Re: [Qemu-devel] [PATCH] monitor: fix object_del for command-line-created objects

[Michael Roth <mdroth@linux.vnet.ibm.com>]

Quoting Dr. David Alan Gilbert (2016-12-01 06:34:10)
> * Daniel P. Berrange (berrange@redhat.com) wrote:
> > On Wed, Nov 30, 2016 at 05:06:16PM -0600, Michael Roth wrote:
> > > Currently objects specified on the command-line are only partially
> > > cleaned up when 'object_del' is issued in either HMP or QMP: the
> > > object itself is fully finalized, but the QemuOpts are not removed.
> > > This results in the following behavior:
> > > 
> > >   x86_64-softmmu/qemu-system-x86_64 -monitor stdio \
> > >     -object memory-backend-ram,id=ram1,size=256M
> > > 
> > >   QEMU 2.7.91 monitor - type 'help' for more information
> > >   (qemu) object_del ram1
> > >   (qemu) object_del ram1
> > >   object 'ram1' not found
> > >   (qemu) object_add memory-backend-ram,id=ram1,size=256M
> > >   Duplicate ID 'ram1' for object
> > >   Try "help object_add" for more information
> > > 
> > > which can be an issue for use-cases like memory hotplug.
> > > 
> > > This happens on the HMP side because hmp_object_add() attempts to
> > > create a temporary QemuOpts entry with ID 'ram1', which ends up
> > > conflicting with the command-line-created entry, since it was never
> > > cleaned up during the previous hmp_object_del() call.
> > > 
> > > We address this by adding checks in {qmp,hmp}_object_del to determine
> > > whether an option group entry matching the object's ID is present,
> > > and removing it if it is.
> > > 
> > > Note that qmp_object_add() never attempts to create a temporary
> > > QemuOpts entry, so it does not encounter the duplicate ID error,
> > > which is why this isn't generally visible in libvirt.
> > > 
> > > Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > Cc: Markus Armbruster <armbru@redhat.com>
> > > Cc: Eric Blake <eblake@redhat.com>
> > > Cc: Daniel Berrange <berrange@redhat.com>
> > > Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> > > ---
> > >  hmp.c | 10 ++++++++++
> > >  qmp.c | 10 ++++++++++
> > >  2 files changed, 20 insertions(+)
> > > 
> > > diff --git a/hmp.c b/hmp.c
> > > index b869617..99b8bf3 100644
> > > --- a/hmp.c
> > > +++ b/hmp.c
> > > @@ -2072,6 +2072,16 @@ void hmp_object_del(Monitor *mon, const QDict *qdict)
> > >  
> > >      user_creatable_del(id, &err);
> > >      hmp_handle_error(mon, &err);
> > > +
> > > +    /* if object was defined on the command-line, remove its corresponding
> > > +     * option group entry
> > > +     */
> > > +    if (err == NULL) {
> > > +        QemuOptsList *opt_group = qemu_find_opts_err("object", &err);
> > > +        if (opt_group) {
> > > +            qemu_opts_del(qemu_opts_find(opt_group, id));
> > > +        }
> > > +    }
> > >  }
> > >  
> > >  void hmp_info_memdev(Monitor *mon, const QDict *qdict)
> > > diff --git a/qmp.c b/qmp.c
> > > index 0028f0b..87c545d 100644
> > > --- a/qmp.c
> > > +++ b/qmp.c
> > > @@ -686,6 +686,16 @@ void qmp_object_add(const char *type, const char *id,
> > >  void qmp_object_del(const char *id, Error **errp)
> > >  {
> > >      user_creatable_del(id, errp);
> > > +
> > > +    /* if object was defined on the command-line, remove its corresponding
> > > +     * option group entry
> > > +     */
> > > +    if (!(errp && *errp)) {
> > > +        QemuOptsList *opt_group = qemu_find_opts_err("object", errp);
> > > +        if (opt_group) {
> > > +            qemu_opts_del(qemu_opts_find(opt_group, id));
> > > +        }
> > > +    }
> > >  }
> > 
> > I rather feel like this code ought to be in user_creatable_del(), since
> > all code paths which call user_creatable_del() need to do deal with
> > this scenario.
> 
> Yes that seems cleaner;  the cleanup that's being done is not the result
> of either of the monitor's actions.

It was also for similar reasons that I was hesitant to put it in
user_creatable_del() :) Putting it higher up in the stack somehow felt
less icky, but given that they both violate encapsulation somewhat I
suppose it does makes sense to take the more practical/safer route and
put it in a common path. Will take this approach for v2.

> 
> Dave
> 
> > Also I'd like to see a unit test added that exposes this problem and
> > demonstrates the fix - could att it to tests/check-qom-proplist.c
> > 
> > Regards,
> > Daniel
> > -- 
> > |: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
> > |: http://libvirt.org              -o-             http://virt-manager.org :|
> > |: http://entangle-photo.org       -o-    http://search.cpan.org/~danberr/ :|
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>