Fix StdioClientSessionTransport shutdown

StreamClientSessionTransport's ReadMessageAsync method does await CleanupAsync in its finally. CleanupAsync awaits _readTask. _readTask _is_ ReadMessageAsync. So it's waiting for itself to complete, which will then always timeout. We may want to refactor how this works more in the future, but for now, make sure it doesn't wait for itself.

Author: stephentoub

src/ModelContextProtocol/Protocol/Transport/StreamClientSessionTransport.cs

@@ -29,9 +29,16 @@ public StreamClientSessionTransport(
         _serverInput = serverInput;
         EndpointName = endpointName;
 
-        // Start reading messages in the background
+        // Start reading messages in the background. We use the rarer pattern of new Task + Start
+        // in order to ensure that the body of the task will always see _readTask initialized.
+        // It is then able to reliably null it out on completion.
         Logger.TransportReadingMessages(endpointName);
-        _readTask = Task.Run(() => ReadMessagesAsync(_shutdownCts.Token), CancellationToken.None);
+        var readTask = new Task<Task>(
+            thisRef => ((StreamClientSessionTransport)thisRef!).ReadMessagesAsync(_shutdownCts.Token), 
+            this,
+            TaskCreationOptions.DenyChildAttach);
+        _readTask = readTask.Unwrap();
+        readTask.Start();
 
         SetConnected(true);
     }
@@ -117,6 +124,7 @@ private async Task ReadMessagesAsync(CancellationToken cancellationToken)
         }
         finally
         {
+            _readTask = null;
             await CleanupAsync(cancellationToken).ConfigureAwait(false);
         }
     }