[PATCH 4 of 7 V3] worker: make waitforworkers reentrant

[Jun Wu]

Excerpts from Augie Fackler's message of 2016-08-05 14:20:15 -0400:
> On Thu, Aug 04, 2016 at 07:29:05PM +0100, Jun Wu wrote:
> > # HG changeset patch
> > # User Jun Wu <quark at fb.com>
> > # Date 1469737019 -3600
> > #      Thu Jul 28 21:16:59 2016 +0100
> > # Node ID 74a3dd438c5645cc1a685e788d86e70698bfe6c9
> > # Parent  521d7fac970d711766e61b7b54d20ddefc4be406
> > # Available At https://bitbucket.org/quark-zju/hg-draft 
> > #              hg pull https://bitbucket.org/quark-zju/hg-draft  -r 74a3dd438c56
> > worker: make waitforworkers reentrant
> >
> > We are going to use it in SIGCHLD handler. The handler will be executed in the
> > main thread with the non-blocking version of waitpid, while the waitforworkers
> > thread runs the blocking version. It's possible that one of them collects a
> > worker and makes the other error out. This patch handles these error: ECHILD
> > is ignored. EINTR needs a retry.
> >
> > diff --git a/mercurial/worker.py b/mercurial/worker.py
> > --- a/mercurial/worker.py
> > +++ b/mercurial/worker.py
> > @@ -96,7 +96,15 @@ def _posixworker(ui, func, staticargs, a
> >                      raise
> >      def waitforworkers(blocking=True):
> >          for pid in pids:
> > -            p, st = os.waitpid(pid, 0 if blocking else os.WNOHANG)
> > +            p = st = 0
> > +            while True:
> > +                try:
> > +                    p, st = os.waitpid(pid, 0 if blocking else os.WNOHANG)
> > +                except OSError as e:
> > +                    if e.errno == errno.EINTR:
> > +                        continue
> > +                    # otherwise it's ECHILD and is ignored
> 
> shouldn't we explicitly look for ECHILD and ignore that? EINVAL could
> also happen if the arguments to os.waitpid are invalid, which could
> lead to some very confusing debugging sessions?

I will add the if condition. os.WNOHANG and 0 are both valid values so
EINVAL is actually not possible.