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

[Yuya Nishihara]

On Wed, 16 Nov 2016 15:18:07 +0000, Jun Wu wrote:
> Excerpts from Yuya Nishihara's message of 2016-11-17 00:00:01 +0900:
> > 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
> 
> Only one "p" will be discard. There could be multiple "p"s that need to be
> discarded.
> 
>   waitforworkers
>     p1: need to be discarded
>       SIGCHLD
>         waitforworkers
>         p2: need to be discarded
>       SIGCHLD
>         waitforworkers

Here waitforworkers() should discard p2 because waitpid(p2) would raise ECHILD.

>         p3: need to be discarded
>       SIGCHLD
>         waitforworkers
>         p4: need to be discarded
>       ....
>     discard p1
>     killworkers
>       # kill p2, p3, p4 unnecessarily
> 
> As long as there is no way to do "atomic waitpid + set.discard", the problem
> can not be solved perfectly.
> 
> > be discarded before killworkers() thanks to ECHILD. And we do unregister the
> > SIGCHLD handler before killworkers().