Don't throw from Observer's OnNext (#1872)

src/Microsoft.AspNetCore.SignalR.Core/Internal/AsyncEnumeratorAdapters.cs

@@ -15,22 +15,35 @@ namespace Microsoft.AspNetCore.SignalR.Internal
         {
             // TODO: Allow bounding and optimizations?
             var channel = Channel.CreateUnbounded<object>();
+            var observer = new ObserverState();
+            var channelObserver = new ChannelObserver<T>(channel.Writer);
+            observer.Subscription = observable.Subscribe(channelObserver);
+            observer.TokenRegistration = cancellationToken.Register(obs => ((ChannelObserver<T>)obs).OnCompleted(), channelObserver);
 
-            var subscription = observable.Subscribe(new ChannelObserver<T>(channel.Writer, cancellationToken));
+            // Make sure the subscription and token registration is disposed when enumeration is completed.
+            return new AsyncEnumerator<object>(channel.Reader, cancellationToken, observer);
+        }
 
-            // Make sure the subscription is disposed when enumeration is completed.
-            return new AsyncEnumerator<object>(channel.Reader, cancellationToken, subscription);
+        // To track and dispose of the Subscription and the cancellation token registration.
+        private class ObserverState : IDisposable
+        {
+            public CancellationTokenRegistration TokenRegistration;
+            public IDisposable Subscription;
+
+            public void Dispose()
+            {
+                TokenRegistration.Dispose();
+                Subscription.Dispose();
+            }
         }
 
         private class ChannelObserver<T> : IObserver<T>
         {
             private readonly ChannelWriter<object> _output;
-            private readonly CancellationToken _cancellationToken;
 
-            public ChannelObserver(ChannelWriter<object> output, CancellationToken cancellationToken)
+            public ChannelObserver(ChannelWriter<object> output)
             {
                 _output = output;
-                _cancellationToken = cancellationToken;
             }
 
             public void OnCompleted()
@@ -45,12 +58,6 @@ namespace Microsoft.AspNetCore.SignalR.Internal
 
             public void OnNext(T value)
             {
-                if (_cancellationToken.IsCancellationRequested)
-                {
-                    // Noop, someone else is handling the cancellation
-                    return;
-                }
-
                 // This will block the thread emitting the object if the channel is bounded and full
                 // I think this is OK, since we want to push the backpressure up. However, we may need
                 // to find a way to force the entire subscription off to a dedicated thread in order to
@@ -60,10 +67,10 @@ namespace Microsoft.AspNetCore.SignalR.Internal
                 while (!_output.TryWrite(value))
                 {
                     // Wait for a spot
-                    if (!_output.WaitToWriteAsync(_cancellationToken).Result)
+                    if (!_output.WaitToWriteAsync().Result)
                     {
-                        // Channel was closed.
-                        throw new InvalidOperationException("Output channel was closed");
+                        // Channel was closed so we just no-op. The observer shouldn't throw.
+                        return;
                     }
                 }
             }

test/Microsoft.AspNetCore.SignalR.Tests/HubConnectionHandlerTests.cs

@@ -148,6 +148,49 @@ namespace Microsoft.AspNetCore.SignalR.Tests
             }
         }
 
+        [Fact]
+        public async Task OberserverDoesntThrowWhenOnNextIsCalledAfterChannelIsCompleted()
+        {
+            var observable = new Observable<int>();
+            var serviceProvider = HubConnectionHandlerTestUtils.CreateServiceProvider(s => s.AddSingleton(observable));
+            var connectionHandler = serviceProvider.GetService<HubConnectionHandler<ObservableHub>>();
+
+            var waitForSubscribe = new TaskCompletionSource<object>();
+            observable.OnSubscribe = o =>
+            {
+                waitForSubscribe.TrySetResult(null);
+            };
+
+            var waitForDispose = new TaskCompletionSource<object>();
+            observable.OnDispose = o =>
+            {
+                waitForDispose.TrySetResult(null);
+            };
+
+            using (var client = new TestClient())
+            {
+                var connectionHandlerTask = await client.ConnectAsync(connectionHandler);
+
+                var subscribeTask = client.StreamAsync(nameof(ObservableHub.Subscribe));
+
+                await waitForSubscribe.Task.OrTimeout();
+
+                Assert.Single(observable.Observers);
+
+                // Disposing the client to complete the observer. Further calls to OnNext should no-op
+                client.Dispose();
+
+                // Calling OnNext after the client has disconnected shouldn't throw.
+                observable.OnNext(1);
+
+                await waitForDispose.Task.OrTimeout();
+
+                Assert.Empty(observable.Observers);
+
+                await connectionHandlerTask.OrTimeout();
+            }
+        }
+
         [Fact]
         public async Task ObservableHubRemovesSubscriptions()
         {