EventLoop: timeout select action is thread unsafe (IOCP/libevent)

[ysbaddaden]

There's an MT error in the IOCP & libevent event loops in stdlib, when resolving select timeouts.

IOCP checks for event.timeout? and fiber.timeout_select_action together, and enqueues the fiber otherwise, while libevent only checks for fiber.timeout_select_action and enqueues otherwise.

Both don't enqueue the fiber when timeout_select_action is set but fail an atomic CAS.

Failing scenario

• thread A starts cancelling the timeout
• thread B dequeues the timeout event
• thread A sets timeout_select_action to nil
• thread B checks timeout_select_action (nil)
• thread A tries to dequeue timeout event (noop: already dequeued)
• thread A enqueues fiber
• thread B enqueues fiber :boom:

I believe this is only working in the current crystal releases because a fiber is tied to a thread and each thread has its own event loop, same if the channel sender/receiver fiber is resumed: they will be executed concurrently, not in parallel, so they can't fail.

But break any of these two assumptions (as we do in ExecutionContext) and :boom:

[ysbaddaden]

Given that we're dealing with threads, we very likely need Fiber#timeout_select_action to become an Atomic(Channel::TimeoutAction?).