Re: [Qemu-devel] [PATCH] GuestAgent: PIDFILE remains when daemon start fails

[Luiz Capitulino]

On Fri, 6 Jan 2012 17:05:53 +0000
"Daniel P. Berrange" <address@hidden> wrote:

> On Fri, Jan 06, 2012 at 11:00:42AM -0600, Michael Roth wrote:
> > On 01/06/2012 04:56 AM, Daniel P. Berrange wrote:
> > >On Thu, Jan 05, 2012 at 06:18:26PM -0600, Michael Roth wrote:
> > >>On 01/05/2012 04:26 PM, MATSUDA, Daiki wrote:
> > >>>Hi, all.
> > >>>
> > >>>I am trying QEMU Guest Agent and encountered a small bug. It is that the
> > >>>PIDFILE remains when daemon start fails. And maybe forgotton to g_free().
> > >>>
> > >>>MATSUDA, Daiki
> > >>>
> > >>
> > >>Thanks for the patch. There was some contention in the past about
> > >>whether or not to clean up pidfiles when there was abnormal
> > >>termination, but personally I like this approach better.

Ok, but can't we use atexit() instead then?

> > >
> > >Yep, this still leaves open the problem of pidfile cleanup when the
> > >daemon crashes. For libvirtd we recently switched over to a crash-safe
> > >pidfile acquisition design, that uses fcntl(F_SETLK) to maintain
> > >exclusive access over the pidfile. With this you don't need to worry
> > >about forgetting to unlink() on termination, since the POSIX lock is
> > >automatically released when process exits (or crashes).
> > 
> > Yup, we did the same at some point via lockf(). An argument was made
> > that stale PID files from unresolved crashes should stick around, so
> > we dropped it. I think we should re-evaluate that decision...libvirt
> > taking the same approach is pretty good precedence for me. I don't
> > expect to have state from crashed programs interrupting attempts to
> > restart them, it's more an unpleasant surprise than a feature, I
> > think.

Ok, I'll agree with you this time. Let's do it.

> 
> Yeah, I think that is rather unpleasant, particularly for something
> like qemu guest agent, which we want to try to ensure is reliably
> running. In any case, if qemu guest agent is being launched by
> something like SystemD, then you can configure whether systemd
> will auto-restart it when it dies with non-zero exit status, so
> I don't think we should delibrately leave stale pidfiles for that
> scenario.
> 
> Regards,
> Daniel