Race condition when running the `StdioServerTransportTests.SendMessageAsync_Should_Send_Message` test

[eiriktsarpalis]

There is a race between the StdioServerTransport.StartListeningAsync() and StdioServerTransport.SendMessageAsync calls since the former spins up a task that switches off the IsConnected flag before SendMessageAsync can finish sending a message. I can only reproduce this in CI.

[xUnit.net 00:00:07.02]     ModelContextProtocol.Tests.Transport.StdioServerTransportTests.SendMessageAsync_Should_Send_Message [FAIL]
  Failed ModelContextProtocol.Tests.Transport.StdioServerTransportTests.SendMessageAsync_Should_Send_Message [11 ms]
  Error Message:
   ModelContextProtocol.Protocol.Transport.McpTransportException : Transport is not connected
  Stack Trace:
     at ModelContextProtocol.Protocol.Transport.StdioServerTransport.SendMessageAsync(IJsonRpcMessage message, CancellationToken cancellationToken) in /home/runner/work/csharp-sdk/csharp-sdk/src/ModelContextProtocol/Protocol/Transport/StdioServerTransport.cs:line 109
   at ModelContextProtocol.Tests.Transport.StdioServerTransportTests.SendMessageAsync_Should_Send_Message() in /home/runner/work/csharp-sdk/csharp-sdk/tests/ModelContextProtocol.Tests/Transport/StdioServerTransportTests.cs:line 75
   at ModelContextProtocol.Tests.Transport.StdioServerTransportTests.SendMessageAsync_Should_Send_Message() in /home/runner/work/csharp-sdk/csharp-sdk/tests/ModelContextProtocol.Tests/Transport/StdioServerTransportTests.cs:line 80

[willibrandon]

Hi @eiriktsarpalis,

While working on the UTF-8 encoding fix in #73, I noticed that we made some changes to the transport connection handling. There's a chance this might have inadvertently addressed this race condition since we improved the connection state management. Would you mind checking if this issue is still reproducible with the latest changes from #73?

Let me know if you'd like me to do any additional testing or make further improvements to help resolve this.

[stephentoub]

The changes I made in the additional commits in #73 will help to mitigate the way this particular issue surfaces, but the underlying problem is still there and can manifest in other ways. The cleanup functionality should only ever be invoked once, but it's currently being invoked multiple times, possibly in parallel, e.g.

csharp-sdk/src/ModelContextProtocol/Protocol/Transport/StdioClientTransport.cs

Line 228 in 729e688

await CleanupAsync(cancellationToken).ConfigureAwait(false);

csharp-sdk/src/ModelContextProtocol/Protocol/Transport/StdioClientTransport.cs

Line 185 in 729e688

await CleanupAsync(CancellationToken.None).ConfigureAwait(false);

and that needs to be fixed.

[eiriktsarpalis]

@willibrandon I see you unskipped the impacted test in your PR, if the issue is still there we'll be seeing it in CI builds.