[libvirt PATCH 6/7] virMutex*: Warn on error

[Peter Krempa]

On Thu, Aug 05, 2021 at 16:33:57 +0100, Daniel P. Berrangé wrote:
> On Thu, Aug 05, 2021 at 05:29:24PM +0200, Peter Krempa wrote:
> > On Thu, Aug 05, 2021 at 16:11:15 +0100, Daniel P. Berrangé wrote:
> > > On Thu, Aug 05, 2021 at 02:58:03PM +0100, Daniel P. Berrangé wrote:
> > > > On Thu, Aug 05, 2021 at 03:33:57PM +0200, Tim Wiederhake wrote:
> > > > > On Thu, 2021-08-05 at 14:24 +0100, Daniel P. Berrangé wrote:
> > > > > > On Thu, Aug 05, 2021 at 03:08:50PM +0200, Tim Wiederhake wrote:
> > 
> > [...]
> > 
> > > The pthread_mutex_destroy call is the only one that I see
> > > returning errors with PTHREAD_MUTEX_NORMAL. So I don't think
> > > there's benefit to adding the code to lock/unlock paths.
> > 
> > And as noted I'd be very careful with that one too. I've got at least
> > one counterexample of a code path which effectively does the same on a
> > very common code path.
> > 
> > Namely 'virDomainObjParseXML' uses:
> > 
> > g_autoptr(virDomainObj) obj = NULL;
> > 
> > and then returns directly on common failures such as XML parsing errors,
> > post-parse callback errors or even errors from the validation callbacks.
> > 
> > Spamming logs in such a common code path is definitely not acceptable.
> 
> Destroying a locked mutex though is a clear bug that should be fixed
> though, not a false positive. If it is a commonly triggered bug that
> is even worse and more reason to fix it.

I definitely agree, but we need a better approach to do this, because it
will keep happening since the g_auto* infrastructure is really
convenient. And this is something that can happen also without it.

Unfortunately I don't think that just replacing

G_DEFINE_AUTOPTR_CLEANUP_FUNC(virDomainObj, virObjectUnref);

by an equivalent of virObjectUnrefAndUnlock is sufficient.

IMO we should do some form of lock state tracking for
'virObjectLockable' so that we can keep using g_auto for it without
either accidentaly unlocking or unrefing a locked object.