Add mailbox client manager.

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

@@ -1,19 +1,22 @@
 package org.briarproject.bramble.api.mailbox;
 
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
 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 {
+@NotNullByDefault
+public 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(
+	public static int getHighestCommonMajorVersion(
 			List<MailboxVersion> client, List<MailboxVersion> server) {
 		TreeSet<Integer> clientVersions = new TreeSet<>();
 		for (MailboxVersion version : client) {

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

@@ -0,0 +1,622 @@
+package org.briarproject.bramble.mailbox;
+
+import org.briarproject.bramble.api.contact.Contact;
+import org.briarproject.bramble.api.contact.ContactId;
+import org.briarproject.bramble.api.contact.ContactManager;
+import org.briarproject.bramble.api.contact.event.ContactRemovedEvent;
+import org.briarproject.bramble.api.db.DatabaseExecutor;
+import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.TransactionManager;
+import org.briarproject.bramble.api.event.Event;
+import org.briarproject.bramble.api.event.EventExecutor;
+import org.briarproject.bramble.api.event.EventListener;
+import org.briarproject.bramble.api.lifecycle.Service;
+import org.briarproject.bramble.api.lifecycle.ServiceException;
+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.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.MailboxPairedEvent;
+import org.briarproject.bramble.api.mailbox.event.MailboxUnpairedEvent;
+import org.briarproject.bramble.api.mailbox.event.MailboxUpdateSentEvent;
+import org.briarproject.bramble.api.mailbox.event.OwnMailboxConnectionStatusEvent;
+import org.briarproject.bramble.api.mailbox.event.RemoteMailboxUpdateEvent;
+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 java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Executor;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.logging.Logger;
+
+import javax.annotation.Nullable;
+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.mailbox.MailboxConstants.CLIENT_SUPPORTS;
+import static org.briarproject.bramble.api.mailbox.MailboxHelper.getHighestCommonMajorVersion;
+import static org.briarproject.bramble.api.nullsafety.NullSafety.requireNonNull;
+import static org.briarproject.bramble.api.plugin.Plugin.State.ACTIVE;
+import static org.briarproject.bramble.api.plugin.TorConstants.ID;
+import static org.briarproject.bramble.util.LogUtils.logException;
+
+@ThreadSafe
+@NotNullByDefault
+class MailboxClientManager implements Service, EventListener {
+
+	private static final Logger LOG =
+			getLogger(MailboxClientManager.class.getName());
+
+	private final Executor eventExecutor, dbExecutor;
+	private final TransactionManager db;
+	private final ContactManager contactManager;
+	private final PluginManager pluginManager;
+	private final MailboxSettingsManager mailboxSettingsManager;
+	private final MailboxUpdateManager mailboxUpdateManager;
+	private final MailboxClientFactory mailboxClientFactory;
+	private final TorReachabilityMonitor reachabilityMonitor;
+	private final AtomicBoolean used = new AtomicBoolean(false);
+
+	// All mutable state must only be accessed on the worker thread
+	private final Map<ContactId, Updates> contactUpdates = new HashMap<>();
+	private final Map<ContactId, MailboxClient> contactClients =
+			new HashMap<>();
+	@Nullable
+	private MailboxProperties ownProperties = null;
+	@Nullable
+	private MailboxClient ownClient = null;
+	private boolean online = false, handleEvents = false;
+
+	@Inject
+	MailboxClientManager(@EventExecutor Executor eventExecutor,
+			@DatabaseExecutor Executor dbExecutor,
+			TransactionManager db,
+			ContactManager contactManager,
+			PluginManager pluginManager,
+			MailboxSettingsManager mailboxSettingsManager,
+			MailboxUpdateManager mailboxUpdateManager,
+			MailboxClientFactory mailboxClientFactory,
+			TorReachabilityMonitor reachabilityMonitor) {
+		this.eventExecutor = eventExecutor;
+		this.dbExecutor = dbExecutor;
+		this.db = db;
+		this.contactManager = contactManager;
+		this.pluginManager = pluginManager;
+		this.mailboxSettingsManager = mailboxSettingsManager;
+		this.mailboxUpdateManager = mailboxUpdateManager;
+		this.mailboxClientFactory = mailboxClientFactory;
+		this.reachabilityMonitor = reachabilityMonitor;
+	}
+
+	@Override
+	public void startService() throws ServiceException {
+		if (used.getAndSet(true)) throw new IllegalStateException();
+		reachabilityMonitor.start();
+		dbExecutor.execute(this::loadMailboxProperties);
+	}
+
+	@DatabaseExecutor
+	private void loadMailboxProperties() {
+		LOG.info("Loading mailbox properties");
+		try {
+			db.transaction(true, txn -> {
+				Map<ContactId, Updates> updates = new HashMap<>();
+				for (Contact c : contactManager.getContacts(txn)) {
+					MailboxUpdate local = mailboxUpdateManager
+							.getLocalUpdate(txn, c.getId());
+					MailboxUpdate remote = mailboxUpdateManager
+							.getRemoteUpdate(txn, c.getId());
+					updates.put(c.getId(), new Updates(local, remote));
+				}
+				MailboxProperties ownProps =
+						mailboxSettingsManager.getOwnMailboxProperties(txn);
+				// Use a commit action so the state in memory remains
+				// consistent with the state in the DB
+				txn.attach(() -> initialiseState(updates, ownProps));
+			});
+		} catch (DbException e) {
+			logException(LOG, WARNING, e);
+		}
+	}
+
+	@EventExecutor
+	private void initialiseState(Map<ContactId, Updates> updates,
+			@Nullable MailboxProperties ownProps) {
+		contactUpdates.putAll(updates);
+		ownProperties = ownProps;
+		Plugin tor = pluginManager.getPlugin(ID);
+		if (tor != null && tor.getState() == ACTIVE) {
+			LOG.info("Online");
+			online = true;
+			createClients();
+		}
+		handleEvents = true;
+	}
+
+	@EventExecutor
+	private void createClients() {
+		LOG.info("Creating clients");
+		for (Entry<ContactId, Updates> e : contactUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = e.getValue();
+			if (isContactMailboxUsable(u.remote)) {
+				// Create and start a client for the contact's mailbox
+				MailboxClient contactClient = createAndStartClient(c);
+				// Assign the contact to the contact's mailbox for upload
+				assignContactToContactMailboxForUpload(c, contactClient, u);
+				if (!isOwnMailboxUsable(ownProperties, u.remote)) {
+					// We don't have a usable mailbox, so assign the contact to
+					// the contact's mailbox for download too
+					assignContactToContactMailboxForDownload(c,
+							contactClient, u);
+				}
+			}
+		}
+		if (ownProperties == null) return;
+		if (!isOwnMailboxUsable(ownProperties)) {
+			LOG.warning("We have a mailbox but we can't use it");
+			return;
+		}
+		// Create and start a client for our mailbox
+		createAndStartClientForOwnMailbox();
+		// Assign contacts to our mailbox for upload/download
+		for (Entry<ContactId, Updates> e : contactUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = e.getValue();
+			if (isOwnMailboxUsable(ownProperties, u.remote)) {
+				// Assign the contact to our mailbox for download
+				assignContactToOwnMailboxForDownload(c, u);
+				if (!isContactMailboxUsable(u.remote)) {
+					// The contact doesn't have a usable mailbox, so assign
+					// the contact to our mailbox for upload too
+					assignContactToOwnMailboxForUpload(c, u);
+				}
+			}
+		}
+	}
+
+	@Override
+	public void stopService() throws ServiceException {
+		CountDownLatch latch = new CountDownLatch(1);
+		eventExecutor.execute(() -> {
+			handleEvents = false;
+			destroyClients();
+			latch.countDown();
+		});
+		reachabilityMonitor.destroy();
+		try {
+			latch.await();
+		} catch (InterruptedException e) {
+			throw new ServiceException(e);
+		}
+	}
+
+	@EventExecutor
+	private void destroyClients() {
+		LOG.info("Destroying clients");
+		for (MailboxClient client : contactClients.values()) {
+			client.destroy();
+		}
+		contactClients.clear();
+		destroyOwnClient();
+	}
+
+	@EventExecutor
+	private void destroyOwnClient() {
+		if (ownClient != null) {
+			ownClient.destroy();
+			ownClient = null;
+		}
+	}
+
+	@Override
+	public void eventOccurred(Event e) {
+		if (!handleEvents) return;
+		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();
+		} else if (e instanceof MailboxPairedEvent) {
+			LOG.info("Mailbox paired");
+			MailboxPairedEvent m = (MailboxPairedEvent) e;
+			onMailboxPaired(m.getProperties(), m.getLocalUpdates());
+		} else if (e instanceof MailboxUnpairedEvent) {
+			LOG.info("Mailbox unpaired");
+			MailboxUnpairedEvent m = (MailboxUnpairedEvent) e;
+			onMailboxUnpaired(m.getLocalUpdates());
+		} else if (e instanceof MailboxUpdateSentEvent) {
+			LOG.info("Contact added");
+			MailboxUpdateSentEvent m = (MailboxUpdateSentEvent) e;
+			onContactAdded(m.getContactId(), m.getMailboxUpdate());
+		} else if (e instanceof ContactRemovedEvent) {
+			LOG.info("Contact removed");
+			ContactRemovedEvent c = (ContactRemovedEvent) e;
+			onContactRemoved(c.getContactId());
+		} else if (e instanceof RemoteMailboxUpdateEvent) {
+			LOG.info("Remote mailbox update");
+			RemoteMailboxUpdateEvent r = (RemoteMailboxUpdateEvent) e;
+			onRemoteMailboxUpdate(r.getContact(), r.getMailboxUpdate());
+		} else if (e instanceof OwnMailboxConnectionStatusEvent) {
+			OwnMailboxConnectionStatusEvent o =
+					(OwnMailboxConnectionStatusEvent) e;
+			onOwnMailboxConnectionStatusChanged(o.getStatus());
+		}
+	}
+
+	@EventExecutor
+	private void onTorActive() {
+		// If we checked the plugin at startup concurrently with the plugin
+		// becoming active then `online` may already be true when we receive
+		// the first TransportActiveEvent, in which case ignore it
+		if (online) return;
+		LOG.info("Online");
+		online = true;
+		createClients();
+	}
+
+	@EventExecutor
+	private void onTorInactive() {
+		// If we checked the plugin at startup concurrently with the plugin
+		// becoming inactive then `online` may already be false when we
+		// receive the first TransportInactiveEvent, in which case ignore it
+		if (!online) return;
+		LOG.info("Offline");
+		online = false;
+		destroyClients();
+	}
+
+	@EventExecutor
+	private void onMailboxPaired(MailboxProperties ownProps,
+			Map<ContactId, MailboxUpdateWithMailbox> localUpdates) {
+		for (Entry<ContactId, MailboxUpdateWithMailbox> e :
+				localUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = contactUpdates.get(c);
+			contactUpdates.put(c, new Updates(e.getValue(), u.remote));
+		}
+		ownProperties = ownProps;
+		if (!online) return;
+		if (!isOwnMailboxUsable(ownProperties)) {
+			LOG.warning("We have a mailbox but we can't use it");
+			return;
+		}
+		// Create and start a client for our mailbox
+		createAndStartClientForOwnMailbox();
+		// Assign contacts to our mailbox for upload/download
+		for (Entry<ContactId, Updates> e : contactUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = e.getValue();
+			boolean isOwnMailboxUsable =
+					isOwnMailboxUsable(ownProperties, u.remote);
+			if (isContactMailboxUsable(u.remote)) {
+				// The contact has a usable mailbox, so the contact should
+				// currently be assigned to the contact's mailbox for upload
+				// and download
+				if (isOwnMailboxUsable) {
+					// Reassign the contact to our mailbox for download
+					MailboxClient contactClient =
+							requireNonNull(contactClients.get(c));
+					contactClient.deassignContactForDownload(c);
+					assignContactToOwnMailboxForDownload(c, u);
+				}
+				// Else the contact remains assigned to the contact's mailbox
+				// for download
+			} else if (isOwnMailboxUsable) {
+				// The contact doesn't have a usable mailbox, so assign the
+				// contact to our mailbox for upload and download
+				assignContactToOwnMailboxForUpload(c, u);
+				assignContactToOwnMailboxForDownload(c, u);
+			}
+			// Else the contact doesn't have a usable mailbox and neither do we,
+			// so the contact remains unassigned for upload and download
+		}
+	}
+
+	@EventExecutor
+	private void onMailboxUnpaired(Map<ContactId, MailboxUpdate> localUpdates) {
+		for (Entry<ContactId, MailboxUpdate> e : localUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates updates = contactUpdates.get(c);
+			contactUpdates.put(c, new Updates(e.getValue(), updates.remote));
+		}
+		MailboxProperties oldOwnProperties = ownProperties;
+		ownProperties = null;
+		if (!online) return;
+		// Destroy the client for our own mailbox, if any
+		destroyOwnClient();
+		// Reassign contacts to their own mailboxes for download where possible
+		for (Entry<ContactId, Updates> e : contactUpdates.entrySet()) {
+			ContactId c = e.getKey();
+			Updates u = e.getValue();
+			if (isContactMailboxUsable(u.remote) &&
+					isOwnMailboxUsable(oldOwnProperties, u.remote)) {
+				// The contact should currently be assigned to our mailbox
+				// for download. Reassign the contact to the contact's
+				// mailbox for download
+				MailboxClient contactClient =
+						requireNonNull(contactClients.get(c));
+				assignContactToContactMailboxForDownload(c, contactClient, u);
+			}
+		}
+	}
+
+	@EventExecutor
+	private void onContactAdded(ContactId c, MailboxUpdate u) {
+		Updates old = contactUpdates.put(c, new Updates(u, null));
+		if (old != null) throw new IllegalStateException();
+		// We haven't yet received an update from the newly added contact,
+		// so at this stage we don't need to assign the contact to any
+		// mailboxes for upload or download
+	}
+
+	@EventExecutor
+	private void onContactRemoved(ContactId c) {
+		Updates updates = requireNonNull(contactUpdates.remove(c));
+		if (!online) return;
+		// Destroy the client for the contact's mailbox, if any
+		MailboxClient client = contactClients.remove(c);
+		if (client != null) client.destroy();
+		// If we have a mailbox and the contact is assigned to it for upload
+		// and/or download, deassign the contact
+		if (ownProperties == null) return;
+		if (isOwnMailboxUsable(ownProperties, updates.remote)) {
+			// We have a usable mailbox, so the contact should currently be
+			// assigned to our mailbox for download. Deassign the contact from
+			// our mailbox for download
+			requireNonNull(ownClient).deassignContactForDownload(c);
+			if (!isContactMailboxUsable(updates.remote)) {
+				// The contact doesn't have a usable mailbox, so the contact
+				// should currently be assigned to our mailbox for upload.
+				// Deassign the contact from our mailbox for upload
+				requireNonNull(ownClient).deassignContactForUpload(c);
+			}
+		}
+	}
+
+	@EventExecutor
+	private void onRemoteMailboxUpdate(ContactId c, MailboxUpdate remote) {
+		Updates old = contactUpdates.get(c);
+		MailboxUpdate oldRemote = old.remote;
+		Updates u = new Updates(old.local, remote);
+		contactUpdates.put(c, u);
+		if (!online) return;
+		// What may have changed?
+		// * Contact's mailbox may be usable now, was unusable before
+		// * Contact's mailbox may be unusable now, was usable before
+		// * Contact's mailbox may have been replaced
+		// * Contact's mailbox may have changed its API versions
+		// * Contact may be able to use our mailbox now, was unable before
+		// * Contact may be unable to use our mailbox now, was able before
+		boolean wasContactMailboxUsable = isContactMailboxUsable(oldRemote);
+		boolean isContactMailboxUsable = isContactMailboxUsable(remote);
+		boolean wasOwnMailboxUsable =
+				isOwnMailboxUsable(ownProperties, oldRemote);
+		boolean isOwnMailboxUsable = isOwnMailboxUsable(ownProperties, remote);
+
+		// Create/destroy/replace the client for the contact's mailbox if needed
+		MailboxClient contactClient = null;
+		boolean clientReplaced = false;
+		if (isContactMailboxUsable) {
+			if (wasContactMailboxUsable) {
+				MailboxProperties oldProps = getMailboxProperties(oldRemote);
+				MailboxProperties newProps = getMailboxProperties(remote);
+				if (oldProps.equals(newProps)) {
+					// The contact previously had a usable mailbox, now has
+					// a usable mailbox, it's the same mailbox, and its API
+					// versions haven't changed. Keep using the existing client
+					contactClient = requireNonNull(contactClients.get(c));
+				} else {
+					// The contact previously had a usable mailbox and now has
+					// a usable mailbox, but either it's a new mailbox or its
+					// API versions have changed. Replace the client
+					requireNonNull(contactClients.remove(c)).destroy();
+					contactClient = createAndStartClient(c);
+					clientReplaced = true;
+				}
+			} else {
+				// The client didn't previously have a usable mailbox but now
+				// has one. Create and start a client
+				contactClient = createAndStartClient(c);
+			}
+		} else if (wasContactMailboxUsable) {
+			// The client previously had a usable mailbox but no longer does.
+			// Destroy the existing client
+			requireNonNull(contactClients.remove(c)).destroy();
+		}
+
+		boolean wasAssignedToOwnMailboxForUpload =
+				wasOwnMailboxUsable && !wasContactMailboxUsable;
+		boolean willBeAssignedToOwnMailboxForUpload =
+				isOwnMailboxUsable && !isContactMailboxUsable;
+
+		boolean wasAssignedToContactMailboxForDownload =
+				!wasOwnMailboxUsable && wasContactMailboxUsable;
+		boolean willBeAssignedToContactMailboxForDownload =
+				!isOwnMailboxUsable && isContactMailboxUsable;
+
+		// Deassign the contact for upload/download if needed
+		if (wasAssignedToOwnMailboxForUpload &&
+				!willBeAssignedToOwnMailboxForUpload) {
+			requireNonNull(ownClient).deassignContactForUpload(c);
+		}
+		if (wasOwnMailboxUsable && !isOwnMailboxUsable) {
+			requireNonNull(ownClient).deassignContactForDownload(c);
+		}
+		// If the client for the contact's mailbox was replaced or destroyed
+		// above then we don't need to deassign the contact for download
+		if (wasAssignedToContactMailboxForDownload &&
+				!willBeAssignedToContactMailboxForDownload &&
+				!clientReplaced && isContactMailboxUsable) {
+			requireNonNull(contactClient).deassignContactForDownload(c);
+		}
+		// We never need to deassign the contact from the contact's mailbox for
+		// upload: this would only be needed if the contact's mailbox were no
+		// longer usable, in which case the client would already have been
+		// destroyed above. Thanks to the linter for spotting this
+
+		// Assign the contact for upload/download if needed
+		if (!wasAssignedToOwnMailboxForUpload &&
+				willBeAssignedToOwnMailboxForUpload) {
+			assignContactToOwnMailboxForUpload(c, u);
+		}
+		if (!wasOwnMailboxUsable && isOwnMailboxUsable) {
+			assignContactToOwnMailboxForDownload(c, u);
+		}
+		if ((!wasContactMailboxUsable || clientReplaced) &&
+				isContactMailboxUsable) {
+			assignContactToContactMailboxForUpload(c, contactClient, u);
+		}
+		if ((!wasAssignedToContactMailboxForDownload || clientReplaced) &&
+				willBeAssignedToContactMailboxForDownload) {
+			assignContactToContactMailboxForDownload(c, contactClient, u);
+		}
+	}
+
+	@EventExecutor
+	private void onOwnMailboxConnectionStatusChanged(MailboxStatus status) {
+		if (!online || ownProperties == null) return;
+		List<MailboxVersion> oldServerSupports =
+				ownProperties.getServerSupports();
+		List<MailboxVersion> newServerSupports = status.getServerSupports();
+		if (!oldServerSupports.equals(newServerSupports)) {
+			LOG.info("Our mailbox's supported API versions have changed");
+			// This potentially affects every assignment of contacts to
+			// mailboxes for upload and download, so just rebuild the clients
+			// and assignments from scratch
+			destroyClients();
+			createClients();
+		}
+	}
+
+	@EventExecutor
+	private void createAndStartClientForOwnMailbox() {
+		if (ownClient != null) throw new IllegalStateException();
+		ownClient = mailboxClientFactory.createOwnMailboxClient(
+				reachabilityMonitor, requireNonNull(ownProperties));
+		ownClient.start();
+	}
+
+	@EventExecutor
+	private MailboxClient createAndStartClient(ContactId c) {
+		MailboxClient client = mailboxClientFactory
+				.createContactMailboxClient(reachabilityMonitor);
+		MailboxClient old = contactClients.put(c, client);
+		if (old != null) throw new IllegalStateException();
+		client.start();
+		return client;
+	}
+
+	@EventExecutor
+	private void assignContactToOwnMailboxForDownload(ContactId c, Updates u) {
+		MailboxProperties localProps = getMailboxProperties(u.local);
+		requireNonNull(ownClient).assignContactForDownload(c,
+				requireNonNull(ownProperties),
+				requireNonNull(localProps.getOutboxId()));
+	}
+
+	@EventExecutor
+	private void assignContactToOwnMailboxForUpload(ContactId c, Updates u) {
+		MailboxProperties localProps = getMailboxProperties(u.local);
+		requireNonNull(ownClient).assignContactForUpload(c,
+				requireNonNull(ownProperties),
+				requireNonNull(localProps.getInboxId()));
+	}
+
+	@EventExecutor
+	private void assignContactToContactMailboxForDownload(ContactId c,
+			MailboxClient contactClient, Updates u) {
+		MailboxProperties remoteProps =
+				getMailboxProperties(requireNonNull(u.remote));
+		contactClient.assignContactForDownload(c, remoteProps,
+				requireNonNull(remoteProps.getInboxId()));
+	}
+
+	@EventExecutor
+	private void assignContactToContactMailboxForUpload(ContactId c,
+			MailboxClient contactClient, Updates u) {
+		MailboxProperties remoteProps =
+				getMailboxProperties(requireNonNull(u.remote));
+		contactClient.assignContactForUpload(c, remoteProps,
+				requireNonNull(remoteProps.getOutboxId()));
+	}
+
+	private MailboxProperties getMailboxProperties(MailboxUpdate update) {
+		if (!(update instanceof MailboxUpdateWithMailbox)) {
+			throw new IllegalArgumentException();
+		}
+		MailboxUpdateWithMailbox mailbox = (MailboxUpdateWithMailbox) update;
+		return mailbox.getMailboxProperties();
+	}
+
+	/**
+	 * Returns true if we can use our own mailbox to communicate with the
+	 * contact that sent the given update.
+	 */
+	private boolean isOwnMailboxUsable(
+			@Nullable MailboxProperties ownProperties,
+			@Nullable MailboxUpdate remote) {
+		if (ownProperties == null || remote == null) return false;
+		return isMailboxUsable(remote.getClientSupports(),
+				ownProperties.getServerSupports());
+	}
+
+	/**
+	 * Returns true if we can use the contact's mailbox to communicate with
+	 * the contact that sent the given update.
+	 */
+	private boolean isContactMailboxUsable(@Nullable MailboxUpdate remote) {
+		if (remote instanceof MailboxUpdateWithMailbox) {
+			MailboxUpdateWithMailbox remoteMailbox =
+					(MailboxUpdateWithMailbox) remote;
+			return isMailboxUsable(remoteMailbox.getClientSupports(),
+					remoteMailbox.getMailboxProperties().getServerSupports());
+		}
+		return false;
+	}
+
+	/**
+	 * Returns true if we can communicate with a contact that has the given
+	 * client-supported API versions via a mailbox with the given
+	 * server-supported API versions.
+	 */
+	private boolean isMailboxUsable(List<MailboxVersion> contactClient,
+			List<MailboxVersion> server) {
+		return getHighestCommonMajorVersion(contactClient, server) >= 0
+				&& getHighestCommonMajorVersion(CLIENT_SUPPORTS, server) >= 0;
+	}
+
+	/**
+	 * Returns true if we're compatible with our own mailbox.
+	 */
+	@SuppressWarnings("BooleanMethodIsAlwaysInverted")
+	private boolean isOwnMailboxUsable(MailboxProperties ownProperties) {
+		return getHighestCommonMajorVersion(CLIENT_SUPPORTS,
+				ownProperties.getServerSupports()) >= 0;
+	}
+
+	private static class Updates {
+
+		private final MailboxUpdate local;
+		@Nullable
+		private final MailboxUpdate remote;
+
+		private Updates(MailboxUpdate local, @Nullable MailboxUpdate remote) {
+			this.local = local;
+			this.remote = remote;
+		}
+	}
+}