[PATCH 6 of 7 V3] worker: test and set problem[0] atomically

[Yuya Nishihara]

On Fri, 5 Aug 2016 14:22:09 -0400, Augie Fackler wrote:
> On Thu, Aug 04, 2016 at 07:29:07PM +0100, Jun Wu wrote:
> > # HG changeset patch
> > # User Jun Wu <quark at fb.com>
> > # Date 1470334096 -3600
> > #      Thu Aug 04 19:08:16 2016 +0100
> > # Node ID d08d72f5d8bf302dd1231be410d0b23dc082eb66
> > # Parent  d179a66b0b15988687ba9d69c5b76e4464a457eb
> > # Available At https://bitbucket.org/quark-zju/hg-draft
> > #              hg pull https://bitbucket.org/quark-zju/hg-draft -r d08d72f5d8bf
> > worker: test and set problem[0] atomically
> >
> > Previously problem[0] is not tested and set atomically:
> >
> >     if st and not problem[0]:
> >         # at this time, another SIGCHILD happens, killworkers may run twice
> >         problem[0] = st
> >         killworkers()
> >
> > This patch addresses the issue by using itertools.count(). Its CPython
> > implementation will make it "atomic" at the Python code level, which is
> > enough for our use-case (signal handler).
> 
> This is very clever, but:
> 
> 1) pypy is already a thing
> 2) It's very clever
> 
> Could we just use a conventional lock of some sort instead?

or could we make killworkers() safe for multiple calls?

  pids = set(...)
  while pids:
      try:
          p = pids.pop()
      except KeyError:
          break
      kill(p)

Anyway, the current killworkers() doesn't care for pid reuse. It could send
SIGTERM to already-waited pids.