Re: [Qemu-devel] [PATCH v2 0/3] HMP/snapshot changes - do not use ID anymore

[Dr. David Alan Gilbert]

* Kevin Wolf (address@hidden) wrote:
> Am 09.01.2019 um 15:54 hat Max Reitz geschrieben:
> > On 09.01.19 15:48, Kevin Wolf wrote:
> > > Am 09.01.2019 um 15:27 hat Max Reitz geschrieben:
> > >> On 09.01.19 15:21, Kevin Wolf wrote:
> > >>> Am 09.01.2019 um 15:10 hat Max Reitz geschrieben:
> > >>>> On 06.09.18 13:11, Daniel Henrique Barboza wrote:
> > >>>>> changes in v2:
> > >>>>> - removed the "RFC" marker;
> > >>>>> - added a new patch (patch 2) that removes
> > >>>>> bdrv_snapshot_delete_by_id_or_name from the code;
> > >>>>> - made changes in patch 1 as suggested by Murilo;
> > >>>>> - previous patch set link:
> > >>>>> https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg04658.html
> > >>>>>
> > >>>>>
> > >>>>> It is not uncommon to see bugs being opened by testers that attempt to
> > >>>>> create VM snapshots using HMP. It turns out that "0" and "1" are quite
> > >>>>> common snapshot names and they trigger a lot of bugs. I gave an 
> > >>>>> example
> > >>>>> in the commit message of patch 1, but to sum up here: QEMU treats the
> > >>>>> input of savevm/loadvm/delvm sometimes as 'ID', sometimes as 'name'. 
> > >>>>> It
> > >>>>> is documented as such, but this can lead to strange situations.
> > >>>>>
> > >>>>> Given that it is strange for an API to consider a parameter to be 2 
> > >>>>> fields
> > >>>>> at the same time, and inadvently treating them as one or the other, 
> > >>>>> and
> > >>>>> that removing the ID field is too drastic, my idea here is to keep the
> > >>>>> ID field for internal control, but do not let the user set it.
> > >>>>>
> > >>>>> I guess there's room for discussion about considering this change an 
> > >>>>> API
> > >>>>> change or not. It doesn't affect users of HMP and it doesn't affect 
> > >>>>> Libvirt,
> > >>>>> but simplifying the meaning of the parameters of savevm/loadvm/delvm.
> > >>>>
> > >>>> (Yes, very late reply, I'm sorry...)
> > >>>>
> > >>>> I do think it affects users of HMP, because right now you can delete
> > >>>> snapshots with their ID, and after this series you cannot.
> > >>>
> > >>> Can there be snapshots that can't be identified by a snapshot name, but
> > >>> only by their ID?
> > >>
> > >> I don't know, but what I meant is that if you have scripts to do all
> > >> this, you might have to adjust them with this change.
> > > 
> > > That's what the H in HMP means.
> > > 
> > >>>> I think we had a short discussion about just disallowing numeric
> > >>>> snapshot names.  How bad would that be?
> > >>>
> > >>> It would be incompatible with existing images and result in a more
> > >>> complex snapshot identifier resolution. Why would it be any better?
> > >>
> > >> It wouldn't be incompatible with existing images if we'd just disallow
> > >> creating such snapshots.  I don't see how the identifier resolution
> > >> would be more complex.
> > >>
> > >> I don't know if it'd be better.  I'd just find it simpler (for us, that
> > >> is -- for users, I'm not sure).
> > > 
> > > Identifier resolution A:
> > > - Find a snapshot that has the given identifier as a name
> > > - If no such snapshot exists, it is an error
> > > 
> > > Identifier resolution B:
> > > - If the identifier is a number, find a snapshot that has the given
> > >   identifier as its ID
> > > - If the identifier is not a number, find a snapshot that has the given
> > >   identifier as a name
> > > - If no such snapshot exists, it is an error
> > 
> > No, my idea was to keep the resolution the same as it is; just to forbid
> > creating new snapshots with numeric names.  This would prevent users
> > from getting into the whole situation.
> 
> That's the version with an even more complex resolution method C. :-)
> 
> I actually think the current behaviour is more confusing than helpful.
> Without looking into the code or trying it out, I couldn't even tell
> whether ID or name takes precedence if there is a matching snapshot for
> both. Considering your proposal, it's probably the ID, but how should a
> user know that? (If against all expectations documentation exists, it
> doesn't count because nobody reads that.)

Would adding a flag to the HMP commands to make it explicit help?

Dave

> Kevin


--
Dr. David Alan Gilbert / address@hidden / Manchester, UK