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

From: Richard W.M. Jones
Subject: Re: [Qemu-devel] [Nbd] [PULL 23/28] nbd: always query export list in fixed new style protocol
Date: Tue, 17 May 2016 17:39:26 +0100
User-agent: Mutt/1.5.20 (2009-12-10)

On Tue, May 17, 2016 at 09:59:02AM -0600, Eric Blake wrote:
> On 05/17/2016 09:52 AM, Eric Blake wrote:
> >>> Perhaps nbdkit should implement NBD_OPT_LIST returning just "" (the
> >>> default name) as its only list entry?
> >>
> >> Or "default".
> > 
> > As I read the protocol, I don't see "default" as a permissible name of
> > the default export, just "".
> > 
> > Also, we currently state that NBD_OPT_LIST has zero or more
> > NBD_REP_SERVER replies, which means that it is valid for the command to
> > succeed with NBD_REP_ACK _without_ advertising any exports at all
> > (rather annoying in that it tells you nothing about whether
> > NBD_OPT_EXPORT_NAME/NBD_OPT_GO will succeed anyways).  Should we reword
> > that to require that if NBD_REP_ACK is sent, then at least one
> > NBD_REP_SERVER reply was given (where "" is the obvious name, but at the
> > same time where "" is not mandatory if some other name is given)?
> Okay, I just downloaded nbdkit code and checked; nbdkit is sending 0
> entries and NBD_REP_ACK (for success), and NOT an error code; so it is
> implying that there are NO known export names but that the command was
> recognized.

Bleah you are right.  I misread the code I wrote :-(

We can add a empty string list entry there easily enough.


> >>
> >> My interpretation of NBD_OPT_LIST failing would be 'this server
> >> doesn't have anything it can export'.
> > 
> > Indeed, and that's why qemu as a client is currently dropping the
> > connection with nbdkit.
> Except that nbdkit is NOT failing NBD_OPT_LIST, just merely listing 0
> replies.
> >  But I would also make that interpretation for
> > NBD_OPT_LIST succeeding with NBD_REP_ACK with 0 NBD_REP_SERVER replies -
> which is the nbdkit behavior
> > so maybe it is worth a note in the protocol how to detect servers that
> > are exporting exactly one volume and don't care what name you pass, then
> > tweaking either nbdkit, qemu, or both to comply to that added protocol
> > wording.
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org

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