[dart2wasm] Asynchronous exception catching isn't working properly

[eyebrowsoffire]

There is some code in flutter framework that is causing unit test failures when compiled to wasm. The issue seems to lie in this file: https://github.com/flutter/flutter/blob/38b2c5bf410b9b2ab52812acf7bbd924bf9386e2/packages/flutter/lib/src/services/platform_channel.dart#L568

I have created a more minimal reproducible sample of code that demonstrates this issue:

class MyException implements Exception {
  MyException([this.message]);

  final String? message;

  @override
  String toString() => 'MyException($message)';
}

class MyOtherException implements Exception {
  MyOtherException([this.message]);

  final String? message;

  @override
  String toString() => 'MyOtherException($message)';
}

Future<String> asynchronouslyThrowException() async {
  throw MyException('throwing an error!');
}

Future<void> main() async {
  await _doIt(() => asynchronouslyThrowException());
}


Future<String?> _doIt(Future<dynamic> Function() handler) async {
  try {
    print('trying thing');
    return 'success: ${await handler()}';
  } on MyOtherException catch (e) {
    return e.message;
  } on MyException {
    print('returning null');
    return null;
  } catch (error) {
    return 'generic error';
  }
}

I don't know exactly what's going on yet, so it might be possible to distill it down further, but this does behave unexpectedly under dart2wasm. Running in the vm, we get the following output:

trying thing
returning null

But in dart2wasm, the MyException escapes out and is not caught by the on MyException block:

trying thing
assert_text.mjs:56 MyException(throwing an error!)
assert_text.mjs:56     at asynchronouslyThrowException inner (http://localhost:8000/assert_text.wasm:wasm-function[747]:0x1518f)
    at asynchronouslyThrowException (http://localhost:8000/assert_text.wasm:wasm-function[746]:0x15151)
    at main closure at file:///Users/jacksongardner/Desktop/scratch/assert_text.dart:25:15 (http://localhost:8000/assert_text.wasm:wasm-function[735]:0x14e82)
    at closure wrapper at file:///Users/jacksongardner/Desktop/scratch/assert_text.dart:25:15 trampoline (http://localhost:8000/assert_text.wasm:wasm-function[741]:0x14fa7)
    at _doIt inner (http://localhost:8000/assert_text.wasm:wasm-function[912]:0x17a12)
    at _doIt (http://localhost:8000/assert_text.wasm:wasm-function[742]:0x14fdc)
    at main inner (http://localhost:8000/assert_text.wasm:wasm-function[736]:0x14efc)
    at main (http://localhost:8000/assert_text.wasm:wasm-function[79]:0xc750)
assert_text.wasm:0xd023 Uncaught Exception {}

[eyebrowsoffire]

A slightly more distilled version:

class MyException implements Exception {
  MyException([this.message]);

  final String? message;

  @override
  String toString() => 'MyException($message)';
}

class MyOtherException implements Exception {
  MyOtherException([this.message]);

  final String? message;

  @override
  String toString() => 'MyOtherException($message)';
}

Future<String> asynchronouslyThrowException() async {
  throw MyException('throwing an error!');
}

Future<String?> main() async {
  try {
    print('trying thing');
    final future = asynchronouslyThrowException();
    print('got future');
    final result = await future;
    return 'success: $result';
  } on MyOtherException catch (e) {
    return e.message;
  } on MyException catch (e) {
    print('got $e, returning null');
    return null;
  } catch (error) {
    return 'generic error';
  }
}

It seems to be specific to having multiple on branches, and the order of the branches. It doesn't reproduce if I change the order of the on branches or if I just remove one of the on branches.

[osa1]

Fix in https://dart-review.googlesource.com/c/sdk/+/361063.