Poll for connections in series, with a delay between attempts.

The delay reduces interference between Bluetooth and wifi.

bramble-android/src/main/java/org/briarproject/bramble/plugin/bluetooth/AndroidBluetoothPlugin.java

@@ -31,6 +31,7 @@ import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Executor;
 import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ScheduledExecutorService;
 import java.util.logging.Logger;
 
 import javax.annotation.Nullable;
@@ -78,11 +79,12 @@ class AndroidBluetoothPlugin extends BluetoothPlugin<BluetoothServerSocket> {
 
 	AndroidBluetoothPlugin(BluetoothConnectionLimiter connectionLimiter,
 			TimeoutMonitor timeoutMonitor, Executor ioExecutor,
-			SecureRandom secureRandom, AndroidExecutor androidExecutor,
+			ScheduledExecutorService scheduler, SecureRandom secureRandom,
-			Context appContext, Clock clock, Backoff backoff,
+			AndroidExecutor androidExecutor, Context appContext, Clock clock,
-			PluginCallback callback, int maxLatency, int maxIdleTime) {
+			Backoff backoff, PluginCallback callback, int maxLatency,
-		super(connectionLimiter, timeoutMonitor, ioExecutor, secureRandom,
+			int maxIdleTime) {
-				backoff, callback, maxLatency, maxIdleTime);
+		super(connectionLimiter, timeoutMonitor, ioExecutor, scheduler,
+				secureRandom, backoff, callback, maxLatency, maxIdleTime);
 		this.androidExecutor = androidExecutor;
 		this.appContext = appContext;
 		this.clock = clock;

bramble-android/src/main/java/org/briarproject/bramble/plugin/bluetooth/AndroidBluetoothPluginFactory.java

@@ -16,6 +16,7 @@ import org.briarproject.bramble.api.system.Clock;
 
 import java.security.SecureRandom;
 import java.util.concurrent.Executor;
+import java.util.concurrent.ScheduledExecutorService;
 
 import javax.annotation.concurrent.Immutable;
 
@@ -32,6 +33,7 @@ public class AndroidBluetoothPluginFactory implements DuplexPluginFactory {
 	private static final double BACKOFF_BASE = 1.2;
 
 	private final Executor ioExecutor;
+	private final ScheduledExecutorService scheduler;
 	private final AndroidExecutor androidExecutor;
 	private final Context appContext;
 	private final SecureRandom secureRandom;
@@ -41,10 +43,12 @@ public class AndroidBluetoothPluginFactory implements DuplexPluginFactory {
 	private final BackoffFactory backoffFactory;
 
 	public AndroidBluetoothPluginFactory(Executor ioExecutor,
-			AndroidExecutor androidExecutor, Context appContext,
+			ScheduledExecutorService scheduler, AndroidExecutor androidExecutor,
-			SecureRandom secureRandom, EventBus eventBus, Clock clock,
+			Context appContext, SecureRandom secureRandom, EventBus eventBus,
-			TimeoutMonitor timeoutMonitor, BackoffFactory backoffFactory) {
+			Clock clock, TimeoutMonitor timeoutMonitor,
+			BackoffFactory backoffFactory) {
 		this.ioExecutor = ioExecutor;
+		this.scheduler = scheduler;
 		this.androidExecutor = androidExecutor;
 		this.appContext = appContext;
 		this.secureRandom = secureRandom;
@@ -71,8 +75,8 @@ public class AndroidBluetoothPluginFactory implements DuplexPluginFactory {
 		Backoff backoff = backoffFactory.createBackoff(MIN_POLLING_INTERVAL,
 				MAX_POLLING_INTERVAL, BACKOFF_BASE);
 		AndroidBluetoothPlugin plugin = new AndroidBluetoothPlugin(
-				connectionLimiter, timeoutMonitor, ioExecutor, secureRandom,
+				connectionLimiter, timeoutMonitor, ioExecutor, scheduler,
-				androidExecutor, appContext, clock, backoff,
+				secureRandom, androidExecutor, appContext, clock, backoff,
 				callback, MAX_LATENCY, MAX_IDLE_TIME);
 		eventBus.addListener(plugin);
 		return plugin;

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

@@ -31,13 +31,17 @@ import org.briarproject.bramble.api.settings.event.SettingsUpdatedEvent;
 import java.io.IOException;
 import java.security.SecureRandom;
 import java.util.Collection;
+import java.util.LinkedList;
 import java.util.UUID;
 import java.util.concurrent.Executor;
+import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.logging.Logger;
 
 import javax.annotation.Nullable;
 
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static java.util.concurrent.TimeUnit.SECONDS;
 import static java.util.logging.Level.INFO;
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
@@ -60,10 +64,18 @@ abstract class BluetoothPlugin<SS> implements DuplexPlugin, EventListener {
 	private static final Logger LOG =
 			getLogger(BluetoothPlugin.class.getName());
 
+	/**
+	 * The delay between connection attempts when polling. This reduces
+	 * interference between Bluetooth and wifi.
+	 */
+	private static final long CONNECTION_ATTEMPT_INTERVAL_MS =
+			SECONDS.toMillis(5);
+
 	final BluetoothConnectionLimiter connectionLimiter;
 	final TimeoutMonitor timeoutMonitor;
 
 	private final Executor ioExecutor;
+	private final ScheduledExecutorService scheduler;
 	private final SecureRandom secureRandom;
 	private final Backoff backoff;
 	private final PluginCallback callback;
@@ -108,11 +120,13 @@ abstract class BluetoothPlugin<SS> implements DuplexPlugin, EventListener {
 
 	BluetoothPlugin(BluetoothConnectionLimiter connectionLimiter,
 			TimeoutMonitor timeoutMonitor, Executor ioExecutor,
-			SecureRandom secureRandom, Backoff backoff,
+			ScheduledExecutorService scheduler, SecureRandom secureRandom,
-			PluginCallback callback, int maxLatency, int maxIdleTime) {
+			Backoff backoff, PluginCallback callback, int maxLatency,
+			int maxIdleTime) {
 		this.connectionLimiter = connectionLimiter;
 		this.timeoutMonitor = timeoutMonitor;
 		this.ioExecutor = ioExecutor;
+		this.scheduler = scheduler;
 		this.secureRandom = secureRandom;
 		this.backoff = backoff;
 		this.callback = callback;
@@ -268,21 +282,31 @@ abstract class BluetoothPlugin<SS> implements DuplexPlugin, EventListener {
 			properties) {
 		if (!isRunning() || !shouldAllowContactConnections()) return;
 		backoff.increment();
-		for (Pair<TransportProperties, ConnectionHandler> p : properties) {
+		LinkedList<Pair<TransportProperties, ConnectionHandler>> connectable =
-			connect(p.getFirst(), p.getSecond());
+				new LinkedList<>();
+		for (Pair<TransportProperties, ConnectionHandler> pair : properties) {
+			TransportProperties p = pair.getFirst();
+			if (isNullOrEmpty(p.get(PROP_ADDRESS))) continue;
+			if (isNullOrEmpty(p.get(PROP_UUID))) continue;
+			connectable.add(pair);
 		}
+		if (!connectable.isEmpty()) poll(connectable);
 	}
 
-	private void connect(TransportProperties p, ConnectionHandler h) {
+	private void poll(LinkedList<Pair<TransportProperties, ConnectionHandler>>
-		String address = p.get(PROP_ADDRESS);
+			connectable) {
-		if (isNullOrEmpty(address)) return;
-		String uuid = p.get(PROP_UUID);
-		if (isNullOrEmpty(uuid)) return;
 		ioExecutor.execute(() -> {
-			DuplexTransportConnection d = createConnection(p);
+			if (!isRunning() || !shouldAllowContactConnections()) return;
+			Pair<TransportProperties, ConnectionHandler> pair =
+					connectable.removeFirst();
+			DuplexTransportConnection d = createConnection(pair.getFirst());
 			if (d != null) {
 				backoff.reset();
-				h.handleConnection(d);
+				pair.getSecond().handleConnection(d);
+			}
+			if (!connectable.isEmpty()) {
+				scheduler.schedule(() -> poll(connectable),
+						CONNECTION_ATTEMPT_INTERVAL_MS, MILLISECONDS);
 			}
 		});
 	}

bramble-java/src/main/java/org/briarproject/bramble/plugin/DesktopPluginModule.java

@@ -10,6 +10,7 @@ import org.briarproject.bramble.api.plugin.PluginConfig;
 import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
 import org.briarproject.bramble.api.plugin.simplex.SimplexPluginFactory;
 import org.briarproject.bramble.api.reliability.ReliabilityLayerFactory;
+import org.briarproject.bramble.api.system.Scheduler;
 import org.briarproject.bramble.plugin.bluetooth.JavaBluetoothPluginFactory;
 import org.briarproject.bramble.plugin.modem.ModemPluginFactory;
 import org.briarproject.bramble.plugin.tcp.LanTcpPluginFactory;
@@ -18,6 +19,7 @@ import org.briarproject.bramble.plugin.tcp.WanTcpPluginFactory;
 import java.security.SecureRandom;
 import java.util.Collection;
 import java.util.concurrent.Executor;
+import java.util.concurrent.ScheduledExecutorService;
 
 import dagger.Module;
 import dagger.Provides;
@@ -30,12 +32,14 @@ public class DesktopPluginModule extends PluginModule {
 
 	@Provides
 	PluginConfig getPluginConfig(@IoExecutor Executor ioExecutor,
+			@Scheduler ScheduledExecutorService scheduler,
 			SecureRandom random, BackoffFactory backoffFactory,
 			ReliabilityLayerFactory reliabilityFactory,
 			ShutdownManager shutdownManager, EventBus eventBus,
 			TimeoutMonitor timeoutMonitor) {
 		DuplexPluginFactory bluetooth = new JavaBluetoothPluginFactory(
-				ioExecutor, random, eventBus, timeoutMonitor, backoffFactory);
+				ioExecutor, scheduler, random, eventBus, timeoutMonitor,
+				backoffFactory);
 		DuplexPluginFactory modem = new ModemPluginFactory(ioExecutor,
 				reliabilityFactory);
 		DuplexPluginFactory lan = new LanTcpPluginFactory(ioExecutor,

bramble-java/src/main/java/org/briarproject/bramble/plugin/bluetooth/JavaBluetoothPlugin.java

@@ -10,6 +10,7 @@ import org.briarproject.bramble.api.plugin.duplex.DuplexTransportConnection;
 import java.io.IOException;
 import java.security.SecureRandom;
 import java.util.concurrent.Executor;
+import java.util.concurrent.ScheduledExecutorService;
 import java.util.logging.Logger;
 
 import javax.annotation.Nullable;
@@ -36,10 +37,11 @@ class JavaBluetoothPlugin extends BluetoothPlugin<StreamConnectionNotifier> {
 
 	JavaBluetoothPlugin(BluetoothConnectionLimiter connectionManager,
 			TimeoutMonitor timeoutMonitor, Executor ioExecutor,
-			SecureRandom secureRandom, Backoff backoff,
+			ScheduledExecutorService scheduler, SecureRandom secureRandom,
-			PluginCallback callback, int maxLatency, int maxIdleTime) {
+			Backoff backoff, PluginCallback callback, int maxLatency,
-		super(connectionManager, timeoutMonitor, ioExecutor, secureRandom,
+			int maxIdleTime) {
-				backoff, callback, maxLatency, maxIdleTime);
+		super(connectionManager, timeoutMonitor, ioExecutor, scheduler,
+				secureRandom, backoff, callback, maxLatency, maxIdleTime);
 	}
 
 	@Override

bramble-java/src/main/java/org/briarproject/bramble/plugin/bluetooth/JavaBluetoothPluginFactory.java

@@ -12,6 +12,7 @@ import org.briarproject.bramble.api.plugin.duplex.DuplexPluginFactory;
 
 import java.security.SecureRandom;
 import java.util.concurrent.Executor;
+import java.util.concurrent.ScheduledExecutorService;
 
 import javax.annotation.concurrent.Immutable;
 
@@ -28,15 +29,18 @@ public class JavaBluetoothPluginFactory implements DuplexPluginFactory {
 	private static final double BACKOFF_BASE = 1.2;
 
 	private final Executor ioExecutor;
+	private final ScheduledExecutorService scheduler;
 	private final SecureRandom secureRandom;
 	private final EventBus eventBus;
 	private final TimeoutMonitor timeoutMonitor;
 	private final BackoffFactory backoffFactory;
 
 	public JavaBluetoothPluginFactory(Executor ioExecutor,
-			SecureRandom secureRandom, EventBus eventBus,
+			ScheduledExecutorService scheduler, SecureRandom secureRandom,
-			TimeoutMonitor timeoutMonitor, BackoffFactory backoffFactory) {
+			EventBus eventBus, TimeoutMonitor timeoutMonitor,
+			BackoffFactory backoffFactory) {
 		this.ioExecutor = ioExecutor;
+		this.scheduler = scheduler;
 		this.secureRandom = secureRandom;
 		this.eventBus = eventBus;
 		this.timeoutMonitor = timeoutMonitor;
@@ -60,8 +64,8 @@ public class JavaBluetoothPluginFactory implements DuplexPluginFactory {
 		Backoff backoff = backoffFactory.createBackoff(MIN_POLLING_INTERVAL,
 				MAX_POLLING_INTERVAL, BACKOFF_BASE);
 		JavaBluetoothPlugin plugin = new JavaBluetoothPlugin(connectionLimiter,
-				timeoutMonitor, ioExecutor, secureRandom, backoff, callback,
+				timeoutMonitor, ioExecutor, scheduler, secureRandom, backoff,
-				MAX_LATENCY, MAX_IDLE_TIME);
+				callback, MAX_LATENCY, MAX_IDLE_TIME);
 		eventBus.addListener(plugin);
 		return plugin;
 	}

briar-android/src/main/java/org/briarproject/briar/android/AppModule.java

@@ -127,8 +127,8 @@ public class AppModule {
 			TimeoutMonitor timeoutMonitor) {
 		Context appContext = app.getApplicationContext();
 		DuplexPluginFactory bluetooth = new AndroidBluetoothPluginFactory(
-				ioExecutor, androidExecutor, appContext, random, eventBus,
+				ioExecutor, scheduler, androidExecutor, appContext, random,
-				clock, timeoutMonitor, backoffFactory);
+				eventBus, clock, timeoutMonitor, backoffFactory);
 		DuplexPluginFactory tor = new AndroidTorPluginFactory(ioExecutor,
 				scheduler, appContext, networkManager, locationUtils, eventBus,
 				torSocketFactory, backoffFactory, resourceProvider,