yield* does not terminate enclosing function on stream cancellation

[alsemenov]

The Dart Programming Language Specification (4th edition draft) version 1.11, chapter 17.16.2 Yield-Each reads:

If m is marked async* (9), then:
• It is a dynamic error if the class of o does not implement Stream. Otherwise
• For each element x of o:
– If the stream u associated with m has been paused, then execution
of m is suspended until u is resumed or canceled.
– If the stream u associated with m has been canceled, then let c be
the finally clause (17.11) of the innermost enclosing try-finally state-
ment, if any. If c is defined, let h be the handler induced by c. If h is
defined, control is transferred to h. If h is undefined, the immediately
enclosing function terminates.
– Otherwise, x is added to the stream associated with m in the order
it appears in o. The function m may suspend.
• If the stream o is done, execution of s is complete.

The following code demonstrates, that execution of immediately enclosing function continues after yield* statement even if the associated stream subscription is cancelled.

import 'dart:async';

Stream<int> generator(Stream<int> input) async* {
  yield* input;
  print("Error!!! Code after yield* should not be executed");
}

main() async {
  StreamController<int> sc = new StreamController<int>();
  Stream<int> s = generator(sc.stream);
  StreamSubscription<int> ss = s.listen(
      (int i){
        print(i);
      }
  );
  sc.add(1);
  sc.add(2);
  sc.add(3);
  await new Future.delayed(new Duration(milliseconds: 100));
  await ss.cancel();
  sc.add(4);
  sc.add(5);
  await new Future.delayed(new Duration(milliseconds: 100));
  await sc.close();
}

Output:

1
2
3
Error!!! Code after yield* should not be executed

Expected output:

1
2
3

Dart VM version: 1.14.1 (Wed Feb 03 11:59:06 2016) on "windows_x64"

[kevmoo]

@floitschG thoughts?

[floitschG]

Looks like a bug in the VM.
Dart2js has the expected output.

[mhausner]

Still reproduces on the VM.

[natebosch]

Closing in favor of #34775 which has more specifics around both pausing and cancelling.