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

[Michael Roth]

On 01/06/2012 01:06 PM, Luiz Capitulino wrote:
> 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?

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)

> > > > 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