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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 06 Jan 2012 14:09:41 -0600
Michael Roth <mdroth@linux.vnet.ibm.com> wrote:

> On 01/06/2012 01:06 PM, Luiz Capitulino wrote:
> > On Fri, 6 Jan 2012 17:05:53 +0000
> > "Daniel P. Berrange"<berrange@redhat.com>  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?
> 
> I guess I prefer it to this patch, but I don't believe that covers 
> segfaults and the like, so maybe a combination of atexit() and F_SETLK 
> would be best (as F_SETLK can still leave stale PID files, they just 
> wouldn't obstruct subsequent instances, but we should still clean them 
> up whenever we can)

Agreed.

> 
> >
> >>>>
> >>>> 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
> >
>