Misleading error when trying to spawn an isolate from a function which accesses local variables

[jakemac53]

Repro program:

import 'dart:async';
import 'dart:isolate';

void main() async {
  var i = 0;
  void childIsolateFn(SendPort sendPort) {
    sendPort.send(i);
  }

  var receivePort = ReceivePort();
  var isolate = await Isolate.spawn(childIsolateFn, receivePort.sendPort);
}

This fails with the error Invalid argument(s): Illegal argument in isolate message: (object is aReceivePort).

It seems valid that you can't refer to local variables in this case - but the error is very misleading and doesn't help you to diagnose the actual problem.

[mraleph]

I think the error is correct. The closure ends up capturing ReceivePort due to current async/await implementation. The limitation on closures should have been lifted as the part of light weight isolates work. You can send closures which capture something, but you can't send receive ports

[natebosch]

I think the error is correct.

It's correct, but I don't think it's very useful. The message was misleading for me - my interpretation was that the message argument to isolate.spawn was a RecievePort.

Can we add more context to the message?

Could we make it say something closer to:

Invalid `entrypoint` argument: Illegal value in state captured by closure: (object is a ReceivePort)

[jakemac53]

The message was misleading for me - my interpretation was that the message argument to isolate.spawn was a RecievePort.

Especially since you only see this fail on your call to Isolate.spawn but you can't inspect the actual implementation which is throwing. So it looks to you like you sent a RecievePort based on the error but you didn't.

So the request here is just to throw a better error (if possible) - something that tells you that you passed a closure which is capturing some local variables which isn't allowed.

[natebosch]

@mraleph - did you intend to close as a duplicate of a different issue?

[mraleph]

Duplicate of #51115

[mraleph]

Good catch @natebosch! Thanks.