Broadcast events asynchronously.

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

@@ -1,10 +1,13 @@
 package org.briarproject.bramble.system;
 
+import org.briarproject.bramble.api.event.EventExecutor;
 import org.briarproject.bramble.api.system.AndroidExecutor;
 import org.briarproject.bramble.api.system.LocationUtils;
 import org.briarproject.bramble.api.system.ResourceProvider;
 import org.briarproject.bramble.api.system.SecureRandomProvider;
 
+import java.util.concurrent.Executor;
+
 import javax.inject.Singleton;
 
 import dagger.Module;
@@ -32,6 +35,13 @@ public class AndroidSystemModule {
 		return androidExecutor;
 	}
 
+	@Provides
+	@Singleton
+	@EventExecutor
+	Executor provideEventExecutor(AndroidExecutor androidExecutor) {
+		return androidExecutor::runOnUiThread;
+	}
+
 	@Provides
 	@Singleton
 	ResourceProvider provideResourceProvider(AndroidResourceProvider provider) {

bramble-api/src/main/java/org/briarproject/bramble/api/event/EventBus.java

@@ -16,7 +16,8 @@ public interface EventBus {
 	void removeListener(EventListener l);
 
 	/**
-	 * Notifies all listeners of an event.
+	 * Asynchronously notifies all listeners of an event. Listeners are
+	 * notified on the {@link EventExecutor}.
 	 */
 	void broadcast(Event e);
 }

bramble-api/src/main/java/org/briarproject/bramble/api/event/EventExecutor.java

@@ -0,0 +1,25 @@
+package org.briarproject.bramble.api.event;
+
+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 executor for broadcasting events. Also used for
+ * annotating methods that should run on the event executor.
+ * <p>
+ * The contract of this executor is that tasks are run in the order they're
+ * submitted, tasks are not run concurrently, and submitting a task will never
+ * block. Tasks must not block. Tasks submitted during shutdown are discarded.
+ */
+@Qualifier
+@Target({FIELD, METHOD, PARAMETER})
+@Retention(RUNTIME)
+public @interface EventExecutor {
+}

bramble-api/src/main/java/org/briarproject/bramble/api/event/EventListener.java

@@ -12,5 +12,6 @@ public interface EventListener {
 	 * Called when an event is broadcast. Implementations of this method must
 	 * not block.
 	 */
+	@EventExecutor
 	void eventOccurred(Event e);
 }

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

@@ -160,13 +160,15 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	public void endTransaction(Transaction transaction) {
 		try {
 			T txn = txnClass.cast(transaction.unbox());
-			if (!transaction.isCommitted()) db.abortTransaction(txn);
+			if (transaction.isCommitted()) {
+				for (Event e : transaction.getEvents()) eventBus.broadcast(e);
+			} else {
+				db.abortTransaction(txn);
+			}
 		} finally {
 			if (transaction.isReadOnly()) lock.readLock().unlock();
 			else lock.writeLock().unlock();
 		}
-		if (transaction.isCommitted())
-			for (Event e : transaction.getEvents()) eventBus.broadcast(e);
 	}
 
 	@Override

bramble-core/src/main/java/org/briarproject/bramble/event/DefaultEventExecutorModule.java

@@ -0,0 +1,32 @@
+package org.briarproject.bramble.event;
+
+import org.briarproject.bramble.api.event.EventExecutor;
+
+import java.util.concurrent.Executor;
+
+import javax.inject.Singleton;
+
+import dagger.Module;
+import dagger.Provides;
+
+import static java.util.concurrent.Executors.newSingleThreadExecutor;
+
+/**
+ * Default implementation of {@link EventExecutor} that uses a dedicated thread
+ * to notify listeners of events. Applications may prefer to supply an
+ * implementation that uses an existing thread, such as the UI thread.
+ */
+@Module
+public class DefaultEventExecutorModule {
+
+	@Provides
+	@Singleton
+	@EventExecutor
+	Executor provideEventExecutor() {
+		return newSingleThreadExecutor(r -> {
+			Thread t = new Thread(r);
+			t.setDaemon(true);
+			return t;
+		});
+	}
+}

bramble-core/src/main/java/org/briarproject/bramble/event/EventBusImpl.java

@@ -7,6 +7,7 @@ import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
 import java.util.Collection;
 import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.Executor;
 
 import javax.annotation.concurrent.ThreadSafe;
 
@@ -16,6 +17,11 @@ class EventBusImpl implements EventBus {
 
 	private final Collection<EventListener> listeners =
 			new CopyOnWriteArrayList<>();
+	private final Executor eventExecutor;
+
+	EventBusImpl(Executor eventExecutor) {
+		this.eventExecutor = eventExecutor;
+	}
 
 	@Override
 	public void addListener(EventListener l) {
@@ -29,6 +35,8 @@ class EventBusImpl implements EventBus {
 
 	@Override
 	public void broadcast(Event e) {
-		for (EventListener l : listeners) l.eventOccurred(e);
+		eventExecutor.execute(() -> {
+			for (EventListener l : listeners) l.eventOccurred(e);
+		});
 	}
 }

bramble-core/src/main/java/org/briarproject/bramble/event/EventModule.java

@@ -1,6 +1,9 @@
 package org.briarproject.bramble.event;
 
 import org.briarproject.bramble.api.event.EventBus;
+import org.briarproject.bramble.api.event.EventExecutor;
+
+import java.util.concurrent.Executor;
 
 import javax.inject.Singleton;
 
@@ -12,7 +15,7 @@ public class EventModule {
 
 	@Provides
 	@Singleton
-	EventBus provideEventBus() {
+	EventBus provideEventBus(@EventExecutor Executor eventExecutor) {
-		return new EventBusImpl();
+		return new EventBusImpl(eventExecutor);
 	}
 }

bramble-java/src/test/java/org/briarproject/bramble/test/BrambleJavaIntegrationTestComponent.java

@@ -2,6 +2,7 @@ package org.briarproject.bramble.test;
 
 import org.briarproject.bramble.BrambleJavaModule;
 import org.briarproject.bramble.battery.DefaultBatteryManagerModule;
+import org.briarproject.bramble.event.DefaultEventExecutorModule;
 import org.briarproject.bramble.event.EventModule;
 import org.briarproject.bramble.plugin.PluginModule;
 import org.briarproject.bramble.plugin.tor.BridgeTest;
@@ -17,8 +18,9 @@ import dagger.Component;
 		BrambleJavaModule.class,
 		TestLifecycleModule.class,
 		DefaultBatteryManagerModule.class,
-		PluginModule.class,  // needed for BackoffFactory
+		DefaultEventExecutorModule.class,
 		EventModule.class,
+		PluginModule.class,  // needed for BackoffFactory
 		SystemModule.class,
 })
 public interface BrambleJavaIntegrationTestComponent {

briar-core/src/test/java/org/briarproject/briar/feed/FeedManagerIntegrationTestComponent.java

@@ -8,6 +8,7 @@ import org.briarproject.bramble.crypto.CryptoExecutorModule;
 import org.briarproject.bramble.crypto.CryptoModule;
 import org.briarproject.bramble.data.DataModule;
 import org.briarproject.bramble.db.DatabaseModule;
+import org.briarproject.bramble.event.DefaultEventExecutorModule;
 import org.briarproject.bramble.event.EventModule;
 import org.briarproject.bramble.identity.IdentityModule;
 import org.briarproject.bramble.lifecycle.LifecycleModule;
@@ -46,6 +47,7 @@ import dagger.Component;
 		FeedModule.class,
 		DataModule.class,
 		DatabaseModule.class,
+		DefaultEventExecutorModule.class,
 		EventModule.class,
 		IdentityModule.class,
 		LifecycleModule.class,

briar-core/src/test/java/org/briarproject/briar/introduction/IntroductionIntegrationTestComponent.java

@@ -6,6 +6,7 @@ import org.briarproject.bramble.crypto.CryptoExecutorModule;
 import org.briarproject.bramble.crypto.CryptoModule;
 import org.briarproject.bramble.data.DataModule;
 import org.briarproject.bramble.db.DatabaseModule;
+import org.briarproject.bramble.event.DefaultEventExecutorModule;
 import org.briarproject.bramble.event.EventModule;
 import org.briarproject.bramble.identity.IdentityModule;
 import org.briarproject.bramble.lifecycle.LifecycleModule;
@@ -45,6 +46,7 @@ import dagger.Component;
 		CryptoExecutorModule.class,
 		DataModule.class,
 		DatabaseModule.class,
+		DefaultEventExecutorModule.class,
 		EventModule.class,
 		ForumModule.class,
 		GroupInvitationModule.class,

briar-core/src/test/java/org/briarproject/briar/messaging/MessageSizeIntegrationTestComponent.java

@@ -6,6 +6,7 @@ import org.briarproject.bramble.crypto.CryptoExecutorModule;
 import org.briarproject.bramble.crypto.CryptoModule;
 import org.briarproject.bramble.data.DataModule;
 import org.briarproject.bramble.db.DatabaseModule;
+import org.briarproject.bramble.event.DefaultEventExecutorModule;
 import org.briarproject.bramble.event.EventModule;
 import org.briarproject.bramble.identity.IdentityModule;
 import org.briarproject.bramble.sync.SyncModule;
@@ -37,6 +38,7 @@ import dagger.Component;
 		CryptoExecutorModule.class,
 		DataModule.class,
 		DatabaseModule.class,
+		DefaultEventExecutorModule.class,
 		EventModule.class,
 		ForumModule.class,
 		IdentityModule.class,

briar-core/src/test/java/org/briarproject/briar/messaging/SimplexMessagingIntegrationTestComponent.java

@@ -13,6 +13,7 @@ import org.briarproject.bramble.contact.ContactModule;
 import org.briarproject.bramble.crypto.CryptoModule;
 import org.briarproject.bramble.data.DataModule;
 import org.briarproject.bramble.db.DatabaseModule;
+import org.briarproject.bramble.event.DefaultEventExecutorModule;
 import org.briarproject.bramble.event.EventModule;
 import org.briarproject.bramble.identity.IdentityModule;
 import org.briarproject.bramble.lifecycle.LifecycleModule;
@@ -46,6 +47,7 @@ import dagger.Component;
 		CryptoModule.class,
 		DataModule.class,
 		DatabaseModule.class,
+		DefaultEventExecutorModule.class,
 		EventModule.class,
 		IdentityModule.class,
 		LifecycleModule.class,

briar-core/src/test/java/org/briarproject/briar/test/BriarIntegrationTestComponent.java

@@ -15,6 +15,7 @@ import org.briarproject.bramble.crypto.CryptoExecutorModule;
 import org.briarproject.bramble.crypto.CryptoModule;
 import org.briarproject.bramble.data.DataModule;
 import org.briarproject.bramble.db.DatabaseModule;
+import org.briarproject.bramble.event.DefaultEventExecutorModule;
 import org.briarproject.bramble.event.EventModule;
 import org.briarproject.bramble.identity.IdentityModule;
 import org.briarproject.bramble.lifecycle.LifecycleModule;
@@ -63,6 +64,7 @@ import dagger.Component;
 		CryptoExecutorModule.class,
 		DataModule.class,
 		DatabaseModule.class,
+		DefaultEventExecutorModule.class,
 		EventModule.class,
 		ForumModule.class,
 		GroupInvitationModule.class,

briar-headless/src/main/java/org/briarproject/briar/headless/HeadlessModule.kt

@@ -21,6 +21,7 @@ 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.battery.DefaultBatteryManagerModule
+import org.briarproject.bramble.event.DefaultEventExecutorModule
 import org.briarproject.bramble.network.JavaNetworkModule
 import org.briarproject.bramble.plugin.tor.CircumventionModule
 import org.briarproject.bramble.plugin.tor.CircumventionProvider
@@ -47,6 +48,7 @@ import javax.net.SocketFactory
         JavaSystemModule::class,
         CircumventionModule::class,
         DefaultBatteryManagerModule::class,
+        DefaultEventExecutorModule::class,
         HeadlessBlogModule::class,
         HeadlessContactModule::class,
         HeadlessEventModule::class,