[Wasm] JS exception not caught in async function

[sma]

I think, this issue is caused by a Dart wasm compiler bug.

This source code (a trivial Futter app) should work, but crashes because the exception on the JavaScript side isn't caught in initialize. Note that it works if initialize isn't an async function. Hence, my guess that the wasm compiler has a problem here.

import 'dart:js_interop';

import 'package:flutter/material.dart';

void main() async {
  await initialize();
  runApp(const MaterialApp(home: Scaffold(body: Text('It works!'))));
}

Future<void> initialize() async {
  try {
    doSomething();
  } catch (e) {
    // ignored
  }
  print('not reached');
}

@JS()
external void doSomething();

For the JavaScript side, I added this to index.html:

    function doSomething() {
      throw new Error('This is a JavaScript error.');
    }

To compile the Flutter app, I use:

flutter build web -t lib/bug.dart --wasm --no-strip-wasm

To run it, I use the httpd command from the documentation. See that other bug report.

My configuration:

$ flutter --version
Flutter 3.22.0-10.0.pre.13 • channel master • https://github.com/flutter/flutter.git
Framework • revision 46fbb73440 (11 hours ago) • 2024-04-12 18:41:57 -0400
Engine • revision 1a13c7d1f4
Tools • Dart 3.5.0 (build 3.5.0-47.0.dev) • DevTools 2.34.1

Some additional details:

Here's the original exception in Chrome dev tools with "break on exceptions":

Here's the WASM exception handler, I guess:

and here's the stack trace:

JavaScriptError
main.dart.mjs:56     at initialize inner (http://localhost:8080/main.dart.wasm:wasm-function[7714]:0x12ea5b)
    at main inner.1 (http://localhost:8080/main.dart.wasm:wasm-function[1817]:0xa96f7)
    at main.1 (http://localhost:8080/main.dart.wasm:wasm-function[1150]:0x99520)
    at bootstrapEngine closure at org-dartlang-sdk:///lib/ui_web/ui_web/initialization.dart:35:16 inner (http://localhost:8080/main.dart.wasm:wasm-function[7725]:0x12ef51)
    at _awaitHelperWithTypeCheck closure at org-dartlang-sdk:///dart-sdk/lib/_internal/wasm/lib/async_patch.dart:97:16 (http://localhost:8080/main.dart.wasm:wasm-function[1721]:0xa7c34)
    at closure wrapper at org-dartlang-sdk:///dart-sdk/lib/_internal/wasm/lib/async_patch.dart:97:16 trampoline (http://localhost:8080/main.dart.wasm:wasm-function[1726]:0xa7c9b)
    at _RootZone.runUnary (http://localhost:8080/main.dart.wasm:wasm-function[985]:0x94c79)
    at _Future._propagateToListeners (http://localhost:8080/main.dart.wasm:wasm-function[964]:0x947c5)

[kevmoo]

CC @eyebrowsoffire – someone else here?

[kevmoo]

Related? #55457

[eyebrowsoffire]

Yep, it's a duplicate.

[eyebrowsoffire]

Closing as a duplicate. Btw, good investigation @sma! We're working on a fix.