWorkers) worker.destroy();
+		if (downloadWorker != null) downloadWorker.destroy();
+		contactListWorker.destroy();
+	}
+
+	@Override
+	public void assignContactForUpload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		LOG.info("Contact assigned for upload");
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		MailboxWorker uploadWorker = workerFactory.createUploadWorker(
+				connectivityChecker, properties, folderId, contactId);
+		synchronized (lock) {
+			MailboxWorker old = uploadWorkers.put(contactId, uploadWorker);
+			if (old != null) throw new IllegalStateException();
+		}
+		uploadWorker.start();
+	}
+
+	@Override
+	public void deassignContactForUpload(ContactId contactId) {
+		LOG.info("Contact deassigned for upload");
+		MailboxWorker uploadWorker;
+		synchronized (lock) {
+			uploadWorker = uploadWorkers.remove(contactId);
+		}
+		if (uploadWorker != null) uploadWorker.destroy();
+	}
+
+	@Override
+	public void assignContactForDownload(ContactId contactId,
+			MailboxProperties properties, MailboxFolderId folderId) {
+		LOG.info("Contact assigned for download");
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		// Create a download worker if we don't already have one. The worker
+		// will use the API to discover which folders have files to download,
+		// so it doesn't need to track the set of assigned contacts
+		MailboxWorker toStart = null;
+		synchronized (lock) {
+			if (!assignedForDownload.add(contactId)) {
+				throw new IllegalStateException();
+			}
+			if (downloadWorker == null) {
+				toStart = workerFactory.createDownloadWorkerForOwnMailbox(
+						connectivityChecker, reachabilityMonitor, properties);
+				downloadWorker = toStart;
+			}
+		}
+		if (toStart != null) toStart.start();
+	}
+
+	@Override
+	public void deassignContactForDownload(ContactId contactId) {
+		LOG.info("Contact deassigned for download");
+		// If there are no more contacts assigned for download, destroy the
+		// download worker
+		MailboxWorker toDestroy = null;
+		synchronized (lock) {
+			if (!assignedForDownload.remove(contactId)) {
+				throw new IllegalStateException();
+			}
+			if (assignedForDownload.isEmpty()) {
+				toDestroy = downloadWorker;
+				downloadWorker = null;
+			}
+		}
+		if (toDestroy != null) toDestroy.destroy();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/OwnMailboxConnectivityChecker.java

@@ -0,0 +1,78 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.TransactionManager;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxSettingsManager;
+import org.briarproject.bramble.api.mailbox.MailboxVersion;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.system.Clock;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.logging.Logger;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@ThreadSafe
+@NotNullByDefault
+class OwnMailboxConnectivityChecker extends ConnectivityCheckerImpl {
+
+	private static final Logger LOG =
+			getLogger(OwnMailboxConnectivityChecker.class.getName());
+
+	private final MailboxApi mailboxApi;
+	private final TransactionManager db;
+	private final MailboxSettingsManager mailboxSettingsManager;
+
+	OwnMailboxConnectivityChecker(Clock clock,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			TransactionManager db,
+			MailboxSettingsManager mailboxSettingsManager) {
+		super(clock, mailboxApiCaller);
+		this.mailboxApi = mailboxApi;
+		this.db = db;
+		this.mailboxSettingsManager = mailboxSettingsManager;
+	}
+
+	@Override
+	ApiCall createConnectivityCheckTask(MailboxProperties properties) {
+		if (!properties.isOwner()) throw new IllegalArgumentException();
+		return () -> {
+			try {
+				return checkConnectivityAndStoreResult(properties);
+			} catch (DbException e) {
+				logException(LOG, WARNING, e);
+				return true; // Retry
+			}
+		};
+	}
+
+	private boolean checkConnectivityAndStoreResult(
+			MailboxProperties properties) throws DbException {
+		try {
+			List<MailboxVersion> serverSupports =
+					mailboxApi.getServerSupports(properties);
+			LOG.info("Own mailbox is reachable");
+			long now = clock.currentTimeMillis();
+			db.transaction(false, txn -> mailboxSettingsManager
+					.recordSuccessfulConnection(txn, now, serverSupports));
+			// Call the observers and cache the result
+			onConnectivityCheckSucceeded(now);
+			return false; // Don't retry
+		} catch (IOException | ApiException e) {
+			LOG.warning("Own mailbox is unreachable");
+			logException(LOG, WARNING, e);
+			long now = clock.currentTimeMillis();
+			db.transaction(false, txn -> mailboxSettingsManager
+					.recordFailedConnectionAttempt(txn, now));
+		}
+		return true; // Retry
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/OwnMailboxContactListWorker.java

@@ -0,0 +1,369 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.contact.event.ContactAddedEvent;
+import org.briarproject.bramble.api.contact.event.ContactRemovedEvent;
+import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.NoSuchContactException;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventExecutor;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.mailbox.MailboxUpdate;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateManager;
+import org.briarproject.bramble.api.mailbox.MailboxUpdateWithMailbox;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.ConnectivityChecker.ConnectivityObserver;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.MailboxContact;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.Queue;
+import java.util.Set;
+import java.util.concurrent.Executor;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@ThreadSafe
+@NotNullByDefault
+class OwnMailboxContactListWorker
+		implements MailboxWorker, ConnectivityObserver, EventListener {
+
+	/**
+	 * When the worker is started it waits for a connectivity check, then
+	 * fetches the remote contact list and compares it to the local contact
+	 * list.
+	 * <p>
+	 * Any contacts that are missing from the remote list are added to the
+	 * mailbox's contact list, while any contacts that are missing from the
+	 * local list are removed from the mailbox's contact list.
+	 * <p>
+	 * Once the remote contact list has been brought up to date, the worker
+	 * waits for events indicating that contacts have been added or removed.
+	 * Each time an event is received, the worker updates the mailbox's
+	 * contact list and then goes back to waiting.
+	 */
+	private enum State {
+		CREATED,
+		CONNECTIVITY_CHECK,
+		FETCHING_CONTACT_LIST,
+		UPDATING_CONTACT_LIST,
+		WAITING_FOR_CHANGES,
+		DESTROYED
+	}
+
+	private static final Logger LOG =
+			getLogger(OwnMailboxContactListWorker.class.getName());
+
+	private final Executor ioExecutor;
+	private final DatabaseComponent db;
+	private final EventBus eventBus;
+	private final ConnectivityChecker connectivityChecker;
+	private final MailboxApiCaller mailboxApiCaller;
+	private final MailboxApi mailboxApi;
+	private final MailboxUpdateManager mailboxUpdateManager;
+	private final MailboxProperties mailboxProperties;
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	private State state = State.CREATED;
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable apiCall = null;
+
+	/**
+	 * A queue of updates waiting to be applied to the remote contact list.
+	 */
+	@GuardedBy("lock")
+	private final Queue<Update> updates = new LinkedList<>();
+
+	OwnMailboxContactListWorker(@IoExecutor Executor ioExecutor,
+			DatabaseComponent db,
+			EventBus eventBus,
+			ConnectivityChecker connectivityChecker,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxUpdateManager mailboxUpdateManager,
+			MailboxProperties mailboxProperties) {
+		if (!mailboxProperties.isOwner()) throw new IllegalArgumentException();
+		this.ioExecutor = ioExecutor;
+		this.db = db;
+		this.connectivityChecker = connectivityChecker;
+		this.mailboxApiCaller = mailboxApiCaller;
+		this.mailboxApi = mailboxApi;
+		this.mailboxUpdateManager = mailboxUpdateManager;
+		this.mailboxProperties = mailboxProperties;
+		this.eventBus = eventBus;
+	}
+
+	@Override
+	public void start() {
+		LOG.info("Started");
+		synchronized (lock) {
+			if (state != State.CREATED) return;
+			state = State.CONNECTIVITY_CHECK;
+		}
+		// Avoid leaking observer in case destroy() is called concurrently
+		// before observer is added
+		connectivityChecker.checkConnectivity(mailboxProperties, this);
+		boolean destroyed;
+		synchronized (lock) {
+			destroyed = state == State.DESTROYED;
+		}
+		if (destroyed) connectivityChecker.removeObserver(this);
+	}
+
+	@Override
+	public void destroy() {
+		LOG.info("Destroyed");
+		Cancellable apiCall;
+		synchronized (lock) {
+			state = State.DESTROYED;
+			apiCall = this.apiCall;
+			this.apiCall = null;
+		}
+		if (apiCall != null) apiCall.cancel();
+		connectivityChecker.removeObserver(this);
+		eventBus.removeListener(this);
+	}
+
+	@Override
+	public void onConnectivityCheckSucceeded() {
+		LOG.info("Connectivity check succeeded");
+		synchronized (lock) {
+			if (state != State.CONNECTIVITY_CHECK) return;
+			state = State.FETCHING_CONTACT_LIST;
+			apiCall = mailboxApiCaller.retryWithBackoff(
+					new SimpleApiCall(this::apiCallFetchContactList));
+		}
+	}
+
+	@IoExecutor
+	private void apiCallFetchContactList() throws IOException, ApiException {
+		synchronized (lock) {
+			if (state != State.FETCHING_CONTACT_LIST) return;
+		}
+		LOG.info("Fetching remote contact list");
+		Collection<ContactId> remote =
+				mailboxApi.getContacts(mailboxProperties);
+		ioExecutor.execute(() -> loadLocalContactList(remote));
+	}
+
+	@IoExecutor
+	private void loadLocalContactList(Collection<ContactId> remote) {
+		synchronized (lock) {
+			if (state != State.FETCHING_CONTACT_LIST) return;
+			apiCall = null;
+		}
+		LOG.info("Loading local contact list");
+		try {
+			db.transaction(true, txn -> {
+				Collection<Contact> local = db.getContacts(txn);
+				// Handle the result on the event executor to avoid races with
+				// incoming events
+				txn.attach(() -> reconcileContactLists(local, remote));
+			});
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@EventExecutor
+	private void reconcileContactLists(Collection<Contact> local,
+			Collection<ContactId> remote) {
+		Set<ContactId> localIds = new HashSet<>();
+		for (Contact c : local) localIds.add(c.getId());
+		remote = new HashSet<>(remote);
+		synchronized (lock) {
+			if (state != State.FETCHING_CONTACT_LIST) return;
+			for (ContactId c : localIds) {
+				if (!remote.contains(c)) updates.add(new Update(true, c));
+			}
+			for (ContactId c : remote) {
+				if (!localIds.contains(c)) updates.add(new Update(false, c));
+			}
+			if (updates.isEmpty()) {
+				LOG.info("Contact list is up to date");
+				state = State.WAITING_FOR_CHANGES;
+			} else {
+				if (LOG.isLoggable(INFO)) {
+					LOG.info(updates.size() + " updates to apply");
+				}
+				state = State.UPDATING_CONTACT_LIST;
+				ioExecutor.execute(this::updateContactList);
+			}
+		}
+	}
+
+	@IoExecutor
+	private void updateContactList() {
+		Update update;
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+			update = updates.poll();
+			if (update == null) {
+				LOG.info("No more updates to process");
+				state = State.WAITING_FOR_CHANGES;
+				apiCall = null;
+				return;
+			}
+		}
+		if (update.add) loadMailboxProperties(update.contactId);
+		else removeContact(update.contactId);
+	}
+
+	@IoExecutor
+	private void loadMailboxProperties(ContactId c) {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+		}
+		LOG.info("Loading mailbox properties for contact");
+		try {
+			MailboxUpdate mailboxUpdate = db.transactionWithResult(true, txn ->
+					mailboxUpdateManager.getLocalUpdate(txn, c));
+			if (mailboxUpdate instanceof MailboxUpdateWithMailbox) {
+				addContact(c, (MailboxUpdateWithMailbox) mailboxUpdate);
+			} else {
+				// Our own mailbox was concurrently unpaired. This worker will
+				// be destroyed soon, so we can stop here
+				LOG.info("Own mailbox was unpaired");
+			}
+		} catch (NoSuchContactException e) {
+			// Contact was removed concurrently. Move on to the next update.
+			// Later we may process a removal update for this contact, which
+			// was never added to the mailbox's contact list. The removal API
+			// call should fail safely with a TolerableFailureException
+			LOG.info("No such contact");
+			updateContactList();
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@IoExecutor
+	private void addContact(ContactId c, MailboxUpdateWithMailbox withMailbox) {
+		MailboxProperties props = withMailbox.getMailboxProperties();
+		MailboxContact contact = new MailboxContact(c, props.getAuthToken(),
+				requireNonNull(props.getInboxId()),
+				requireNonNull(props.getOutboxId()));
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+			apiCall = mailboxApiCaller.retryWithBackoff(new SimpleApiCall(() ->
+					apiCallAddContact(contact)));
+		}
+	}
+
+	@IoExecutor
+	private void apiCallAddContact(MailboxContact contact)
+			throws IOException, ApiException, TolerableFailureException {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+		}
+		LOG.info("Adding contact to remote contact list");
+		mailboxApi.addContact(mailboxProperties, contact);
+		updateContactList();
+	}
+
+	@IoExecutor
+	private void removeContact(ContactId c) {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+			apiCall = mailboxApiCaller.retryWithBackoff(new SimpleApiCall(() ->
+					apiCallRemoveContact(c)));
+		}
+	}
+
+	@IoExecutor
+	private void apiCallRemoveContact(ContactId c)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST) return;
+		}
+		LOG.info("Removing contact from remote contact list");
+		try {
+			mailboxApi.deleteContact(mailboxProperties, c);
+		} catch (TolerableFailureException e) {
+			// Catch this so we can continue to the next update
+			LOG.warning("Contact does not exist");
+		}
+		updateContactList();
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof ContactAddedEvent) {
+			LOG.info("Contact added");
+			onContactAdded(((ContactAddedEvent) e).getContactId());
+		} else if (e instanceof ContactRemovedEvent) {
+			LOG.info("Contact removed");
+			onContactRemoved(((ContactRemovedEvent) e).getContactId());
+		}
+	}
+
+	@EventExecutor
+	private void onContactAdded(ContactId c) {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST &&
+					state != State.WAITING_FOR_CHANGES) {
+				return;
+			}
+			updates.add(new Update(true, c));
+			if (state == State.WAITING_FOR_CHANGES) {
+				state = State.UPDATING_CONTACT_LIST;
+				ioExecutor.execute(this::updateContactList);
+			}
+		}
+	}
+
+	@EventExecutor
+	private void onContactRemoved(ContactId c) {
+		synchronized (lock) {
+			if (state != State.UPDATING_CONTACT_LIST &&
+					state != State.WAITING_FOR_CHANGES) {
+				return;
+			}
+			updates.add(new Update(false, c));
+			if (state == State.WAITING_FOR_CHANGES) {
+				state = State.UPDATING_CONTACT_LIST;
+				ioExecutor.execute(this::updateContactList);
+			}
+		}
+	}
+
+	/**
+	 * An update that should be applied to the remote contact list.
+	 */
+	private static class Update {
+
+		/**
+		 * True if the contact should be added, false if the contact should be
+		 * removed.
+		 */
+		private final boolean add;
+		private final ContactId contactId;
+
+		private Update(boolean add, ContactId contactId) {
+			this.add = add;
+			this.contactId = contactId;
+		}
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/OwnMailboxDownloadWorker.java

@@ -0,0 +1,148 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.mailbox.MailboxFolderId;
+import org.briarproject.bramble.api.mailbox.MailboxProperties;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.MailboxFile;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Queue;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.Collections.shuffle;
+import static java.util.logging.Level.INFO;
+
+@ThreadSafe
+@NotNullByDefault
+class OwnMailboxDownloadWorker extends MailboxDownloadWorker {
+
+	/**
+	 * The maximum number of files that will be downloaded before checking
+	 * again for folders with available files. This ensures that if a file
+	 * arrives during a download cycle, its folder will be checked within a
+	 * reasonable amount of time even if some other folder has a very large
+	 * number of files.
+	 * <p>
+	 * Package access for testing.
+	 */
+	static final int MAX_ROUND_ROBIN_FILES = 1000;
+
+	OwnMailboxDownloadWorker(
+			ConnectivityChecker connectivityChecker,
+			TorReachabilityMonitor torReachabilityMonitor,
+			MailboxApiCaller mailboxApiCaller,
+			MailboxApi mailboxApi,
+			MailboxFileManager mailboxFileManager,
+			MailboxProperties mailboxProperties) {
+		super(connectivityChecker, torReachabilityMonitor, mailboxApiCaller,
+				mailboxApi, mailboxFileManager, mailboxProperties);
+		if (!mailboxProperties.isOwner()) throw new IllegalArgumentException();
+	}
+
+	@Override
+	protected ApiCall createApiCallForDownloadCycle() {
+		return new SimpleApiCall(this::apiCallListFolders);
+	}
+
+	private void apiCallListFolders() throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		LOG.info("Listing folders with available files");
+		List<MailboxFolderId> folders =
+				mailboxApi.getFolders(mailboxProperties);
+		if (folders.isEmpty()) onDownloadCycleFinished();
+		else listNextFolder(new LinkedList<>(folders), new HashMap<>());
+	}
+
+	/**
+	 * Removes the next folder from `queue` and starts a task to list the
+	 * files in the folder and add them to `available`.
+	 */
+	private void listNextFolder(Queue<MailboxFolderId> queue,
+			Map<MailboxFolderId, Queue<MailboxFile>> available) {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+			MailboxFolderId folder = queue.remove();
+			apiCall = mailboxApiCaller.retryWithBackoff(new SimpleApiCall(() ->
+					apiCallListFolder(folder, queue, available)));
+		}
+	}
+
+	private void apiCallListFolder(MailboxFolderId folder,
+			Queue<MailboxFolderId> queue,
+			Map<MailboxFolderId, Queue<MailboxFile>> available)
+			throws IOException, ApiException {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		LOG.info("Listing folder");
+		try {
+			List<MailboxFile> files =
+					mailboxApi.getFiles(mailboxProperties, folder);
+			if (!files.isEmpty()) {
+				available.put(folder, new LinkedList<>(files));
+			}
+		} catch (TolerableFailureException e) {
+			LOG.warning("Folder does not exist");
+		}
+		if (queue.isEmpty()) {
+			LOG.info("Finished listing folders");
+			if (available.isEmpty()) onDownloadCycleFinished();
+			else createDownloadQueue(available);
+		} else {
+			listNextFolder(queue, available);
+		}
+	}
+
+	/**
+	 * Visits the given folders in round-robin order to create a queue of up to
+	 * {@link #MAX_ROUND_ROBIN_FILES} to download.
+	 */
+	private void createDownloadQueue(
+			Map<MailboxFolderId, Queue<MailboxFile>> available) {
+		synchronized (lock) {
+			if (state == State.DESTROYED) return;
+		}
+		if (LOG.isLoggable(INFO)) {
+			LOG.info(available.size() + " folders have available files");
+		}
+		Queue<FolderFile> queue = createRoundRobinQueue(available);
+		if (LOG.isLoggable(INFO)) {
+			LOG.info("Downloading " + queue.size() + " files");
+		}
+		downloadNextFile(queue);
+	}
+
+	// Package access for testing
+	Queue<FolderFile> createRoundRobinQueue(
+			Map<MailboxFolderId, Queue<MailboxFile>> available) {
+		List<MailboxFolderId> roundRobin = new ArrayList<>(available.keySet());
+		// Shuffle the folders so we don't always favour the same folders
+		shuffle(roundRobin);
+		Queue<FolderFile> queue = new LinkedList<>();
+		while (queue.size() < MAX_ROUND_ROBIN_FILES && !available.isEmpty()) {
+			Iterator<MailboxFolderId> it = roundRobin.iterator();
+			while (queue.size() < MAX_ROUND_ROBIN_FILES && it.hasNext()) {
+				MailboxFolderId folder = it.next();
+				Queue<MailboxFile> files = available.get(folder);
+				MailboxFile file = files.remove();
+				queue.add(new FolderFile(folder, file.name));
+				if (files.isEmpty()) {
+					available.remove(folder);
+					it.remove();
+				}
+			}
+		}
+		return queue;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/SimpleApiCall.java

@@ -0,0 +1,55 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.mailbox.MailboxApi.ApiException;
+import org.briarproject.bramble.mailbox.MailboxApi.TolerableFailureException;
+
+import java.io.IOException;
+import java.util.logging.Logger;
+
+import static java.util.logging.Level.INFO;
+import static java.util.logging.Level.WARNING;
+import static java.util.logging.Logger.getLogger;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+/**
+ * Convenience class for making simple API calls that don't return values.
+ */
+@NotNullByDefault
+class SimpleApiCall implements ApiCall {
+
+	private static final Logger LOG = getLogger(SimpleApiCall.class.getName());
+
+	private final Attempt attempt;
+
+	SimpleApiCall(Attempt attempt) {
+		this.attempt = attempt;
+	}
+
+	@Override
+	public boolean callApi() {
+		try {
+			attempt.tryToCallApi();
+			return false; // Succeeded, don't retry
+		} catch (IOException | ApiException e) {
+			logException(LOG, WARNING, e);
+			return true; // Failed, retry with backoff
+		} catch (TolerableFailureException e) {
+			logException(LOG, INFO, e);
+			return false; // Failed tolerably, don't retry
+		}
+	}
+
+	interface Attempt {
+
+		/**
+		 * Makes a single attempt to call an API endpoint. If this method
+		 * throws an {@link IOException} or an {@link ApiException}, the call
+		 * will be retried. If it throws a {@link TolerableFailureException}
+		 * or returns without throwing an exception, the call will not be
+		 * retried.
+		 */
+		void tryToCallApi()
+				throws IOException, ApiException, TolerableFailureException;
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitor.java

@@ -0,0 +1,51 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.Plugin;
+
+import javax.annotation.concurrent.ThreadSafe;
+
+import static java.util.concurrent.TimeUnit.MINUTES;
+
+@ThreadSafe
+@NotNullByDefault
+interface TorReachabilityMonitor {
+
+	/**
+	 * How long the Tor plugin needs to be continuously
+	 * {@link Plugin.State#ACTIVE active} before we assume our contacts can
+	 * reach our hidden service.
+	 */
+	long REACHABILITY_PERIOD_MS = MINUTES.toMillis(10);
+
+	/**
+	 * Starts the monitor.
+	 */
+	void start();
+
+	/**
+	 * Destroys the monitor.
+	 */
+	void destroy();
+
+	/**
+	 * Adds an observer that will be called when our Tor hidden service becomes
+	 * reachable. If our hidden service is already reachable, the observer is
+	 * called immediately.
+	 * <p>
+	 * Observers are removed after being called, or when the monitor is
+	 * {@link #destroy() destroyed}.
+	 */
+	void addOneShotObserver(TorReachabilityObserver o);
+
+	/**
+	 * Removes an observer that was added via
+	 * {@link #addOneShotObserver(TorReachabilityObserver)}.
+	 */
+	void removeObserver(TorReachabilityObserver o);
+
+	interface TorReachabilityObserver {
+
+		void onTorReachable();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/TorReachabilityMonitorImpl.java

@@ -0,0 +1,137 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.Cancellable;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.IoExecutor;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.Plugin;
+import org.briarproject.bramble.api.plugin.PluginManager;
+import org.briarproject.bramble.api.plugin.event.TransportActiveEvent;
+import org.briarproject.bramble.api.plugin.event.TransportInactiveEvent;
+import org.briarproject.bramble.api.system.TaskScheduler;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Executor;
+
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
+
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+import static org.briarproject.bramble.api.plugin.TorConstants.ID;
+
+@ThreadSafe
+@NotNullByDefault
+class TorReachabilityMonitorImpl
+		implements TorReachabilityMonitor, EventListener {
+
+	private final Executor ioExecutor;
+	private final TaskScheduler taskScheduler;
+	private final PluginManager pluginManager;
+	private final EventBus eventBus;
+	private final Object lock = new Object();
+
+	@GuardedBy("lock")
+	private boolean reachable = false, destroyed = false;
+
+	@GuardedBy("lock")
+	private final List<TorReachabilityObserver> observers = new ArrayList<>();
+
+	@GuardedBy("lock")
+	@Nullable
+	private Cancellable task = null;
+
+	@Inject
+	TorReachabilityMonitorImpl(
+			@IoExecutor Executor ioExecutor,
+			TaskScheduler taskScheduler,
+			PluginManager pluginManager,
+			EventBus eventBus) {
+		this.ioExecutor = ioExecutor;
+		this.taskScheduler = taskScheduler;
+		this.pluginManager = pluginManager;
+		this.eventBus = eventBus;
+	}
+
+	@Override
+	public void start() {
+		eventBus.addListener(this);
+		Plugin plugin = pluginManager.getPlugin(ID);
+		if (plugin != null && plugin.getState() == ACTIVE) onTorActive();
+	}
+
+	@Override
+	public void destroy() {
+		eventBus.removeListener(this);
+		synchronized (lock) {
+			destroyed = true;
+			if (task != null) task.cancel();
+			task = null;
+			observers.clear();
+		}
+	}
+
+	@Override
+	public void addOneShotObserver(TorReachabilityObserver o) {
+		boolean callNow = false;
+		synchronized (lock) {
+			if (destroyed) return;
+			if (reachable) callNow = true;
+			else observers.add(o);
+		}
+		if (callNow) o.onTorReachable();
+	}
+
+	@Override
+	public void removeObserver(TorReachabilityObserver o) {
+		synchronized (lock) {
+			if (destroyed) return;
+			observers.remove(o);
+		}
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (e instanceof TransportActiveEvent) {
+			TransportActiveEvent t = (TransportActiveEvent) e;
+			if (t.getTransportId().equals(ID)) onTorActive();
+		} else if (e instanceof TransportInactiveEvent) {
+			TransportInactiveEvent t = (TransportInactiveEvent) e;
+			if (t.getTransportId().equals(ID)) onTorInactive();
+		}
+	}
+
+	private void onTorActive() {
+		synchronized (lock) {
+			if (destroyed || task != null) return;
+			task = taskScheduler.schedule(this::onTorReachable, ioExecutor,
+					REACHABILITY_PERIOD_MS, MILLISECONDS);
+		}
+	}
+
+	private void onTorInactive() {
+		synchronized (lock) {
+			reachable = false;
+			if (task != null) task.cancel();
+			task = null;
+		}
+	}
+
+	@IoExecutor
+	private void onTorReachable() {
+		List<TorReachabilityObserver> observers;
+		synchronized (lock) {
+			if (destroyed) return;
+			reachable = true;
+			observers = new ArrayList<>(this.observers);
+			this.observers.clear();
+			task = null;
+		}
+		for (TorReachabilityObserver o : observers) o.onTorReachable();
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/UrlConverter.java

@@ -0,0 +1,17 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+/**
+ * An interface for converting an onion address to an HTTP URL, allowing the
+ * conversion to be customised for integration tests.
+ */
+@NotNullByDefault
+interface UrlConverter {
+
+   /**
+    * Converts a raw onion address, excluding the .onion suffix, into an
+    * HTTP URL.
+    */
+   String convertOnionToBaseUrl(String onion);
+}

bramble-core/src/main/java/org/briarproject/bramble/mailbox/UrlConverterImpl.java

@@ -0,0 +1,18 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.inject.Inject;
+
+@NotNullByDefault
+class UrlConverterImpl implements UrlConverter {
+
+	@Inject
+	UrlConverterImpl() {
+	}
+
+	@Override
+	public String convertOnionToBaseUrl(String onion) {
+		return "http://" + onion + ".onion";
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/plugin/PollerImpl.java

@@ -1,5 +1,6 @@
 package org.briarproject.bramble.plugin;
 
+import org.briarproject.bramble.api.Cancellable;
 import org.briarproject.bramble.api.Pair;
 import org.briarproject.bramble.api.connection.ConnectionManager;
 import org.briarproject.bramble.api.connection.ConnectionRegistry;
@@ -27,7 +28,6 @@ import org.briarproject.bramble.api.properties.TransportProperties;
 import org.briarproject.bramble.api.properties.TransportPropertyManager;
 import org.briarproject.bramble.api.system.Clock;
 import org.briarproject.bramble.api.system.TaskScheduler;
-import org.briarproject.bramble.api.system.TaskScheduler.Cancellable;
 import org.briarproject.bramble.api.system.Wakeful;
 import org.briarproject.bramble.api.system.WakefulIoExecutor;
 

bramble-core/src/main/java/org/briarproject/bramble/plugin/bluetooth/AbstractBluetoothPlugin.java

@@ -427,7 +427,13 @@ abstract class AbstractBluetoothPlugin<S, SS> implements BluetoothPlugin,
 		BdfList descriptor = new BdfList();
 		descriptor.add(TRANSPORT_ID_BLUETOOTH);
 		String address = getBluetoothAddress();
-		if (address != null) descriptor.add(macToBytes(address));
+		if (address != null) {
+			try {
+				descriptor.add(macToBytes(address));
+			} catch (FormatException e) {
+				throw new RuntimeException();
+			}
+		}
 		return new BluetoothKeyAgreementListener(descriptor, ss);
 	}
 

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/AbstractRemovableDrivePlugin.java

@@ -67,6 +67,7 @@ abstract class AbstractRemovableDrivePlugin implements SimplexPlugin {
 	public void start() {
 		callback.mergeLocalProperties(
 				new TransportProperties(singletonMap(PROP_SUPPORTED, "true")));
+		callback.pluginStateChanged(ACTIVE);
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/FilePlugin.java

@@ -11,6 +11,7 @@ import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.io.OutputStream;
 import java.util.logging.Logger;
 
 import static java.util.logging.Level.WARNING;
@@ -29,11 +30,6 @@ abstract class FilePlugin implements SimplexPlugin {
 	protected final PluginCallback callback;
 	protected final long maxLatency;
 
-	protected abstract void writerFinished(File f, boolean exception);
-
-	protected abstract void readerFinished(File f, boolean exception,
-			boolean recognised);
-
 	FilePlugin(PluginCallback callback, long maxLatency) {
 		this.callback = callback;
 		this.maxLatency = maxLatency;
@@ -50,9 +46,8 @@ abstract class FilePlugin implements SimplexPlugin {
 		String path = p.get(PROP_PATH);
 		if (isNullOrEmpty(path)) return null;
 		try {
-			File file = new File(path);
-			FileInputStream in = new FileInputStream(file);
-			return new FileTransportReader(file, in, this);
+			FileInputStream in = new FileInputStream(path);
+			return new TransportInputStreamReader(in);
 		} catch (IOException e) {
 			logException(LOG, WARNING, e);
 			return null;
@@ -70,8 +65,8 @@ abstract class FilePlugin implements SimplexPlugin {
 				LOG.info("Failed to create file");
 				return null;
 			}
-			FileOutputStream out = new FileOutputStream(file);
-			return new FileTransportWriter(file, out, this);
+			OutputStream out = new FileOutputStream(file);
+			return new TransportOutputStreamWriter(this, out);
 		} catch (IOException e) {
 			logException(LOG, WARNING, e);
 			return null;

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/FileTransportReader.java

@@ -1,39 +0,0 @@
-package org.briarproject.bramble.plugin.file;
-
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.plugin.TransportConnectionReader;
-
-import java.io.File;
-import java.io.InputStream;
-import java.util.logging.Logger;
-
-import static java.util.logging.Level.WARNING;
-import static org.briarproject.bramble.util.IoUtils.tryToClose;
-
-@NotNullByDefault
-class FileTransportReader implements TransportConnectionReader {
-
-	private static final Logger LOG =
-			Logger.getLogger(FileTransportReader.class.getName());
-
-	private final File file;
-	private final InputStream in;
-	private final FilePlugin plugin;
-
-	FileTransportReader(File file, InputStream in, FilePlugin plugin) {
-		this.file = file;
-		this.in = in;
-		this.plugin = plugin;
-	}
-
-	@Override
-	public InputStream getInputStream() {
-		return in;
-	}
-
-	@Override
-	public void dispose(boolean exception, boolean recognised) {
-		tryToClose(in, LOG, WARNING);
-		plugin.readerFinished(file, exception, recognised);
-	}
-}

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/FileTransportWriter.java

@@ -1,54 +0,0 @@
-package org.briarproject.bramble.plugin.file;
-
-import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
-import org.briarproject.bramble.api.plugin.TransportConnectionWriter;
-
-import java.io.File;
-import java.io.OutputStream;
-import java.util.logging.Logger;
-
-import static java.util.logging.Level.WARNING;
-import static org.briarproject.bramble.util.IoUtils.tryToClose;
-
-@NotNullByDefault
-class FileTransportWriter implements TransportConnectionWriter {
-
-	private static final Logger LOG =
-			Logger.getLogger(FileTransportWriter.class.getName());
-
-	private final File file;
-	private final OutputStream out;
-	private final FilePlugin plugin;
-
-	FileTransportWriter(File file, OutputStream out, FilePlugin plugin) {
-		this.file = file;
-		this.out = out;
-		this.plugin = plugin;
-	}
-
-	@Override
-	public long getMaxLatency() {
-		return plugin.getMaxLatency();
-	}
-
-	@Override
-	public int getMaxIdleTime() {
-		return plugin.getMaxIdleTime();
-	}
-
-	@Override
-	public boolean isLossyAndCheap() {
-		return plugin.isLossyAndCheap();
-	}
-
-	@Override
-	public OutputStream getOutputStream() {
-		return out;
-	}
-
-	@Override
-	public void dispose(boolean exception) {
-		tryToClose(out, LOG, WARNING);
-		plugin.writerFinished(file, exception);
-	}
-}

bramble-core/src/main/java/org/briarproject/bramble/plugin/file/MailboxPlugin.java

@@ -0,0 +1,73 @@
+package org.briarproject.bramble.plugin.file;
+
+import org.briarproject.bramble.api.Pair;
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+import org.briarproject.bramble.api.plugin.ConnectionHandler;
+import org.briarproject.bramble.api.plugin.PluginCallback;
+import org.briarproject.bramble.api.plugin.PluginException;
+import org.briarproject.bramble.api.plugin.TransportId;
+import org.briarproject.bramble.api.properties.TransportProperties;
+
+import java.util.Collection;
+
+import static org.briarproject.bramble.api.mailbox.MailboxConstants.ID;
+import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+
+@NotNullByDefault
+class MailboxPlugin extends FilePlugin {
+
+	MailboxPlugin(PluginCallback callback, long maxLatency) {
+		super(callback, maxLatency);
+	}
+
+	@Override
+	public TransportId getId() {
+		return ID;
+	}
+
+	@Override
+	public int getMaxIdleTime() {
+		// Unused for simplex transports
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void start() throws PluginException {
+		callback.pluginStateChanged(ACTIVE);
+	}
+
+	@Override
+	public void stop() throws PluginException {
+	}
+
+	@Override
+	public State getState() {
+		return ACTIVE;
+	}
+
+	@Override
+	public int getReasonsDisabled() {
+		return 0;
+	}
+
+	@Override
+	public boolean shouldPoll() {
+		return false;
+	}
+
+	@Override
+	public int getPollingInterval() {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public void poll(
+			Collection<Pair<TransportProperties, ConnectionHandler>> properties) {
+		throw new UnsupportedOperationException();
+	}
+
+	@Override
+	public boolean isLossyAndCheap() {
+		return false;
+	}
+}