Implementation of async-await transformation on js ast.

sdk/lib/_internal/compiler/js_lib/js_helper.dart

Index: sdk/lib/_internal/compiler/js_lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_helper.dart b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
index 7a75198df3ccaab7fb7ed24c9c56433c0fcc11ed..7f75fb91162b9162de78c370d61244a4b59afccc 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -25,7 +25,14 @@ import 'dart:_isolate_helper' show
     enterJsAsync,
     isWorker;
 
-import 'dart:async' show Future, DeferredLoadException, Completer;
+import 'dart:async'

[floitsch, 2015/02/05 20:17:23]

maybe not your code, but should be easy:
move "show" to the import line. (see below for the foreign_helper).
and sort the shows.

[sigurdm, 2015/02/06 14:26:34]

Done.

+  show Future,
+  DeferredLoadException,
+  Completer,
+  StreamController,
+  Stream,
+  StreamSubscription,
+  scheduleMicrotask;
 
 import 'dart:_foreign_helper' show
     DART_CLOSURE_TO_JS,
@@ -3462,3 +3469,249 @@ void badMain() {
 void mainHasTooManyParameters() {
   throw new MainError("'main' expects too many parameters.");
 }
+
+/// Runtime support for async-await transformation.
+///
+/// This function is called by a transformed function on each await and return
+/// in the untransformed function, and before starting.
+///
+/// If [object] is not a future it will be wrapped in a `new Future.value`.
+///
+/// If [helperCallback] is null it indicates a return from the async function,
+/// and we complete the completer with object.
+///
+/// Otherwise [helperCallback] is set up to be called when the future is
+/// successfull and [errorCallback] if it is completed with an error.
+///
+/// If helperCallback or errorCallback throws we complete the completer with the
+/// error.
+///
+/// Returns the future of the completer for convenience of the first call.
+dynamic thenHelper(dynamic object,
+                   dynamic /* js function */ helperCallback,
+                   Completer completer,
+                   dynamic /* js function */ errorCallback) {
+  if (helperCallback == null) {
+    completer.complete(object);
+    return;
+  }
+  Future future = object is Future ? object : new Future.value(object);
+  future.then(_wrapJsFunctionForThenHelper(helperCallback, completer),
+              onError: (errorCallback == null)
+                  ? null
+                  : _wrapJsFunctionForThenHelper(errorCallback, completer));
+  return completer.future;
+}
+
+Function _wrapJsFunctionForThenHelper(dynamic /* js function */ function,
+                                      Completer completer) {
+  return (result) {
+    try {
+      JS('', '#(#)', function, result);
+    } catch (e, st) {
+      completer.completeError(e, st);
+    }
+  };
+}
+
+
+/// Implements the runtime support for async* functions.
+///
+/// Called by the transformed function for each original return, await, yield,
+/// yield* and before starting the function.
+///
+/// When the async* function wants to return it call this with

[floitsch, 2015/02/05 20:17:23]

calls this functions. The streamHelper takes this as signal to close the stream.

[sigurdm, 2015/02/06 14:26:34]

Done.

+/// [helperCallback] == null, and the stream is closed.
+///
+/// If the async* function wants to do a yield or yield* it will call this with

[floitsch, 2015/02/05 20:17:23]

it calls this function

[sigurdm, 2015/02/06 14:26:34]

Done.

+/// [object] being an [IterationMarker]. In this case [errorCallback] has a
+/// special meaning; it is a callback that will run all enclosing finalizers.
+///
+/// In the case of a yield or yield*, if the stream subscription has been
+/// canceled [errorCallback] is scheduled.
+///
+/// If [object] is a single-yield [IterationMarker], add the value of the

[floitsch, 2015/02/05 20:17:23]

adds

[sigurdm, 2015/02/06 14:26:34]

Done.

+/// [IterationMarker] to the stream, next: if the stream subscription has been

[floitsch, 2015/02/05 20:17:23]

to the stream. If the stream...

(unless I'm misunderstanding something).

[sigurdm, 2015/02/06 14:26:34]

Done.

+/// paused, return early. Otherwise schedule the helper function to be
+/// executed again.
+///
+/// If [object] is a yield-star [IterationMarker], start listening to the

[floitsch, 2015/02/05 20:17:23]

starts ... and adds ... schedules

[sigurdm, 2015/02/06 14:26:34]

Done.

+/// yielded stream, and add all events and errors to our own controller (taking
+/// care if the subscription has been paused or canceled) - when the sub-stream
+/// is done, schedule [helperCallback] again.
+///
+/// If the async* function wants to do an await it will call this with [object]

[floitsch, 2015/02/05 20:17:23]

calls this function with [object] not an [IterationMarker] ?

[sigurdm, 2015/02/06 14:26:34]

Done.

+/// not being an [IterationMarker].
+///
+/// If [object] is not a [Future], it is wrapped in a `Future.value`.
+/// Then set up [helperCallback] to be called on successfull completion of the

[floitsch, 2015/02/05 20:17:23]

The [helperCallback] is called on successful completion of the future.

[sigurdm, 2015/02/06 14:26:34]

Done.

+/// future.
+///
+/// If [helperCallback] or [errorCallback] throws the error is added to the
+/// stream.
+dynamic streamHelper(dynamic object,
+                     dynamic /* js function */ helperCallback,
+                     AsyncStarStreamController controller,
+                     dynamic /* js function */ errorCallback) {
+  if (helperCallback == null) {
+    // This happens on return from the async* function.
+    controller.close();
+    return null;
+  }
+
+  if (object is IterationMarker) {
+    if (controller.stopRunning) {
+      _wrapJsFunctionForStream(errorCallback, controller)();
+      return null;
+    }
+    if (object.state == IterationMarker.YIELD_SINGLE) {
+      controller.add(object.value);
+      if (controller.isPaused) {
+        return null;
+      }
+      new Future.value(null).then(

[floitsch, 2015/02/05 20:17:23]

Why?
Do we have to wait a microtask before we can continue?
If yes, just use the scheduleMicrotask instead of a Future.value (unless I'm
misunderstanding).

[sigurdm, 2015/02/06 14:26:35]

I thought that it was necessary to suspend execution when yielding. But I was
wrong. Execution should just continue.
I leave it for now, with a TODO.
It was a hack, because _wrapJsFunctionForStream() takes one argument.
Fixed.

+          _wrapJsFunctionForStream(helperCallback, controller));
+      return;
+    } else if (object.state == IterationMarker.YIELD_STAR) {
+      Stream stream = object.value;
+      controller.isAdding = true;
+      controller.addStream(stream).then((_) {

[floitsch, 2015/02/05 20:17:23]

You might need to listen to the error too. I'm not sure if that can happen, or
if this future is always without error.
If it never has an error (because the errors are just piped through) then add a
comment.

Please add it to your TODO list, to determine if errors from yield* are
canceling the stream, or just passed through verbatim. (For now doing what
`addStream` does, is enough).

[sigurdm, 2015/02/06 14:26:34]

As you saw below, I explicitly ask errors to be passed through. Added a comment.
As far as I understand the spec, errors are passed through like we do here. But
the spec just says: ..., Next, for each element x of o: ...
Not mentioning errors explicitly. We should mention this to Gilad.
Added TODO.

+        controller.isAdding = false;
+        _wrapJsFunctionForStream(helperCallback, controller)(null);
+      });
+      return null;
+    }
+  }
+
+  Future future = object is Future ? object : new Future.value(object);
+  future.then(_wrapJsFunctionForStream(helperCallback, controller),
+              onError: errorCallback == null
+                  ? null
+                  : _wrapJsFunctionForStream(errorCallback, controller));
+  return controller.stream;
+}
+
+/// A wrapper around a [StreamController] that remembers if that controller
+/// got a cancel.
+///
+/// Also has a subSubscription that when not null will provide events for the
+/// stream, and will be paused and resumed along with this controller.
+class AsyncStarStreamController {
+  StreamController controller;
+  Stream get stream => controller.stream;
+  bool stopRunning = false;
+  bool isAdding = false;
+  bool get isPaused => controller.isPaused;
+  add(event) => controller.add(event);
+  addStream(Stream stream) {
+    return controller.addStream(stream, cancelOnError: false);

[floitsch, 2015/02/05 20:17:23]

I see. So I guess the future cannot have an error.

[sigurdm, 2015/02/06 14:26:34]

Done.

+  }
+  addError(error, stackTrace) => controller.addError(error, stackTrace);
+  close() => controller.close();
+
+  AsyncStarStreamController(helperCallback) {
+    controller = new StreamController(
+      onResume: () {
+        if (!isAdding) {

[floitsch, 2015/02/05 20:17:23]

Check with Lasse, if 'isPaused' on the controller is as good as having an
"onPause" callback.

[floitsch, 2015/02/05 20:17:23]

Add comment.

[sigurdm, 2015/02/06 14:26:34]

Done.

[sigurdm, 2015/02/06 14:26:34]

According to him, it is just as good.

+          streamHelper(null, helperCallback, this, null);
+        }
+      }, onCancel: () {
+        stopRunning = true;

[floitsch, 2015/02/05 20:17:23]

Please make sure (test), that an incoming stream is correctly canceled. That is,
if you cancel during a yield* that the source-stream is canceled.

[sigurdm, 2015/02/06 14:26:34]

Modified asyncstar_yieldstar_test to do that.

+      });
+  }
+}
+
+makeAsyncStarController(helperCallback) {
+  return new AsyncStarStreamController(helperCallback);
+}
+
+Function _wrapJsFunctionForStream(dynamic /* js function */ function,
+                                  AsyncStarStreamController controller) {
+  return (result) {
+    try {
+      JS('', '#(#)', function, result);
+    } catch (e, st) {
+      controller.addError(e, st);
+    }
+  };
+}
+
+
+class IterationMarker {
+  static const YIELD_SINGLE = 0;
+  static const YIELD_STAR = 1;
+  static const ITERATION_ENDED = 2;
+
+  final value;
+  final int state;
+
+  IterationMarker._(this.state, this.value);
+
+  static yieldStar(dynamic /* Iterable or Stream */ yielded) {

[floitsch, 2015/02/05 20:17:23]

maybe just "values" ?

[sigurdm, 2015/02/06 14:26:34]

Done.

+    return new IterationMarker._(YIELD_STAR, yielded);
+  }
+
+  static endOfIteration() {
+    return new IterationMarker._(ITERATION_ENDED, null);
+  }
+
+  static yieldSingle(dynamic value) {
+    return new IterationMarker._(YIELD_SINGLE, value);
+  }
+
+  toString() => "IterationMarker($state, $value)";
+}
+
+class SyncStarIterator implements Iterator {
+  final Function _helper;
+
+  // If [runningNested] this is the nested iterator, otherwise it is the
+  // current value.
+  dynamic _current = null;
+  bool _runningNested = false;
+
+  get current => _runningNested ? _current.current : _current;
+
+  SyncStarIterator(helper)
+      : _helper = ((arg) => JS('', '#(#)', helper, arg));
+
+  bool moveNext() {
+    if (_runningNested) {
+      if (_current.moveNext()) {
+        return true;
+      } else {
+        _runningNested = false;
+      }
+    }
+    _current = _helper(null);
+    if (_current is IterationMarker) {
+      if (_current.state == IterationMarker.ITERATION_ENDED) {
+        _current = null;
+        // Rely on [_helper] to repeatedly return `ITERATION_ENDED`.
+        return false;
+      } else {
+        assert(_current.state == IterationMarker.YIELD_STAR);
+        _current = _current.value.iterator;
+        _runningNested = true;
+        return moveNext();
+      }
+    }
+    return true;
+  }
+}
+
+/// An Iterable corresponding to a sync* method.
+///
+/// Each invocation of a sync* method will return a new instance of this class.
+class SyncStarIterable extends IterableBase {
+  // This is a function that will return a helper function that does the
+  // iteration of the sync*.
+  //
+  // Each invocation should give a helper with fresh state.
+  final dynamic /* js function */ _outerHelper;
+
+  SyncStarIterable(this._outerHelper);
+
+  Iterator get iterator => new SyncStarIterator(JS('', '#()', _outerHelper));
+}
+