[PATCH 7 of 8 V4] worker: add a SIGCHLD handler to collect worker immediately

[Jun Wu]

Interesting. Thanks for the very careful review (and also for the
setproctitle series)!

I did some simple manual tests but didn't cover all corners here. I will
investigate.

Excerpts from Yuya Nishihara's message of 2016-11-14 22:15:24 +0900:
> On Sat, 12 Nov 2016 03:11:38 +0000, Jun Wu wrote:
> > # HG changeset patch
> > # User Jun Wu <quark at fb.com>
> > # Date 1478920042 0
> > #      Sat Nov 12 03:07:22 2016 +0000
> > # Node ID 7492186538a69cdfad28cb93093d7f7077942ca3
> > # Parent  596cee9d5c12dcfa2d272b5441d5060a26a6440c
> > # Available At https://bitbucket.org/quark-zju/hg-draft 
> > #              hg pull https://bitbucket.org/quark-zju/hg-draft  -r 7492186538a6
> > worker: add a SIGCHLD handler to collect worker immediately
> > 
> > As planned by previous patches, add a SIGCHLD handler to get notifications
> > about worker exits, and deals with worker failure immediately.
> > 
> > diff --git a/mercurial/worker.py b/mercurial/worker.py
> > --- a/mercurial/worker.py
> > +++ b/mercurial/worker.py
> > @@ -126,8 +126,12 @@ def _posixworker(ui, func, staticargs, a
> >                  problem[0] = st
> >                  killworkers()
> > +    def sigchldhandler(signum, frame):
> > +        waitforworkers(blocking=False)
> > +    oldchldhandler = signal.signal(signal.SIGCHLD, sigchldhandler)
> >      for pargs in partition(args, workers):
> >          pid = os.fork()
> >          if pid == 0:
> >              signal.signal(signal.SIGINT, oldhandler)
> > +            signal.signal(signal.SIGCHLD, oldchldhandler)
> >              try:
> >                  os.close(rfd)
> > @@ -147,4 +151,5 @@ def _posixworker(ui, func, staticargs, a
> >          signal.signal(signal.SIGINT, oldhandler)
> >          t.join()
> > +        signal.signal(signal.SIGCHLD, oldchldhandler)
> 
> I got strange error:
> 
> % hg up --trace
> Traceback (most recent call last):
>   ...
>   File "mercurial/merge.py", line 1187, in applyupdates
>     for i, item in prog:
>   File "mercurial/worker.py", line 159, in _posixworker
>     for line in fp:
> IOError: [Errno 0] Error
> abort: Error
> 
> Maybe something goes wrong in the buffered iterator?
> iter(fp.readline, '') worked fine.