[PATCH 3 of 4 V2] chgcache: implement the background preloading thread

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2017-03-01 21:56:08 +0900:
> On Wed, 22 Feb 2017 18:16:10 -0800, Jun Wu wrote:
> > # HG changeset patch
> > # User Jun Wu <quark at fb.com>
> > # Date 1487813979 28800
> > #      Wed Feb 22 17:39:39 2017 -0800
> > # Node ID 28571825744fb4f4b424385f55afa9484532ef43
> > # Parent  630457e88fa0485bce7822345ea640b5cdcb9b8e
> > # Available At https://bitbucket.org/quark-zju/hg-draft 
> > #              hg pull https://bitbucket.org/quark-zju/hg-draft  -r 28571825744f
> > chgcache: implement the background preloading thread
> > 
> > The background preloading thread is meant to used by chg master, and it sets
> > up the IPC channel.
> 
> > +    def _preloadloop(self, interval=0.5):
> > +        while not self._shouldexit.is_set():
> > +            try:
> > +                atimestr, path = self._ipc.recv().split(' ', 1)
> > +                atime = float(atimestr)
> > +            except Exception: # ignore errors
> > +                pass
> > +            else:
> > +                if path in _repocaches and _repocaches[path][0] >= atime:
> > +                    # this repocache is up-to-date, no need to update
> > +                    continue
> > +                now = time.time()
> > +                # TODO update repocache properly
> 
> Another concern just came up. Do you know what will happen if the process
> is fork()ed while the preloader thread is actively loading cache?
> 
> Only the main thread (which called fork()) would be copied, so the preloader
> would stop. That should be okay, but the other resources such as files and
> mutexes (if any) would be copied.

Right. It's possible to leak some fds, where I think it's probably fine. I
guess Python will deal with mutexes created by Python code.