VM should pause or cancel at a yield if the listener pauses or cancels in response to that event.

[lrhn]

The current specification of async* states that for yield value, the implementation must deliver the value (meaning the stream's subscription's onData handler must be called) before it continues after the yield. If the subscription has been paused or cancelled immediately by that call (synchronously), then the implementation should pause or cancel, respectively, and not continue with the following code.

The VM does continue.
Example:

Stream<int> foo() async* {
  try {
    print("a1");
    yield 1;
    print("a2");
    yield 2;
    print("a3");
    yield 3;
  } finally {
    print("af");
  }
}

void main() {
  var s = foo().listen(null);
  s.onData((int v) {
    print("e$v");
    if (v == 1) {
      var delay = Future.delayed(Duration(seconds: 1)).whenComplete(() {
        print("resume");
      });
      print("pause");
      s.pause(delay);
    } else if (v == 2) {
      s.cancel().whenComplete(() {
        print("ef");
      });
    }
  });
}

This should print in the following order: a1, e1, pause, resume, a2, e2, af, ef

It actually prints: a1, e1, pause, a2, resume, e2, a3, af, ef

That is, neither pause nor cancel is respected until the next yield is reached.

(Dart2js gets half if this right - it does pause in the correct place, but does not cancel at the second yield).

[CosmicPangolin]

@lrhn
Out of curiosity, is this being addressed anytime soon? :)

[sgrekhov]

Please note that this issue exists for dartk, dartdevk and dartkp as well

[a-siva]

Can this issue be closed ?

[lrhn]

The VM now behaves correctly on the original example, so LGTM.