async* stream never ends if the generator's return is caused by an exception

[vexcat]

Dart SDK version: 2.14.2 (stable) (Wed Sep 15 12:32:06 2021 +0200) on "linux_x64"
OS: Linux
Platforms: desktop (working), web on chrome (bug)

Code:

int idx = 0;
Future<int> getValueExceptionOnEnd() async {
  if (idx < 4) {
    return idx++;
  } else {
    throw RangeError("no more ints for you!");
  }
}

Stream<int> onlyWorksOnDesktop() async* {
  try {
    while (true) {
      yield await getValueExceptionOnEnd();
    }
  } on RangeError {
    print("all done! dart should close the stream after this.");
    return;
  }
}

Future<int?> _allPlatformsHelper() async {
  try {
    return await getValueExceptionOnEnd();
  } on RangeError {
    return null;
  }
}

Stream<int> worksOnAllPlatforms() async* {
  while (true) {
    final value = await _allPlatformsHelper();
    if (value == null) break;
    yield value;
  }
  print("all done! dart should close the stream after this.");
}

void bugDemo() async {
  idx = 0;
  await for (var value in worksOnAllPlatforms()) {
    print(value);
  }
  print("worksOnAllPlatforms()'s stream closed.");

  idx = 0;
  await for (var value in onlyWorksOnDesktop()) {
    print(value);
  }
  //This never runs when developing for the web with flutter
  //(though, interestingly, it does work with dart2js.)
  print("onlyWorksOnDesktop()'s stream closed.");
}

bugDemo is being run in a Flutter app targeting desktop & the web.

Desktop output (expected):

0
1
2
3
all done! dart should close the stream after this.
worksOnAllPlatforms()'s stream closed.
0
1
2
3
all done! dart should close the stream after this.
onlyWorksOnDesktop()'s stream closed.

Web output:

0
1
2
3
all done! dart should close the stream after this.
worksOnAllPlatforms()'s stream closed.
0
1
2
3
all done! dart should close the stream after this.

[lrhn]

I'm not seeing the reported web output in dart2js (dartpad.dev or tip-of-tree dart2js).

Are you using dartdevc to compile for the web?

[vexcat]

Yes, this only affects dartdevc. I also cannot reproduce the issue when using dart2js.

[nshahan]

@vexcat Thanks for the report and the reproduction! This looks like a duplicate of #35624.

We have been tracking issues related to async timings in DDC compiled applications for some time but fixes have always been at a lower priority. To help us prioritize can I ask how you ran into this bug? Did you discover it through the use of some public package? Are you working with a flutter web project or using another other dart web framework?

[vexcat]

I was working on a flutter web project for analyzing a binary file. I encountered the issue while writing my own code using streams; this isn't an issue with a library function. I had written a class, AsyncByteStream, which made a Stream<ByteData> accessible via a function of signature Future<ByteData> load(int length), making parsing the byte stream a bit more convenient by letting the caller decide what size chunks it needs. I decided to handle EOF of the wrapped Stream<ByteData> in this class by throwing an exception from load(). I wanted to go from my Stream<ByteData> input -> AsyncByteStream for parsing -> Stream<ParsedData> generated using an async* function as output.

It looked something like this:

Stream<ParsedData> parse(AsyncByteStream stream) async* {
  try {
    while(true) {
      final header = await stream.load(16);
      final length = header.getUint32(0, Endian.little);
      //parsing happens in this constructor
      yield ParsedData(await stream.load(length));
    }
  } on RangeError {
    //EOF
    return;
  }
}

I ended up abandoning the idea of an AsyncByteStream class and just had parse() accumulate data from a Stream itself and yield when enough data was present, avoiding this exception issue.