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


From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 12/14] nbd/client: Work around 3.0 bug for listing meta contexts
Date: Fri, 7 Dec 2018 09:21:58 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

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



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