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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

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 

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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