Cancel GroupBy when all substreams cancel

(cherry picked from commit 4584f59)

Author: ismaelhamed

src/core/Akka.Streams.Tests/Dsl/FlowGroupBySpec.cs

@@ -574,6 +574,50 @@ public void GroupBy_must_work_if_pull_is_exercised_from_multiple_substreams_whil
             }, Materializer);
         }
 
+        [Fact]
+        public void GroupBy_must_cancel_if_downstream_has_cancelled_and_all_substreams_cancel()
+        {
+            this.AssertAllStagesStopped(() =>
+            {
+                var upstream = this.CreatePublisherProbe<int>();
+                var downstreamMaster = this.CreateSubscriberProbe<Source<int, NotUsed>>();
+
+                Source.FromPublisher(upstream)
+                    .Via(new GroupBy<int, int>(10, element => element))
+                    .RunWith(Sink.FromSubscriber(downstreamMaster), Materializer);
+
+                var substream1 = this.CreateSubscriberProbe<int>();
+                downstreamMaster.Request(1);
+                upstream.SendNext(1);
+                downstreamMaster.ExpectNext().RunWith(Sink.FromSubscriber(substream1), Materializer);
+
+                var substream2 = this.CreateSubscriberProbe<int>();
+                downstreamMaster.Request(1);
+                upstream.SendNext(2);
+                downstreamMaster.ExpectNext().RunWith(Sink.FromSubscriber(substream2), Materializer);
+
+                // Cancel downstream
+                downstreamMaster.Cancel();
+
+                // Both substreams still work
+                substream1.Request(1);
+                substream1.ExpectNext(1);
+                substream2.Request(1);
+                substream2.ExpectNext(2);
+
+                // New keys are ignored
+                upstream.SendNext(3);
+                upstream.SendNext(4);
+
+                // Cancel all substreams
+                substream1.Cancel();
+                substream2.Cancel();
+
+                // Upstream gets cancelled
+                upstream.ExpectCancellation();
+            }, Materializer);
+        }
+
         [Fact]
         public void GroupBy_must_work_with_random_demand()
         {

src/core/Akka.Streams/Implementation/Fusing/StreamOfStreams.cs

@@ -15,7 +15,6 @@
 using Akka.Streams.Implementation.Stages;
 using Akka.Streams.Stage;
 using Akka.Streams.Supervision;
-using Akka.Streams.Util;
 using Akka.Util;
 using Akka.Util.Internal;
 
@@ -464,9 +463,7 @@ public void OnUpstreamFinish()
 
             public void OnDownstreamFinish()
             {
-                if (_activeSubstreams.Count == 0)
-                    CompleteStage();
-                else
+                if (!TryCancel()) 
                     SetKeepGoing(true);
             }
 
@@ -493,6 +490,18 @@ private bool TryCompleteAll()
                 return false;
             }
 
+            private bool TryCancel()
+            {
+                // if there's no active substreams or there's only one but it's not been pushed yet
+                if (_activeSubstreams.Count == 0 || (_activeSubstreams.Count == 1 && _substreamWaitingToBePushed.HasValue))
+                {
+                    CompleteStage();
+                    return true;
+                }
+
+                return false;
+            }
+
             private void Fail(Exception ex)
             {
                 foreach (var value in _activeSubstreams.Values)
@@ -602,15 +611,12 @@ public void OnPull()
 
                 public void OnDownstreamFinish()
                 {
-                    if(_logic.HasNextElement && _logic._nextElementKey.Equals(Key))
-                        _logic.ClearNextElement();
-                    if (FirstPush)
-                        _logic._firstPushCounter--;
+                    if(_logic.HasNextElement && _logic._nextElementKey.Equals(Key)) _logic.ClearNextElement();
+                    if (FirstPush) _logic._firstPushCounter--;
                     CompleteSubStream();
-                    if (_logic.IsClosed(_logic._stage.In))
-                        _logic.TryCompleteAll();
-                    else if (_logic.NeedToPull)
-                        _logic.Pull(_logic._stage.In);
+                    if (_logic.IsClosed(_logic._stage.Out)) _logic.TryCancel();
+                    if (_logic.IsClosed(_logic._stage.In)) _logic.TryCompleteAll(); 
+                    else if (_logic.NeedToPull) _logic.Pull(_logic._stage.In);
                 }
             }
         }