`FocusManager.instance.toStringDeep` hangs when flutter test specifies --wasm --web-renderer=skwasm

[HansMuller]

Attempting to run a flutter/flutter FocusManager test on MacOS using the master branch:

(cd packages/flutter; flutter test --reporter=expanded -v --platform=chrome --wasm --web-renderer=skwasm --dart-define=DART_HHH_BOT=false test/widgets/focus_manager_test.dart)

The test hangs. The test runs OK if --wasm --web-renderer=skwasm is not specified.

The "debugDescribeFocusTree produces correct output" test consistently hangs here:

String debugDescribeFocusTree() {
  String? result;
  assert(() {
    result = FocusManager.instance.toStringDeep(); //  -- hang -- 
    return true;
  }());
  return result ?? '';
}

Strangely, adding a reference to the toStringDeep method before it's called fixes the problem:

String debugDescribeFocusTree() {
  String? result;
  assert(() {
    FocusManager.instance.toStringDeep; // huh?
    result = FocusManager.instance.toStringDeep(); // does not hang
    return true;
  }());
  return result ?? '';
}

FTR, just adding a reference to FocusManager.instance does not fix the problem.

This issue is currently holding up landing flutter/flutter#151145. I'm going to land the huh? workaround there for now.

[dart-github-bot]

Summary: The FocusManager.instance.toStringDeep method hangs when running Flutter tests with --wasm --web-renderer=skwasm flags. Adding a reference to toStringDeep before calling it unexpectedly resolves the issue.

[mkustermann]

Thanks for the report @HansMuller !

I believe the underlying bug is due incorrect handling of optional parameters in very specific situations (where global type flow inferred specific types for an optional parameter). Uploaded cl/374565 which I think will fix this issue.

The way this manifests is that it hits a ref.cast <type> wasm instruction that fails. I wonder why the test framework doesn't pick this error up and report it as such (e.g. via window.onerror or other mechanisms). It would be so much easier to get an exception & stack reported - instead of it just hanging. Maybe this is a bug i test framework?