Debugger treats Streams that throw and have an error handler supplied through .listen() as uncaught

[jellynoone]

Consider the following example:

void main() async {
  _stream().listen(print, onError: print);
}

Stream<int> _stream() async* {
  for (var i = 0; i < 10; i++) {
    yield i;
  }

  throw Exception('Debugger always treats it as uncaught!');
}

Steps to reproduce:

1. Run dart run --observe --pause-isolates-on-start streams.dart
2. Open DevTools debugger
3. Select Stop on uncaught exceptions
4. Continue
5. Observe the debugger stop at throw even though the error is handled via listen.

However, if we rewrite the example to:

void main() async {
  try {
    await for (var item in _stream()) {
      print(item);
    }
  } catch (e) {
    print(e);
  }
}

Stream<int> _stream() async* {
  for (var i = 0; i < 10; i++) {
    yield i;
  }

  throw Exception('Debugger always treats it as uncaught!');
}

The debugger does not pause. As expected...

If we yet again, however, rewrite the example to use .asBroadcastStream():

void main() async {
  try {
    await for (var item in _stream().asBroadcastStream()) {
      print(item);
    }
  } catch (e) {
    print(e);
  }
}

Stream<int> _stream() async* {
  for (var i = 0; i < 10; i++) {
    yield i;
  }

  throw Exception('Debugger always treats it as uncaught!');
}

There is a pause again on the throw.

Personally, I've encountered this issue when using some firebase packages, and
thinking I did some error handling wrong, only to figure out after some time,
that the errors were in fact handled, but the debugger reported them as not.

Dart version:
Dart SDK version: 3.2.6 (stable) (Wed Jan 24 13:41:58 2024 +0000) on "macos_arm64"

[devoncarew]

This may be working as expected, but I'll let @bkonyi confirm.

[aam]

cc @mraleph (given https://dart.googlesource.com/sdk/+/a52f2b96179ebdf29c38cab47e73dd1c6ec6bdff)

[aam]

perhaps

sdk/runtime/vm/stack_trace.cc

Line 414 in 12841a6

void AsyncAwareStackUnwinder::UnwindFrameToStreamListener() {

should have code similar to

sdk/runtime/vm/stack_trace.cc

Line 410 in 12841a6

awaiter_frame_.has_catch_error = true;

[mraleph]

I have been sitting on a CL which should help here, but I have been delaying landing it because I have been slowly thinking through what we should even do with this? question.

CL is here: https://dart-review.googlesource.com/c/sdk/+/322720

[aam]

what we should even do with this? question.

It might be that in this issue the problem is simpler and clearer: we don't recognize that particular stream listener has an exception handler.

[a-siva]

what we should even do with this? question.

It might be that in this issue the problem is simpler and clearer: we don't recognize that particular stream listener has an exception handler.

Agree we should probably get the immediate issue fixed while @mraleph works on the CL listed above