Don't schedule work before a database is opened

moor/lib/src/utils/synchronized.dart

@@ -2,13 +2,13 @@ import 'dart:async';
 
 /// A single asynchronous lock implemented by future-chaining.
 class Lock {
-  Future<void> _last = Future.value();
+  Future<void>? _last;
 
   /// Waits for previous [synchronized]-calls on this [Lock] to complete, and
   /// then calls [block] before further [synchronized] calls are allowed.
   Future<T> synchronized<T>(FutureOr<T> Function() block) {
     final previous = _last;
-    // This controller may not be sync: It must complete just after
+    // This completer may not be sync: It must complete just after
     // callBlockAndComplete completes.
     final blockCompleted = Completer<void>();
     _last = blockCompleted.future;
@@ -21,6 +21,10 @@ class Lock {
       }
     }
 
-    return previous.then((_) => callBlockAndComplete());
+    if (previous != null) {
+      return previous.then((_) => callBlockAndComplete());
+    } else {
+      return callBlockAndComplete();
+    }
   }
 }

moor/test/integration_tests/regress_1235_test.dart

@@ -0,0 +1,32 @@
+import 'dart:async';
+
+import 'package:moor/src/ffi/vm_database.dart';
+import 'package:test/scaffolding.dart';
+
+import '../data/tables/todos.dart';
+
+void main() {
+  test('creating a database instance does not schedule async work', () {
+    // See https://github.com/simolus3/moor/issues/1235. We shouldn't run async
+    // work without users being aware of it, and no one expects creating an
+    // instance to schedule new microtasks.
+    noAsync(() => TodoDb(VmDatabase.memory()));
+  });
+}
+
+T noAsync<T>(T Function() fun) {
+  return runZoned(
+    fun,
+    zoneSpecification: ZoneSpecification(
+      scheduleMicrotask: (self, parent, zone, function) {
+        throw StateError('Not allowed: scheduleMicrotask');
+      },
+      createTimer: (self, parent, zone, duration, callback) {
+        throw StateError('Not allowed: createTimer');
+      },
+      createPeriodicTimer: (self, parent, zone, duration, callback) {
+        throw StateError('Not allowed: createPeriodicTimer');
+      },
+    ),
+  );
+}