Return futures on Stream.cancel when possible.

sdk/lib/async/stream_transformers.dart

Index: sdk/lib/async/stream_transformers.dart
diff --git a/sdk/lib/async/stream_transformers.dart b/sdk/lib/async/stream_transformers.dart
index e0982da4260ec621621a82b526aa21dac87f2be4..32fe28dfd820fab22c8b71e0099104cc59a28a89 100644
--- a/sdk/lib/async/stream_transformers.dart
+++ b/sdk/lib/async/stream_transformers.dart
@@ -33,6 +33,9 @@ class _SinkTransformerStreamSubscription<S, T>
   /// The subscription to the input stream.
   StreamSubscription<S> _subscription;
 
+  /// The cancelFuture (if any).
+  Future _cancelFuture;

[Lasse Reichstein Nielsen, 2016/08/01 15:23:26]

Why a field? I only see it used inside one method?

[floitsch, 2016/08/01 21:00:37]

Acknowledged.

+
   _SinkTransformerStreamSubscription(Stream<S> source,
                                      _SinkMapper<S, T> mapper,
                                      void onData(T data),
@@ -109,9 +112,9 @@ class _SinkTransformerStreamSubscription<S, T>
     if (_isSubscribed) {
       StreamSubscription subscription = _subscription;
       _subscription = null;
-      subscription.cancel();
+      _cancelFuture = subscription.cancel();

[Lasse Reichstein Nielsen, 2016/08/01 15:23:26]

That is, why is _cancelFuture not a local variable here.

[floitsch, 2016/08/01 15:55:55]

In case someone calls `cancel` multiple times.

[Lasse Reichstein Nielsen, 2016/08/01 16:49:38]

Possibly a good point.
I'm not sure _onCancel will be called more than once, though.
In the default _BufferingStreamSubscription implementation (stream_impl.dart,
line 191) we check whether we are already canceled, and only call _cancel (which
calls _onCancel) if we arent'. We also call _cancel in _sendDone and _sendError
(if cancelOnError), which can also only happen once. The implementation buffers
the cancelFuture there, so you don't need to do it here.

[floitsch, 2016/08/01 21:00:37]

You are right.
Changed a bit more...

     }
-    return null;
+    return _cancelFuture ?? new Future.value(null);
   }
 
   void _handleData(S data) {