Use crossbeam-channel instead of stdlib

[matklad]

Hi! My understanding is that currently notify hard-codes the usage of std::mpsc, and this is a bit unfortunate: there are many channel implementations in Rust (crossbeam-channel most notably), and the std version is not really great: it lacks select, which severally restricts its applicability.

It would be cool if notify had a trait EventSink { fn send(&mut self, Event); } and used that instead of any particular channel implementation.

Though, for practical purposes, perhaps just betting on crossbeam-channel would be even better?

[passcod]

v4 is in maintenance only, v5 (aiming for end of this year) uses a significantly different architecture based on Tokio and Futures. Internally, v5 uses multiqueue channels for some plumbing which I believe use crossbeam. Externally, in v5 you'll just subscribe to a Stream which will plug into the kernel eventing as appropriate.

[matklad]

Excellent, that should be even better! Thanks!

[netvl]

If v5 is still some time away, are there any chances for this to be implemented in v4?

[passcod]

Hmm, I'll take an implementation if someone wants to.

The remaining question I think would be whether to be generic over only the channel exposed through the public API (which would be simpler), or to be generic over every channel use inside this library (which would likely help the performance characteristic further than just the last hop, varying on internals).

[passcod]

Shifting this to be purely crossbeam-channel as originally alternately proposed. If there's interest in making it generic I'm happy to take an implementation later.