state make more threadsafe + close started chanel

[tantra35]

When i begin use this library i found 2 issues:

1. state in playback not thread safe
2. started chanel must be closed when Close method of playback is invoked, anotherway we can get deadlock with Start method

[tantra35]

Another PR #36

partialy make the same, it only maek state as sync.Atomic, but not close started chanell

[jfreymuth]

I've merged a different PR for the data race on PlaybackStream.state. I'm not sure I understand what the deadlock is, can you provide an example?

[tantra35]

@jfreymuth Ok I wil try to explain:

1. We neeed to close started chanel https://github.com/jfreymuth/pulse/blob/master/playback.go#L20, in Close method, because this can reduce problems when We call Start simultaneously(we never hung in <-p.started code - https://github.com/jfreymuth/pulse/blob/master/playback.go#L198C3-L198C14) To tell the truth, it won't matter if all operations with the state are done atomically. But for semantic accuracy, it's still worth correcting.

2. Aproach that you accept is naive. For example Look at Start method, It racy because state doesn't changes atomically, more correct if you doesn't want use atomic, use mutex on whole method Start, or with use of atomic fix it like this:

func (p *PlaybackStream) Start() {
	if p.state.CompareAndSwap(int32(idle), int32(running)) {  // we go here only when succesfuly chnage state from idle to running, and run rest of code only in this case
		p.c.c.Request(&proto.FlushPlaybackStream{StreamIndex: p.index}, nil)
		p.err = nil
		p.request <- int(p.createReply.BufferTargetLength)
		p.underflow = false
		p.c.c.Request(&proto.CorkPlaybackStream{StreamIndex: p.index, Corked: false}, nil)
		<-p.started
	}
}

but start now looks like this

func (p *PlaybackStream) Start() {
	if p.state.is(idle) { // <------
		p.c.c.Request(&proto.FlushPlaybackStream{StreamIndex: p.index}, nil) 
		p.state.set(running) // <--- between marked calls we have undefined behaviour in other goroutines, because state doesn't changes atomically
		p.err = nil
		p.request <- int(p.createReply.BufferTargetLength)
		p.underflow = false
		p.c.c.Request(&proto.CorkPlaybackStream{StreamIndex: p.index, Corked: false}, nil)
		<-p.started
	}
}

[aykevl]

I made a similar PR a while ago, but didn't make it ready yet. It has a similar approach to this PR.
Code: db8ebac