worker: fix simpleworker Kill

worker: fix simpleworker Kill gccgo does not like it when you close a channel twice. It's not a very nice thing anyway. Fixes lp:1308852 https://code.launchpad.net/~axwalk/juju-core/lp1308852-worker-simple-gccgo/+merge/220563 (do not edit description out of merge proposal)

[axw, 2014-05-22 05:29:49]

Please take a look.

[wallyworld, 2014-05-22 06:13:30]

LGTM

[rog, 2014-05-22 08:38:34]

https://codereview.appspot.com/98480046/diff/1/worker/simpleworker.go File worker/simpleworker.go (right): https://codereview.appspot.com/98480046/diff/1/worker/simpleworker.go#newcode35 worker/simpleworker.go:35: case w.stopc <- struct{}{}: This isn't quite right because it changes the NewSimpleWorker contract, which states that stopCh is closed when the worker is killed, which means that we can read from the channel any number of times after it's been closed. If we want to avoid the recover check (which is something that gccgo really should allow, but that's a different issue) the correct solution is to have a mutex that guards the close and do something like this: w.killMutex.Lock() defer w.killMutex.Unlock() select { case <-w.stopc: return // stopc is already closed default: close(w.stopc) }