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..e9407bb846453897a7f89d2a3faaea21292ca41a 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -25,7 +25,13 @@ import 'dart:_isolate_helper' show
     enterJsAsync,
     isWorker;
 
-import 'dart:async' show Future, DeferredLoadException, Completer;
+import 'dart:async'
+  show Future,
+  DeferredLoadException,
+  Completer,
+  StreamController,
+  Stream,
+  StreamSubscription;
 
 import 'dart:_foreign_helper' show
     DART_CLOSURE_TO_JS,
@@ -3462,3 +3468,262 @@ 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(_wrapJsFunction(helperCallback, completer),
+              onError: _wrapJsFunction(errorCallback, completer));

[floitsch, 2015/02/04 12:31:29]

this loses the stacktrace.

[sigurdm, 2015/02/05 14:06:06]

True - this won't work. We have to find a good solution. The problem is the
tension between js's Error with stacktrace and Darts's error next to stacktrace.

+  return completer.future;
+}
+
+// Used to avoid creating two identical closure classes in [thenHelper].

[floitsch, 2015/02/04 12:31:29]

Remove comment.

[sigurdm, 2015/02/05 14:06:05]

Done.

+Function _wrapJsFunction(dynamic /* js function */ function,

[floitsch, 2015/02/04 12:31:29]

name is too general.

[sigurdm, 2015/02/05 14:06:05]

Made it more specific.

+                         Completer completer) {
+  return (result) {
+    try {
+      JS('', '#(#)', function, result);
+    } catch (e, st) {
+      completer.completeError(e, st);
+    }
+  };
+}
+
+
+/// Runtime support for async*.
+///
+/// Called by the transformed function for each original return, await, yield,

[floitsch, 2015/02/04 12:31:29]

Start by explaining what it does, not who uses it.

"Implements the runtime support for async* functions". (or better).

[sigurdm, 2015/02/05 14:06:05]

Done.

+/// yield* and before starting the function.
+///
+/// On a return call with helperCallback == null, and we close the stream.

[floitsch, 2015/02/04 12:31:29]

?

[sigurdm, 2015/02/05 14:06:06]

Reworded

+///
+/// If object is an [IterationMarker] it indicates a yield or yield*.

[floitsch, 2015/02/04 12:31:29]

what indicates?

[floitsch, 2015/02/04 12:31:29]

reference [object]

[sigurdm, 2015/02/05 14:06:05]

Done.

[sigurdm, 2015/02/05 14:06:05]

Done.

[sigurdm, 2015/02/05 14:06:06]

Tried to reword.

+/// If the stream subscription has been canceled we schedule the errorCallback

[floitsch, 2015/02/04 12:31:29]

don't use "we" (here and in the rest of the documentation).
If the stream subscription has been canceled, schedules the errorCallback ...

[floitsch, 2015/02/04 12:31:29]

has been canceled when?

[floitsch, 2015/02/04 12:31:29]

new line before? or "In that case..." ?

[sigurdm, 2015/02/05 14:06:05]

Done.

[sigurdm, 2015/02/05 14:06:06]

I will try to avoid it in the future. I use it to avoid too much passive.
done

+/// that should run through enclosing finally blocks before exiting.

[floitsch, 2015/02/04 12:31:29]

that feels weird. A 'cancel' is not an error. Why do we call the errorCallback?
If cancel is like an error (for implementation purposes), mention that.

[sigurdm, 2015/02/05 14:06:05]

errorCallback is used for running the finallies in the case of a yield if there
has been a cancel. I tried to explain it better.

+///
+/// If it is a single-yield we add the value to the stream.

[floitsch, 2015/02/04 12:31:29]

If what is a single-yield? I assume it's the [object], but it's not clear.

[sigurdm, 2015/02/05 14:06:05]

Reworded

+/// If the stream subscription has been paused we return early, otherwise
+/// we schedule the helper function to be executed again.
+///
+/// If it is a yield* we start listening to the 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 we schedule the
+/// helper function again.
+///
+/// If object is not and [IterationMarker] it indicates an await.

[floitsch, 2015/02/04 12:31:29]

not what?
Reference [object].

(Whenever you would want dartdoc to be highlighted when hovering over an
argument you should reference the argument)

[sigurdm, 2015/02/05 14:06:04]

Acknowledged.

+/// 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
+/// error
+///
+/// If helperCallback or errorCallback throws we add the error to the stream
+/// If the object is a YIELD_SINGLE we add the object to the stream, and if the
+///    stream has been canceled call the errorCallBack.
+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;
+  }
+
+  if (object is IterationMarker) {
+    if (controller.stopRunning) {
+      _wrapJsFunctionForStream(errorCallback, controller)();
+      return;
+    }
+    if (object.state == IterationMarker.YIELD_SINGLE) {
+      controller.add(object.value);
+    } else if (object.state == IterationMarker.YIELD_STAR) {
+      Stream stream = object.value;

[floitsch, 2015/02/04 12:31:29]

can't you just controller.addStream(object.value).then((...)) ?

[sigurdm, 2015/02/05 14:06:04]

Brilliant - had not seen that!

+      controller.subSubscription = stream.listen((value) {
+        if (controller.stopRunning) {
+          controller.subSubscription.cancel();
+          _wrapJsFunctionForStream(errorCallback, controller)(null);
+          return;
+        }
+        controller.add(value);
+      }, onError: (error, stackTrace) {
+        if (controller.stopRunning) {
+          controller.subSubscription.cancel();
+          // This will run the enclosing finally blocks.
+          _wrapJsFunctionForStream(errorCallback, controller)(null);
+          return;
+        }
+        controller.addError(error, stackTrace);
+        if (controller.isPaused) {
+          controller.subSubscription.pause();
+        }
+      }, onDone: () {
+        controller.subSubscription = null;
+        // Resume producing.
+        streamHelper(null, helperCallback, controller, null);
+      });
+      return;
+    }
+    if (controller.isPaused) {
+      return;
+    }
+    object = 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;
+  StreamSubscription subSubscription = null;
+  Stream get stream => controller.stream;
+  bool stopRunning = false;
+  bool get isPaused => controller.isPaused;
+  add(event) => controller.add(event);
+  addError(error, stackTrace) => controller.addError(error, stackTrace);
+  close() => controller.close();
+
+  AsyncStarStreamController(helperCallBack) {
+    controller = new StreamController(
+      onPause: () {
+        if (subSubscription != null) {
+          subSubscription.pause();
+        }
+      },
+      onResume: () {
+        if (subSubscription == null) {
+          helperCallBack(null, helperCallBack, controller, null);
+        } else {
+          subSubscription.resume();
+        }
+      }, onCancel: () {
+        stopRunning = true;
+      });
+  }
+}
+
+makeAsyncStarController(helperCallback) {
+  return new AsyncStarStreamController(helperCallback);
+}
+
+// Used to avoid creating identical closure classes in [streamHelper].

[floitsch, 2015/02/04 12:31:29]

Remove comment.

[sigurdm, 2015/02/05 14:06:04]

Done.

+Function _wrapJsFunctionForStream(dynamic /* js function */ function,
+                                  AsyncStarStreamController controller) {
+  return (result) {
+    try {
+      JS('', '#(#)', function, result);
+    } catch (e, st) {

[floitsch, 2015/02/04 12:31:29]

I believe that errors in async* are fatal.
Maybe Lasse or Kevin knows (without needing to go through the spec).

[sigurdm, 2015/02/05 14:06:05]

Acknowledged.
We will have to find out.

+      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(Iterable iterable) {
+    return new IterationMarker._(YIELD_STAR, iterable);
+  }
+
+  static endOfIteration() {
+    return new IterationMarker._(ITERATION_ENDED, null);
+  }
+
+  static yieldSingle(value) {
+    return new IterationMarker._(YIELD_SINGLE, value);
+  }
+
+  toString() => "IterationMarker($state, $value)";
+}
+
+class SyncStarIterator implements Iterator {
+  final Function _helper;
+
+  // if [runningNested] this will be the nested iterator, otherwise it will be

[floitsch, 2015/02/04 12:31:29]

no need for future tense.
Start sentence with capital character.

[sigurdm, 2015/02/05 14:06:06]

Done.

+  // 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;
+        // We 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));
+}
+