Functional impact

• Clients are unable to establish SignalR connections when someone attaches to the PreSendRequestHeaders event.

Minimal repro steps

• Install SignalR 2.1.0
• Attach to the PreSendRequestHeaders event.
• Create a PersistentConnection or Hub.
• Try to connect to the PersistentConnection or Hub with the JS or .NET client.

Expected result

• The /start request should complete immediately after OnConnected completes, and the SignalR client should move into the connected state.

Actual result

• 2.1.0 client: No transport can be initialized successfully, because all the transports time out waiting on the start request. After all the transports time out, the client moves to the disconnected state.
• Client after PR Initiate /start request after a transport is selected in the JS client #3105 is applied: The SignalR client hangs in the connecting state indefinitely waiting on the /start request to complete.

This issue was originally reported on the ASP.NET forums.