(bug) Possible deadlock in the websocket service

[Vizualni]

Observed behaviour

Apologizes for not testing this, but rather I am simply reporting it as I've noticed this while exploring the codebase. It seems that there might be a deadlocking situation happening in the websocket service.

go-feature-flag/cmd/relayproxy/service/websocket.go

Lines 42 to 51 in 7eeeaf1

	func (w *websocketServiceImpl) BroadcastFlagChanges(diff notifier.DiffCache) {
	w.mutex.RLock()
	defer w.mutex.RUnlock()
	for c := range w.clients {
	err := c.WriteJSON(diff)
	if err != nil {
	w.Deregister(c)
	}
	}
	}

In the BroadcastFlagChanges we are RLock-ing the mutex, but if writing the message to a client fails, we will Deregister them, and the Deregister is getting the Lock:

go-feature-flag/cmd/relayproxy/service/websocket.go

Lines 61 to 65 in 7eeeaf1

	func (w *websocketServiceImpl) Deregister(c WebsocketConn) {
	w.mutex.Lock()
	defer w.mutex.Unlock()
	delete(w.clients, c)
	}

As you can't upgrade the RWLock, this can block the whole server.

Expected Behavior

No deadlock to happen.

Steps to reproduce

No response

[thomaspoignant]

Thanks @Vizualni for the issue, I've been able to reproduce the deadlock in a unit test.
I've open this PR #3146 to fix the issue, I would appreciate a quick look at the fix before I merge the PR.