Fix crash when using a no-op transaction (#361)

extras/integration_tests/tests/lib/suite/transactions.dart

@@ -52,4 +52,9 @@ void transactionTests(TestExecutor executor) {
 
     await db.close();
   });
+
+  test('can use no-op transactions', () async {
+    final db = Database(executor.createExecutor());
+    await db.transaction(() => Future.value(null));
+  });
 }

moor/CHANGELOG.md

@@ -5,6 +5,7 @@
   has been updated to explain how to use them.
 - Support table-valued functions (like `json_each` and `json_tree`) in moor files 
   [#260](https://github.com/simolus3/moor/issues/260).
+- Fix a crash when opening a transaction without using it ([#361](https://github.com/simolus3/moor/issues/361))
 
 ## 2.3.0
 

moor/lib/src/runtime/api/query_engine.dart

@@ -26,7 +26,7 @@ mixin QueryEngine on DatabaseConnectionUser {
   /// Here, the `update` method would be called on the [GeneratedDatabase]
   /// although it is very likely that the user meant to call it on the
   /// [Transaction] t. We can detect this by calling the function passed to
-  /// `transaction` in a forked [Zone] storing the transaction in
+  /// `transaction` in a forked [Zone].
   @protected
   bool get topLevel => false;
 

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

@@ -104,9 +104,17 @@ class BatchedStatement {
 abstract class TransactionExecutor extends QueryExecutor {
   /// Completes the transaction. No further queries may be sent to to this
   /// [QueryExecutor] after this method was called.
+  ///
+  /// This may be called before [ensureOpen] was awaited, implementations must
+  /// support this. That state implies that no query was sent, so it should be
+  /// a no-op.
   Future<void> send();
 
   /// Cancels this transaction. No further queries may be sent ot this
   /// [QueryExecutor] after this method was called.
+  ///
+  /// This may be called before [ensureOpen] was awaited, implementations must
+  /// support this. That state implies that no query was sent, so it should be
+  /// a no-op.
   Future<void> rollback();
 }

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

@@ -176,6 +176,9 @@ class _TransactionExecutor extends TransactionExecutor
 
   @override
   Future<void> send() async {
+    // don't do anything if the transaction completes before it was opened
+    if (_openingCompleter == null) return;
+
     if (_sendOnCommit != null) {
       await runCustom(_sendOnCommit, const []);
       _db.delegate.isInTransaction = false;
@@ -186,6 +189,9 @@ class _TransactionExecutor extends TransactionExecutor
 
   @override
   Future<void> rollback() async {
+    // don't do anything if the transaction completes before it was opened
+    if (_openingCompleter == null) return;
+
     if (_sendOnRollback != null) {
       await runCustom(_sendOnRollback, const []);
       _db.delegate.isInTransaction = false;

moor/lib/src/runtime/isolate/client.dart

@@ -158,13 +158,19 @@ class _TransactionIsolateExecutor extends _BaseExecutor
   }
 
   @override
-  Future<void> rollback() {
-    return _sendAction(_TransactionControl.rollback);
+  Future<void> rollback() async {
+    // don't do anything if the transaction isn't open yet
+    if (_pendingOpen == null) return;
+
+    return await _sendAction(_TransactionControl.rollback);
   }
 
   @override
-  Future<void> send() {
-    return _sendAction(_TransactionControl.commit);
+  Future<void> send() async {
+    // don't do anything if the transaction isn't open yet
+    if (_pendingOpen == null) return;
+
+    return await _sendAction(_TransactionControl.commit);
   }
 }
 

moor/test/isolate_test.dart

@@ -114,7 +114,14 @@ void _runTests(
     expect(result, isNotEmpty);
   });
 
-  test('transactions have an isolate view on data', () async {
+  test('supports no-op transactions', () async {
+    final database = TodoDb.connect(isolateConnection);
+    await database.transaction(() {
+      return Future.value(null);
+    });
+  });
+
+  test('transactions have an isolated view on data', () async {
     // regression test for https://github.com/simolus3/moor/issues/324
     final db = TodoDb.connect(isolateConnection);