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

+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

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

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

@@ -9,7 +9,9 @@ import org.briarproject.bramble.api.contact.event.ContactVerifiedEvent;
 import org.briarproject.bramble.api.crypto.SecretKey;
 import org.briarproject.bramble.api.db.ContactExistsException;
 import org.briarproject.bramble.api.db.DatabaseComponent;
+import org.briarproject.bramble.api.db.DbCallable;
 import org.briarproject.bramble.api.db.DbException;
+import org.briarproject.bramble.api.db.DbRunnable;
 import org.briarproject.bramble.api.db.Metadata;
 import org.briarproject.bramble.api.db.MigrationListener;
 import org.briarproject.bramble.api.db.NoSuchContactException;
@@ -166,6 +168,43 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 			for (Event e : transaction.getEvents()) eventBus.broadcast(e);
 	}
 
+	@Override
+	public void transaction(boolean readOnly, DbRunnable<DbException> task)
+			throws DbException {
+		throwingTransaction(readOnly, task);
+	}
+
+	@Override
+	public <R> R transactionWithResult(boolean readOnly,
+			DbCallable<R, DbException> task) throws DbException {
+		return throwingTransactionWithResult(readOnly, task);
+	}
+
+	@Override
+	public <E extends Exception> void throwingTransaction(boolean readOnly,
+			DbRunnable<E> task) throws DbException, E {
+		final Transaction txn = startTransaction(readOnly);
+		try {
+			task.run(txn);
+			commitTransaction(txn);
+		} finally {
+			endTransaction(txn);
+		}
+	}
+
+	@Override
+	public <R, E extends Exception> R throwingTransactionWithResult(
+			boolean readOnly, DbCallable<R, E> task) throws DbException, E {
+		final Transaction txn = startTransaction(readOnly);
+		try {
+			R result = task.call(txn);
+			commitTransaction(txn);
+			return result;
+		} finally {
+			endTransaction(txn);
+		}
+	}
+
 	private T unbox(Transaction transaction) {
 		if (transaction.isCommitted()) throw new IllegalStateException();
 		return txnClass.cast(transaction.unbox());