Ability to rollback transactions

moor/lib/src/runtime/executor/executor.dart

@@ -87,4 +87,8 @@ abstract class TransactionExecutor extends QueryExecutor {
   /// Completes the transaction. No further queries may be sent to to this
   /// [QueryExecutor] after this method was called.
   Future<void> send();
+
+  /// Cancels this transaction. No further queries may be sent ot this
+  /// [QueryExecutor] after this method was called.
+  Future<void> rollback();
 }

moor/lib/src/runtime/executor/helpers/delegates.dart

@@ -106,8 +106,15 @@ class NoTransactionDelegate extends TransactionDelegate {
   /// The statement that commits a transaction on this database engine.
   final String commit;
 
-  const NoTransactionDelegate(
-      {this.start = 'BEGIN TRANSACTION', this.commit = 'COMMIT TRANSACTION'});
+  /// The statement that will perform a rollback of a transaction on this
+  /// database engine.
+  final String rollback;
+
+  const NoTransactionDelegate({
+    this.start = 'BEGIN TRANSACTION',
+    this.commit = 'COMMIT TRANSACTION',
+    this.rollback = 'ROLLBACK TRANSACTION',
+  });
 }
 
 /// A [TransactionDelegate] for database APIs which do support creating and

moor/lib/src/runtime/executor/helpers/engines.dart

@@ -98,6 +98,7 @@ class _TransactionExecutor extends TransactionExecutor
   Completer<bool> _openingCompleter;
 
   String _sendOnCommit;
+  String _sendOnRollback;
 
   Future get completed => _sendCalled.future;
 
@@ -130,6 +131,7 @@ class _TransactionExecutor extends TransactionExecutor
         impl = _db.delegate;
         await impl.runCustom(transactionManager.start, const []);
         _sendOnCommit = transactionManager.commit;
+        _sendOnRollback = transactionManager.rollback;
 
         transactionStarted.complete();
 
@@ -162,6 +164,16 @@ class _TransactionExecutor extends TransactionExecutor
 
     _sendCalled.complete();
   }
+
+  @override
+  Future<void> rollback() async {
+    if (_sendOnRollback != null) {
+      await impl.runCustom(_sendOnRollback, const []);
+    }
+
+    _sendCalled.completeError(
+        Exception('artificial exception to rollback the transaction'));
+  }
 }
 
 class _BeforeOpeningExecutor extends QueryExecutor