Do not propagate a CancellationToken from a method arg to a long-live…

…d state machine

This fixes #414, which is a hang that occurs when the `CancellationToken` passed to `MultiplexingStream.CreateAsync` is cancelled after `CreateAsync` has completed.

src/Nerdbank.Streams.Tests/MultiplexingStreamTests.cs

@@ -307,6 +307,25 @@ public async Task CreateChannelAsync_CanceledBeforeAcceptance()
         await Assert.ThrowsAnyAsync<OperationCanceledException>(() => channel1Task).WithCancellation(this.TimeoutToken);
     }
 
+    [Fact]
+    public async Task CreateAsync_CancellationToken()
+    {
+        var cts = new CancellationTokenSource();
+        var streamPair = FullDuplexStream.CreatePair();
+        Task<MultiplexingStream> mx1Task = MultiplexingStream.CreateAsync(streamPair.Item1, new MultiplexingStream.Options { ProtocolMajorVersion = this.ProtocolMajorVersion }, cts.Token);
+        Task<MultiplexingStream> mx2Task = MultiplexingStream.CreateAsync(streamPair.Item2, new MultiplexingStream.Options { ProtocolMajorVersion = this.ProtocolMajorVersion }, CancellationToken.None);
+        MultiplexingStream mx1 = await mx1Task;
+        MultiplexingStream mx2 = await mx2Task;
+
+        // At this point the cancellation token really shouldn't have any effect on mx1 now that the connection is established.
+        cts.Cancel();
+
+        Task<MultiplexingStream.Channel> ch1Task = mx1.OfferChannelAsync(string.Empty, this.TimeoutToken);
+        Task<MultiplexingStream.Channel> ch2Task = mx2.AcceptChannelAsync(string.Empty, this.TimeoutToken);
+
+        await Task.WhenAll(ch1Task, ch2Task).WithCancellation(this.TimeoutToken);
+    }
+
     [Fact]
     public async Task CreateChannelAsync()
     {

src/Nerdbank.Streams/MultiplexingStream.cs

@@ -284,7 +284,9 @@ public static async Task<MultiplexingStream> CreateAsync(Stream stream, Options?
                 throw new NotSupportedException(Strings.SeededChannelsRequireV3Protocol);
             }
 
-            var streamWriter = stream.UsePipeWriter(cancellationToken: cancellationToken);
+            // Do NOT specify our own cancellationToken parameter in UsePipeWriter, since this PipeWriter
+            // must outlive this method and therefore should not be canceled later if that token is eventually canceled.
+            var streamWriter = stream.UsePipeWriter(cancellationToken: CancellationToken.None);
 
             var formatter = options.ProtocolMajorVersion switch
             {