How to use drift background tasks with shareAcrossIsolates

[cos-overclock]

I am considering an application that uses Bluetooth in the background, writes its connection status to a database, and reflects it in the UI.

I learned that drift has an option to share database connections between multiple isolates called shareAcrossIsolates.
However, an exception occurs when accessing the database from a background entry point.
I would like to know the problem with the following code and the reason for the exception.

I use below package

• flutter_foreground_task
• get

void main() {
  FlutterForegroundTask.initCommunicationPort();

  // for main isolate
  Get.put<LocalDatabase>(LocalDatabase());

  runApp(const ProviderScope(child: MainApp()));
}

@pragma('vm:entry-point')
void startCallback() {
  FlutterForegroundTask.setTaskHandler(MyTaskHandler());
}

class MyTaskHandler extends TaskHandler {
  // Called when the task is started.
  @override
  void onStart(DateTime timestamp) {
    // for background isolate
    Get.put<LocalDatabase>(LocalDatabase());

    print('onStart');
  }
}

@DriftDatabase(tables: [
  AppSettings,
  BleDevices,
])
class LocalDatabase extends _$LocalDatabase {
  LocalDatabase()
      : super(driftDatabase(
          name: 'local_database',
          native: const DriftNativeOptions(shareAcrossIsolates: true),
        ));

  @override
  int get schemaVersion => 1;
}

E/flutter (31430): [ERROR:flutter/runtime/dart_isolate.cc(1315)] Unhandled exception:
E/flutter (31430): Invalid argument: is a regular instance reachable via : Instance of 'Request'
E/flutter (31430): #0      _SendPort._sendInternal (dart:isolate-patch/isolate_patch.dart:250:43)
E/flutter (31430): #1      _SendPort.send (dart:isolate-patch/isolate_patch.dart:231:5)
E/flutter (31430): #2      _RootZone.runUnaryGuarded (dart:async/zone.dart:1594:10)
E/flutter (31430): #3      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:365:11)
E/flutter (31430): #4      _BufferingStreamSubscription._add (dart:async/stream_impl.dart:297:7)
E/flutter (31430): #5      _SyncStreamControllerDispatch._sendData (dart:async/stream_controller.dart:784:19)
E/flutter (31430): #6      _StreamController._add (dart:async/stream_controller.dart:658:7)
E/flutter (31430): #7      _StreamController.add (dart:async/stream_controller.dart:606:5)
E/flutter (31430): #8      _RootZone.runUnaryGuarded (dart:async/zone.dart:1594:10)
E/flutter (31430): #9      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:365:11)
E/flutter (31430): #10     _BufferingStreamSubscription._add (dart:async/stream_impl.dart:297:7)
E/flutter (31430): #11     _SyncStreamControllerDispatch._sendData (dart:async/stream_controller.dart:784:19)
E/flutter (31430): #12     _StreamController._add (dart:async/stream_controller.dart:658:7)
E/flutter (31430): #13     _StreamController.add (dart:async/stream_controller.dart:606:5)
E/flutter (31430): #14     _StreamSinkWrapper.add (dart:async/stream_controller.dart:877:13)
E/flutter (31430): #15     _GuaranteeSink.add (package:stream_channel/src/guarantee_channel.dart:125:12)
E/flutter (31430): #16     DriftCommunication._send (package:drift/src/remote/communication.dart:136:19)
E/flutter (31430): #17     DriftCommunication.notify (package:drift/src/remote/communication.dart:123:5)
E/flutter (31430): #18     ServerImplementation.serve (package:drift/src/remote/server_impl.dart:76:10)
E/flutter (31430): #19     new RunningDriftServer.<anonymous closure> (package:drift/src/isolate.dart:128:16)
E/flutter (31430): #20     _RootZone.runUnaryGuarded (dart:async/zone.dart:1594:10)
E/flutter (31430): #21     _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:365:11)
E/flutter (31430): #22     _BufferingStreamSubscription._add (dart:async/stream_impl.dart:297:7)
E/flutter (31430): #23     _SyncStreamControllerDispatch._sendData (dart:async/stream_controller.dart:784:19)
E/flutter (31430): #24     _StreamController._add (dart:async/stream_controller.dart:658:7)
E/flutter (31430): #25     _StreamController.add (dart:async/stream_controller.dart:606:5)
E/flutter (31430): #26     _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)

[cos-overclock]

In fact, both UI and background tasks perform queries such as watch.

[simolus3]

Thanks for the report! I was under the assumption that isolates sharing an IsolateNameServer would always be able to send arbitrary objects between them. Apparently that's not the case.

Drift can in principle work around this issue by serializing messages sent between different isolates into simple Dart objects that can always be sent. That flag is just somewhat hidden and not exported by drift_flutter. In e1d20c5 and e614f94, I've changed drift_flutter to enable serialization only if necessary instead of always disabling it.

This isn't easy to test since it requires independent isolate groups, could you check whether adding these dependency overrides fixes the issue for you?

dependency_overrides:
  drift:
    git:
      url: https://github.com/simolus3/drift.git
      ref: develop
      path: drift
  drift_flutter:
    git:
      url: https://github.com/simolus3/drift.git
      ref: develop
      path: drift_flutter

[cos-overclock]

Thank you for your response.

We have tried the version you provided and the same exception occurred.

[simolus3]

Argh, I had them the wrong way around. This was fixed in 14202c8, I also managed to find an easier way to test this. Let me know if you run into any other issues with that.

[cos-overclock]

I tried again and confirmed that it works fine.
Thank you for your support.