Use race against the AsyncContext timeout

[rossabaker]

Races the service against the async servlet context timeout.

• Eliminates a verbose and unsafe callback.
• Cancels a losing, potentially expensive response.

[rossabaker]

There is more we can do with Concurrent. This is my simple example to spur discussion. @jmcardon has expressed worry about it due to the inability to implement it in scalaz8 IO.

I think it's a non-starter to forgo the benefits of Concurrent altogether, and highly unpalatable to use something like the Priority pattern, requiring two paths. My hope is that all the effects people want to use can be made Concurrent, either through clever implementation or negotiation on the laws. Time is running very short on the latter for cats-effect-1.0.

[aeons]

The new implementation looks a lot nicer.

[jcranky]

Which F.never?

[rossabaker]

One that's coming in cats-effect-1.0.

[jcranky]

Ok, but from the comment I thought is was already being used here, and I don't see it anywhere 😀

[rossabaker]

Yeah. I wrote the comment before I coded the F.never and remembered we don't have it yet. Consider it a TODO to replace it. :)

[jmcardon]

We should leave it in IMHO. It will be never very soon.

[jmcardon]

So far this looks good to me.

Note re: ConcurrentEffect: Race is part of scalaz8 IO. It's definitely expressible in terms of it, it's the cancellable business that is not. The problem I have is with anything built off of cancellable and runCancellable, being that I still do believe it conflates resource safety with asynchrony really hard.

[jmcardon]

We should leave it in IMHO. It will be never very soon.

[rossabaker]

Hell. This one is broken by #1887, too.

Made some changes due to bitrot.