Errors in async blocks are propagated too slowly

[nex3]

If an error is thrown in an async block, it propagates to the zone's error handler much more slowly than one thrown in a non-async context. For example:

    import 'dart:async';

    void main() {
      runZoned(() async {
        scheduleMicrotask(() => throw 'in microtask');
        throw 'in async function';
      }, onError: (error, stackTrace) {
        print("caught $error");
      });
    }

this prints:

    in microtask
    in async function

which is not expected, since "in async function" is thrown before "in microtask". Compare to the version without async:

    import 'dart:async';

    void main() {
      runZoned(() {
        scheduleMicrotask(() => throw 'in microtask');
        throw 'in async function';
      }, onError: (error, stackTrace) {
        print("caught $error");
      });
    }

and the version that runs the entire body in a microtask:

    import 'dart:async';

    void main() {
      runZoned(() {
        scheduleMicrotask(() {
          scheduleMicrotask(() => throw 'in microtask');
          throw 'in async function';
        });
      }, onError: (error, stackTrace) {
        print("caught $error");
      });
    }

both of which print

    in async function
    in microtask

as expected.

This is very important for testing (among other things), where errors have to be handled specially and users have strong expectations for their consistent ordering.

[iposva-google]

Removed Priority-High label.
Added Priority-Unassigned label.

[floitschG]

This might be trivial to fix, by switching to sync-completers.
I'm too tired to think too deeply about it, and it will need more people thinking this through, but at the moment I think that should do it.

For dart2js the patch would be (assuming that the completer-constructor lookup is only used for the async-rewrite):

diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 986a313..f3067d1 100644

--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1852,7 +1852,7 @­@ class JavaScriptBackend extends Backend {
   Element getCompleterConstructor() {
     ClassElement classElement = find(compiler.asyncLibrary, "Completer");
     classElement.ensureResolved(compiler);

• return classElement.lookupConstructor("");
• return classElement.lookupConstructor("sync");
     }
   
     Element getASyncStarController() {

---

cc @lrhn.
cc @sigurdm.

[floitschG]

Fixed in f2d0814.

[kevmoo]

Thanks, @floitschG !!!