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Re: [Qemu-devel] [Nbd] [PULL 23/28] nbd: always query export list in fix

From: Wouter Verhelst
Subject: Re: [Qemu-devel] [Nbd] [PULL 23/28] nbd: always query export list in fixed new style protocol
Date: Sat, 21 May 2016 23:53:12 +0200
User-agent: Mutt/1.6.0 (2016-04-01)

On Tue, May 17, 2016 at 10:50:01AM -0600, Eric Blake wrote:
> Option 2: An alternative solution would be to allow nbdkit to fail
> NBD_OPT_LIST with NBD_REP_ERR_UNSUP, at which point qemu client of 2.6
> should just ignore the failure and proceed on to NBD_OPT_EXPORT_NAME.
> It is the fact that it is returning NBD_REP_ACK with 0 names that is
> giving qemu grief.

I think this makes most sense. If you don't look at export names, you
effectively don't support them, and you can't be expected to send a list
of "supported" names, because *everything* is supported (or, put a
different way, nothing is).

I note that nbdkit has been patched to now send the empty name, which
is also fine as a way to fix interoperability in this particular case --
but for the general case, I think if we want to define ways for a server
to explicitly say that it doesn't support export names, returning
ERR_UNSUP to OPT_LIST seems cleaner.

(That does mean I'll have to fix nbd-client, since it currently assumes
that fixed newstyle implies support for OPT_LIST -- but that should be
an easy enough patch)

> But to date, I think ALL of the options (except NBD_OPT_EXPORT_NAME)
> _are_ optional.  In fact, I'm arguing that per Option 2, we WANT
> NBD_OPT_LIST to be optional for servers that don't care about names.

Sortof. Like I said, nbd-client assumes that servers which support fixed
newstyle will also support OPT_LIST -- but that's mostly an artefact of
the fact that OPT_LIST was used as a test case for fixed newstyle, and
isn't necessarily a good enough reason to make that a required part of
the protocol.

< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12

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