Await on async work within await for loop causing unwanted stream event

[tvolkert]

Consider the following code:

Future<void> main() async {
  await for (int i in foo()) {
    print('received $i');
    if (i > 1) {
      break;
    }
  }
}

Stream<int> foo() async* {
  for (int i = 0; true; i++) {
    print('yielding $i');
    yield i;
  }
}

This produces the following output:

yielding 0
received 0
yielding 1
received 1
yielding 2
received 2
yielding 3

Now, if we change the code to insert an async task in the await for loop, like so:

Future<void> main() async {
  await for (int i in foo()) {
    print('received $i');
    await bar();
    if (i > 1) {
      break;
    }
  }
}

Stream<int> foo() async* {
  for (int i = 0; true; i++) {
    print('yielding $i');
    yield i;
  }
}

Future<void> bar() async {}

It instead produces the following output:

yielding 0
received 0
yielding 1
received 1
yielding 2
received 2
yielding 3
yielding 4

For some reason, the introduction of an extra await on async work within the await for loop caused us to yield an extra value.

[lrhn]

This is a bug in the implementation of async* methods.

In Dart 1, the specfication was not precise about what should happen at a yield.
The Dart 2 specification now states that it must deliver the event and then check for pauses (and be ready to handle a cancel while paused), and an await for should receive the event and then pause its subscription if it does any await. Together those should prevent the behavior you see here.

The implementations have not moved to the specified behavior yet, though. That's #34775.