[libvirt] QMP: Supporting off tree APIs

[Anthony Liguori]

On 01/05/2012 09:35 AM, Eric Blake wrote:
> On 01/05/2012 07:16 AM, Luiz Capitulino wrote:
>>> I know.  We're stuck in a hard place here again because NotSupported
>>> has been in the Image Streaming API spec and hence implemented in
>>> libvirt for a while now.  If we change this then an old client which
>>> only understands NotSupported will not know what to do with the
>>> Unsupported error.
>>>
>>> (Unsupported was not in QEMU when the Image Streaming API was defined.)
>>
>> Let me try to understand it: is libvirt relying on an off tree API and
>> we are now required to have stable guarantees to off tree APIs?
>
> No.  Libvirt recognizes the off-tree spelling, but does not rely on it -
> after all, the goal of libvirt is to provide the high level action,
> using whatever underlying mechanism(s) necessary to get to that action,
> even if it means using several different attempts until one actually works.
>
> If a user has the older libvirt, which only expects the off-tree
> spelling, then that user's setup will break if they upgrade qemu but not
> libvirt.  But that's not a severe problem - they could have only been
> relying on the situation if they were using an off-tree build in the
> first place, so they should be aware that upgrading qemu is a
> potentially risky scenario, and that they may have to deal with the pieces.

Right, this is the difference between ABI compatibility and strict backwards 
compatibility.

To achieve ABI compatibility, we need to not overload BLOCK_JOB_COMPLETED to 
mean something other than libvirt what expects it to mean.

We MUST provide ABI compatibility and SHOULD provide backwards compatibility 
whenever possible.

In this case, I'd suggest that in the very least, we should add 
BLOCK_JOB_COMPLETED to qapi-schema.json with gen=False set.  That way it's 
codified in the schema to ensure we maintain ABI compatibility.

That said, I'm inclined to say that we should just use the BLOCK_JOB_COMPLETED 
name because I don't think we gain a lot by using QMP_JOB_COMPLETED (not that we 
shouldn't introduce it, but using it here isn't going to make or break anything).

With respect to libvirt relying on interfaces before they exist in QEMU, we need 
to be a bit flexible here.  We want to get better at co-development to help make 
libvirt support QEMU features as the bleeding edge.

Forcing libvirt to wait until a feature is fully baked in QEMU will ensure 
there's always a feature gap in libvirt which is in none of our best interests.

Now that we have gen=False support in qapi-schema.json, we can agree to an API 
and add it to QEMU before we fully implement it.  This gives libvirt something 
to work off of that they can rely upon.

Regards,

Anthony Liguori

>
> Newer libvirt can be easily taught to recognize both the off-tree and
> stable spellings of the error (checking the stable first, of course,
> since that will be more likely as the off-tree qemu builds filter out
> over time).  At which point, using either the off-tree qemu or the
> stable qemu should both work with the newer libvirt.
>