Add interface for DB tasks will nullable results.

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

@@ -95,7 +95,7 @@ public interface DatabaseComponent {
 	 */
 	@Nullable
 	<R, E extends Exception> R transactionWithNullableResult(boolean readOnly,
-			DbCallable<R, E> task) throws DbException, E;
+			NullableDbCallable<R, E> task) throws DbException, E;
 
 	/**
 	 * Stores a contact associated with the given local and remote pseudonyms,

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

@@ -2,11 +2,8 @@ package org.briarproject.bramble.api.db;
 
 import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
 
-import javax.annotation.Nullable;
-
 @NotNullByDefault
 public interface DbCallable<R, E extends Exception> {
 
-	@Nullable
 	R call(Transaction txn) throws DbException, E;
 }

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

@@ -0,0 +1,12 @@
+package org.briarproject.bramble.api.db;
+
+import org.briarproject.bramble.api.nullsafety.NotNullByDefault;
+
+import javax.annotation.Nullable;
+
+@NotNullByDefault
+public interface NullableDbCallable<R, E extends Exception> {
+
+	@Nullable
+	R call(Transaction txn) throws DbException, E;
+}

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

@@ -19,6 +19,7 @@ import org.briarproject.bramble.api.db.NoSuchGroupException;
 import org.briarproject.bramble.api.db.NoSuchLocalAuthorException;
 import org.briarproject.bramble.api.db.NoSuchMessageException;
 import org.briarproject.bramble.api.db.NoSuchTransportException;
+import org.briarproject.bramble.api.db.NullableDbCallable;
 import org.briarproject.bramble.api.db.Transaction;
 import org.briarproject.bramble.api.event.Event;
 import org.briarproject.bramble.api.event.EventBus;
@@ -183,14 +184,20 @@ class DatabaseComponentImpl<T> implements DatabaseComponent {
 	@Override
 	public <R, E extends Exception> R transactionWithResult(boolean readOnly,
 			DbCallable<R, E> task) throws DbException, E {
-		R result = transactionWithNullableResult(readOnly, task);
+		Transaction txn = startTransaction(readOnly);
-		if (result == null) throw new NullPointerException();
+		try {
-		return result;
+			R result = task.call(txn);
+			commitTransaction(txn);
+			return result;
+		} finally {
+			endTransaction(txn);
+		}
 	}
 
 	@Override
 	public <R, E extends Exception> R transactionWithNullableResult(
-			boolean readOnly, DbCallable<R, E> task) throws DbException, E {
+			boolean readOnly, NullableDbCallable<R, E> task)
+			throws DbException, E {
 		Transaction txn = startTransaction(readOnly);
 		try {
 			R result = task.call(txn);

bramble-core/src/test/java/org/briarproject/bramble/sync/SimplexOutgoingSessionTest.java

@@ -50,12 +50,12 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 			oneOf(eventBus).addListener(session);
 			// No acks to send
 			oneOf(db).transactionWithNullableResult(with(false),
-					withDbCallable(noAckTxn));
+					withNullableDbCallable(noAckTxn));
 			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
 			will(returnValue(null));
 			// No messages to send
 			oneOf(db).transactionWithNullableResult(with(false),
-					withDbCallable(noMsgTxn));
+					withNullableDbCallable(noMsgTxn));
 			oneOf(db).generateBatch(with(noMsgTxn), with(contactId),
 					with(any(int.class)), with(MAX_LATENCY));
 			will(returnValue(null));
@@ -84,25 +84,25 @@ public class SimplexOutgoingSessionTest extends BrambleMockTestCase {
 			oneOf(eventBus).addListener(session);
 			// One ack to send
 			oneOf(db).transactionWithNullableResult(with(false),
-					withDbCallable(ackTxn));
+					withNullableDbCallable(ackTxn));
 			oneOf(db).generateAck(ackTxn, contactId, MAX_MESSAGE_IDS);
 			will(returnValue(ack));
 			oneOf(recordWriter).writeAck(ack);
 			// One message to send
 			oneOf(db).transactionWithNullableResult(with(false),
-					withDbCallable(msgTxn));
+					withNullableDbCallable(msgTxn));
 			oneOf(db).generateBatch(with(msgTxn), with(contactId),
 					with(any(int.class)), with(MAX_LATENCY));
 			will(returnValue(singletonList(message)));
 			oneOf(recordWriter).writeMessage(message);
 			// No more acks
 			oneOf(db).transactionWithNullableResult(with(false),
-					withDbCallable(noAckTxn));
+					withNullableDbCallable(noAckTxn));
 			oneOf(db).generateAck(noAckTxn, contactId, MAX_MESSAGE_IDS);
 			will(returnValue(null));
 			// No more messages
 			oneOf(db).transactionWithNullableResult(with(false),
-					withDbCallable(noMsgTxn));
+					withNullableDbCallable(noMsgTxn));
 			oneOf(db).generateBatch(with(noMsgTxn), with(contactId),
 					with(any(int.class)), with(MAX_LATENCY));
 			will(returnValue(null));

bramble-core/src/test/java/org/briarproject/bramble/test/DbExpectations.java

@@ -2,6 +2,7 @@ package org.briarproject.bramble.test;
 
 import org.briarproject.bramble.api.db.DbCallable;
 import org.briarproject.bramble.api.db.DbRunnable;
+import org.briarproject.bramble.api.db.NullableDbCallable;
 import org.briarproject.bramble.api.db.Transaction;
 import org.jmock.Expectations;
 
@@ -21,4 +22,11 @@ public class DbExpectations extends Expectations {
 		return null;
 	}
 
+	protected <R, E extends Exception> NullableDbCallable<R, E> withNullableDbCallable(
+			Transaction txn) {
+		addParameterMatcher(any(NullableDbCallable.class));
+		currentBuilder().setAction(new RunTransactionWithResultAction(txn));
+		return null;
+	}
+
 }