MultiplexingStream.Dispose races with itself and throws NRE

[AArnott]

When calling MultiplexingStream.Dispose I witnessed an NRE with this callstack:

 	mscorlib.dll!System.IO.BufferedStream.Dispose(bool disposing) Line 275	C#
 	mscorlib.dll!System.IO.Stream.Close() Line 249	C#
 	mscorlib.dll!System.IO.Stream.Dispose() Line 261	C#
 	Microsoft.VisualStudio.LiveShare.Rpc.Json.dll!Microsoft.Cascade.Rpc.Json.SafeWriteBufferedStream.DisposeWritable() Line 46	C#
 	Microsoft.VisualStudio.LiveShare.Rpc.Json.dll!Microsoft.Cascade.Rpc.Json.WrappedStream.Dispose(bool disposing) Line 104	C#
 	mscorlib.dll!System.IO.Stream.Close() Line 249	C#
 	mscorlib.dll!System.IO.Stream.Dispose() Line 261	C#
 	Nerdbank.Streams.dll!Nerdbank.Streams.MultiplexingStream.Dispose(bool disposing)	Unknown
 	Nerdbank.Streams.dll!Nerdbank.Streams.MultiplexingStream.Dispose()	Unknown
>	Microsoft.VisualStudio.Shell.UI.Internal.dll!Microsoft.VisualStudio.Services.LiveShareServiceBrokerIntegration.ServiceBrokerHost.CreateServiceAsync.AnonymousMethod__1() Line 245	C#

The NRE would be caused by calling BufferedStream.Dispose concurrently on two threads. Could MultiplexingStream be responsible for calling Dispose on itself from another thread while we are calling Dispose on it? If so, we should ensure it's safe to call Dispose on it explicitly.

[AArnott]

Yes: MultiplexingStream calls dispose on itself:

https://github.com/AArnott/Nerdbank.Streams/blob/f751349ae7f5e9b68258d307fe71def015f39bc2/src/Nerdbank.Streams/MultiplexingStream.cs#L726

What's more, it could protect the underlying stream from being disposed on two threads by just rearranging the order of these two lines:

https://github.com/AArnott/Nerdbank.Streams/blob/f751349ae7f5e9b68258d307fe71def015f39bc2/src/Nerdbank.Streams/MultiplexingStream.cs#L541-L542

[AArnott]

In fact I even caught the self-closing callstack as the NRE throwing stack in another occurrence:

>	mscorlib.dll!System.IO.BufferedStream.Dispose(bool disposing) Line 275	C#
 	mscorlib.dll!System.IO.Stream.Close() Line 249	C#
 	mscorlib.dll!System.IO.Stream.Dispose() Line 261	C#
 	Microsoft.VisualStudio.LiveShare.Rpc.Json.dll!Microsoft.Cascade.Rpc.Json.SafeWriteBufferedStream.DisposeWritable() Line 46	C#
 	Microsoft.VisualStudio.LiveShare.Rpc.Json.dll!Microsoft.Cascade.Rpc.Json.WrappedStream.Dispose(bool disposing) Line 104	C#
 	mscorlib.dll!System.IO.Stream.Close() Line 249	C#
 	mscorlib.dll!System.IO.Stream.Dispose() Line 261	C#
 	Nerdbank.Streams.dll!Nerdbank.Streams.MultiplexingStream.Dispose(bool disposing) Line 542	C#
 	Nerdbank.Streams.dll!Nerdbank.Streams.MultiplexingStream.ReadStreamAsync() Line 726	C#