Re: [Qemu-devel] [libvirt] QMP: Supporting off tree APIs

[Eric Blake <eblake@redhat.com>]

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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.

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.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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