Add transactional DB interface.

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

@@ -76,6 +76,32 @@ public interface DatabaseComponent {
 	 */
 	void endTransaction(Transaction txn);
 
+	/**
+	 * Runs the given task within a transaction.
+	 */
+	void transaction(boolean readOnly, DbRunnable<DbException> task)
+			throws DbException;
+
+	/**
+	 * Runs the given task within a transaction and returns the result of the
+	 * task.
+	 */
+	<R> R transactionWithResult(boolean readOnly,
+			DbCallable<R, DbException> task) throws DbException;
+
+	/**
+	 * Runs the given task within a transaction.
+	 */
+	<E extends Exception> void throwingTransaction(boolean readOnly,
+			DbRunnable<E> task) throws DbException, E;
+
+	/**
+	 * Runs the given task within a transaction and returns the result of the
+	 * task.
+	 */
+	<R, E extends Exception> R throwingTransactionWithResult(boolean readOnly,
+			DbCallable<R, E> task) throws DbException, E;
+
 	/**
 	 * Stores a contact associated with the given local and remote pseudonyms,
 	 * and returns an ID for the contact.

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

@@ -0,0 +1,6 @@
+package org.briarproject.bramble.api.db;
+
+public interface DbCallable<R, E extends Exception> {
+
+	R call(Transaction txn) throws DbException, E;
+}

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

@@ -0,0 +1,6 @@
+package org.briarproject.bramble.api.db;
+
+public interface DbRunnable<E extends Exception> {
+
+	void run(Transaction txn) throws DbException, E;
+}