Merge pull request #331 from AArnott/fix209

Block transmission of frames regarding terminated channels

Author: AArnott

src/Nerdbank.Streams.Tests/MultiplexingStreamTests.cs

@@ -649,15 +649,15 @@ public async Task PartialFrameSentWithoutExplicitFlush()
     [InlineData(false)]
     public async Task CancelChannelOfferBeforeAcceptance(bool cancelFirst)
     {
-        // TODO: We need to test both the race condition where acceptance is sent before cancellation is received,
-        //       and the case where cancellation is received before we call AcceptChannelAsync.
         var cts = new CancellationTokenSource();
         var offer = this.mx1.OfferChannelAsync(string.Empty, cts.Token);
         cts.Cancel();
         await Assert.ThrowsAnyAsync<OperationCanceledException>(() => offer).WithCancellation(this.TimeoutToken);
         Stream? acceptedStream = null;
         try
         {
+            // We need to test both the race condition where acceptance is sent before cancellation is received,
+            // and the case where cancellation is received before we call AcceptChannelAsync.
             if (cancelFirst)
             {
                 // Increase the odds that cancellation will be processed before acceptance.

src/Nerdbank.Streams/MultiplexingStream.cs

@@ -1086,6 +1086,12 @@ private async Task SendFrameAsync(FrameHeader header, ReadOnlySequence<byte> pay
                     }
                     else
                     {
+                        if (!this.openChannels.ContainsKey(qualifiedChannelId))
+                        {
+                            // Drop frames regarding channels that are not open.
+                            return;
+                        }
+
                         if (this.channelsPendingTermination.Contains(qualifiedChannelId))
                         {
                             // We shouldn't ever send a frame about a channel after we've transmitted a ChannelTermination frame for it.