WebSocket: backend request left open when the client leaves before the upgrade completes #155

Version

5.2.1 (also current main)

Context

The client and backend sockets of a proxied WebSocket are only joined in ReverseProxy.Proxy#sendResponse once the
backend answers 101. Nothing watches the client connection before that, so a client that goes away during the
upgrade leaves the request to the backend open:

1. Backend has not answered yet: the backend connection stays open for as long as the backend does not answer
   (indefinitely for a backend that never answers).
2. Backend answers 101 after the client left: proxiedRequest().toNetSocket() fails, the failure branch prints
   Handle this case and a stack trace to System.err, and the backend socket is never closed.
3. Client already gone when the origin is resolved (e.g. an OriginRequestProvider doing an async lookup): the
   upgrade is still sent to the backend.

Proxied HTTP requests do not have this problem: ProxiedRequest#sendRequest resets the backend request from the
client response's exception handler. The WebSocket branch in ReverseProxy.Proxy#sendRequest calls
request.connect() directly and does not set it.

Steps to reproduce

1. Proxy to a backend that reads the upgrade request and never answers.
2. Send a WebSocket upgrade through the proxy, then close the client connection.
3. The backend connection is never closed.

Do you have a reproducer?

Yes, two tests in WebSocketTest in the linked PR; both fail on main (the first by timing out, the second with the
upgrade reaching the backend).