Fix Association and Stream closure

[mafredri]

Description

• Always close Association on writeLoop exit

The connection will now always be closed on writeLoop exit because it
will ensure that readLoop exits, which is needed to propagate the
closing of Streams.

• Guard against creating Streams after Association close

It was possible for new Streams to be created after readLoop has
exited and called unregisterStream on the existing ones. The new
Streams would never close.

This also guards against a potential panic due to send on nil channel
(acceptCh).

Reference issue

Fixes pion/webrtc#2098 (with high probability)

Notes

Included a few fixes to tests to improve stability when run with go test -race.

[Sean-Der]

Really great job @mafredri ! Would you mind squashing these into one commit?

I will add you to the Pion org so your tests will always run. @enobufs does this match with the patterns already?

[codecov]

Codecov Report

Merging #236 (a0a2c67) into master (d0b7cf3) will increase coverage by 0.00%.
The diff coverage is 73.33%.

❗ Current head a0a2c67 differs from pull request most recent head e4d7a2f. Consider uploading reports for the commit e4d7a2f to get more accurate results

@@           Coverage Diff           @@
##           master     #236   +/-   ##
=======================================
  Coverage   80.86%   80.87%           
=======================================
  Files          48       48           
  Lines        3962     3968    +6     
=======================================
+ Hits         3204     3209    +5     
- Misses        619      620    +1     
  Partials      139      139           

Flag	Coverage Δ
go	80.87% <73.33%> (+<0.01%)	⬆️
wasm	66.75% <46.66%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
association.go	84.68% <73.33%> (-0.01%)	⬇️

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[mafredri]

@Sean-Der sure thing, done!

[forcom]

This PR breaks some tests in pion/webrtc.
The error message in the broken tests is datachannel_test.go:26: Failed to close PeerConnection: dtls fatal: conn is closed.
In the situation that the association is disconnected without calling Close(), when (*Association).Close() is called explicitly, the double closure of netConn emits such error.

I suggest that make (*Association).close() be called only once like below.

func (a *Association) close() error {
	a.log.Debugf("[%s] closing association..", a.name)

	a.closeOnce.Do(func() {
		a.setState(closed)

		err := a.netConn.Close()
		if err != nil {
			a.closeErr.Store(err)
		}

		a.closeAllTimers()

		// awake writeLoop to exit
		a.closeWriteLoopOnce.Do(func() { close(a.closeWriteLoopCh) })
	})

	err := a.closeErr.Load()
	if err != nil {
		return err.(error)
	}
	return nil
}

[edaniels]

hi @mafredri, are you still interested in pushing this PR forward?