[PATCH 6 of 8 V5] worker: make sure killworkers runs at most once

[Yuya Nishihara]

On Wed, 16 Nov 2016 13:57:58 +0000, Jun Wu wrote:
> Excerpts from Yuya Nishihara's message of 2016-11-16 21:25:45 +0900:
> > I meant set.discard(). My idea is:
> > 
> >  a. discard pid by anyone who notices it no longer exists
> >  b. then, kill the rest
> 
> This looks good. Although it's still possible to kill processes multiple
> times (because atomic "wait-and-drop-pid" is impossible).

Hmm, signal handlers are stacked at arbitrary code location, there's no
time-based preemption like threading, I think. So even if the wait-discard(p)
sequence is interrupted by another waitforworkers(), the process "p" would
be discarded before killworkers() thanks to ECHILD. And we do unregister the
SIGCHLD handler before killworkers().

I'll queue the series without the patch 6.