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

[Kevin Wolf <kwolf@redhat.com>]

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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.)

Kevin

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