Re: [Qemu-devel] [PATCH 12/14] nbd/client: Work around 3.0 bug for listing meta contexts

[Eric Blake <eblake@redhat.com>]

On 12/7/18 5:21 AM, Vladimir Sementsov-Ogievskiy wrote:
> 01.12.2018 1:03, Eric Blake wrote:
>> Commit 3d068aff forgot to advertise available qemu: contexts
>> when the client requests a list with 0 queries. Furthermore,
>> 3.0 shipped with a qemu-img hack of x-dirty-bitmap (commit
>> 216ee365) that _silently_ acts as though the entire image is
>> clean if a requested bitmap is not present.  Both bugs have
>> been recently fixed to give full output from the start, and
>> to refuse a connection if a requested x-dirty-bitmap was not
>> found.
>>
>> Still, it is likely that there will be users that have to
>> work with a mix of old and new qemu versions, depending on
>> which features get backported where, at which point being
>> able to rely on 'qemu-img --list' output to know for sure
>> whether a given NBD export has the desired dirty bitmap is
>> much nicer than blindly connecting and risking that the
>> entire image may appear clean.  We can make our --list code
>> smart enough to work around buggy servers by tracking
>> whether we've seen any qemu: replies in the original 0-query
>> list; if not, recurse to a single query on "qemu:" (which
>> may still have no replies, but then we know for sure we
>> didn't trip up on the server bug).
> 
> related thing:
> should we document these bugs with corresponding version numbers in docs/interop/nbd.txt?

Might be useful.  I'll add that as another patch in my v2 series.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org