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

[Kevin Wolf]

Am 10.01.2019 um 12:41 hat Dr. David Alan Gilbert geschrieben:
> * Kevin Wolf (address@hidden) wrote:
> > Am 09.01.2019 um 20:02 hat Eric Blake geschrieben:
> > > On 1/9/19 12:51 PM, Kevin Wolf wrote:
> > > 
> > > >> Indeed, and libvirt IS using 'savevm' via HMP via QMP's
> > > >> human-monitor-command, since there is no QMP counterpart for internal
> > > >> snapshot.  Even though lately we consistently tell people that internal
> > > >> snapshots are underdeveloped and you should use external snapshots, it
> > > >> does not get away from the fact that libvirt has been using 'savevm' to
> > > >> drive internal snapshots for years now, and that we MUST consider
> > > >> back-compat and/or add an introspectible QMP interface before making
> > > >> changes that would break libvirt.
> > > > 
> > > > Okay, so what does libvirt do when you request a snapshot with a
> > > > numerical name? Without having looked at the code, the best case I would
> > > > expect that it forbids them, and more realistically I suspect that we
> > > > may actually fix a bug for libvirt by changing the semantics.
> > > > 
> > > > Or does libvirt really use snapshot IDs rather than names?
> > > 
> > > At the moment, libvirt does not place any restriction on internal
> > > snapshot names, but passes the user's name through without thought of
> > > whether it is an id or a name.
> > > 
> > > So yes, arguably tightening the semantics fixes a libvirt bug for
> > > libvirt having allowed internal snapshots to get into an inconsistent
> > > state.
> > 
> > So there are two scenarios to consider with respect to breaking
> > backwards compatibility:
> > 
> > 1. There may be code out there that relies on numeric arguments being
> >    interpreted as IDs. This code will break if we make this change and
> >    numeric snapshot names exist. That such code exists is speculation
> >    (even though plausible) and we don't know how widespread it is.
> > 
> > 2. There may be code out there that blindly assumes that whatever it
> >    passes is interpreted as a name. Nobody considered that with a
> >    numeric snapshot name, it maybe misinterpreted as an ID. We know that
> >    this is true for libvirt, the single most used management tool for
> >    QEMU. More code like this may (and probably does) exist.
> > 
> > Essentially the same two categories exist for human users: those who
> > somehow found out that QEMU accepts IDs as monitor command arguments and
> > are using those (mitigated by not displaying IDs any more), and those
> > who are trapped because they wanted to access a numeric name, but
> > surprisingly got it interpreted as an ID. Both are speculation to some
> > degree, but my guess is that the latter group is larger.
> > 
> > Given all this, this is a no-brainer for me. We simplify the interface,
> > we simplify the code, we make things less confusing for manual users and
> > we fix the management tool that everybody uses. How could we not make
> > this change?
> > 
> > > But again, it falls in the category of "properly fixing this
> > > requires a lot of auditing, time, mental power, and unit tests", which
> > > makes it a rather daunting task to state for certain.
> > 
> > Fix the world before we can fix anything?
> 
> Can't we break this loop for the savevm command fairly easily; it's
> currently documnted as:
> 
> savevm [tag|id] 
> 
> If we made that:
> 
> savevm [-t] [-i] [tag|id]
> 
> then:
>   a) with neither -t or -i  it would behave in the same roulette way
>      as it does in the moment, and it might be a tag or id
> 
>   b) with -t we'd explicitly treat the parameter as a tag and it
>      would error if it wasn't found
> 
>   c) With -i we'd explicitly treat the parameter as an id and
>      it would error if it wasn't found
> 
> Since we still allow (a) it doesn't break any existing code.

If you can explain why we need both tag and id?

And by keeping the current behaviour, we might not break hypothetically
existing correct code, but we leave currently actually existing broken
code like libvirt broken.

Kevin