Limit the number of database threads.

briar-core/src/net/sf/briar/db/DatabaseModule.java

 package net.sf.briar.db;
 
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+
 import java.sql.Connection;
+import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.Executor;
-import java.util.concurrent.Executors;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.ThreadPoolExecutor;
 
 import net.sf.briar.api.clock.Clock;
 import net.sf.briar.api.clock.SystemClock;
@@ -17,11 +21,24 @@ import com.google.inject.Singleton;
 
 public class DatabaseModule extends AbstractModule {
 
+	/**
+	 * The maximum number of database threads. When a task is submitted to the
+	 * database executor and no thread is available to run it, the task will be
+	 * queued.
+	 */
+	private static final int MAX_DB_THREADS = 10;
+
+	/** How many milliseconds to keep idle threads alive. */
+	private static final int DB_KEEPALIVE = 60 * 1000;
+
 	@Override
 	protected void configure() {
 		bind(DatabaseCleaner.class).to(DatabaseCleanerImpl.class);
+		// Use an unbounded queue to prevent deadlock between submitted tasks
+		BlockingQueue<Runnable> queue = new LinkedBlockingQueue<Runnable>();
 		bind(Executor.class).annotatedWith(DatabaseExecutor.class).toInstance(
-				Executors.newCachedThreadPool());
+				new ThreadPoolExecutor(MAX_DB_THREADS, MAX_DB_THREADS,
+						DB_KEEPALIVE, MILLISECONDS, queue));
 	}
 
 	@Provides