condchan: simplify ChanPool

[bcmills]

I do not guarantee that this is free from bugs, but the general idea should be sound.

[bcmills]

I should note that this implementation slightly weakens the “Close should interrupt Get” property: it does reliably interrupt Get eventually, but pending Get calls will block until the number of active connections drops below the limit.

That could be fixed (at a small cost in LoC) by changing the closed field from a boolean to a channel.

[cespare]

That could be fixed (at a small cost in LoC) by changing the closed field from a boolean to a channel.

Can you describe that in a bit more detail? I don't see how it would work, since you can't select on a channel and a semaphore acquisition at the same time. (Or do you also mean spawning an extra goroutine per Get?)

[cespare]

Thanks for this, btw!

[bcmills]

I don't see how it would work, since you can't select on a channel and a semaphore acquisition at the same time.

Hmm, good point. Will think a bit and get back to you.

The cheating way to do it would be to store a context.Context in the Pool struct and use that instead of context.TODO(): then you can just cancel it in Close, but you can't use a real Context.

[bcmills]

The slightly-less-cheaty way to do it would be to add a Close method to *semaphore.Weighted that would terminate pending calls to Acquire (and cause future calls to Acquire and TryAcquire to fail).

[cespare]

Maybe we need semaphore.AcquireChan ;)

[bcmills]

I was wrong; it's actually shorter with closed as a channel. :)

[bcmills]

Maybe we need semaphore.AcquireChan ;)

That would be pretty dangerous: if the caller abandons the Acquire they need to give up their slot in the queue. (If we returned a channel they could wait on, we would also have to return an “abandon” callback, and it would be really easy to accidentally omit or duplicate the abandon() call.)

[cespare]

Hey, I like this one a lot more. Nice.

[cespare]

Point made. We have some internal connection pools which use a pattern like this these days :)