Re: [Qemu-devel] [Nbd] [PULL 23/28] nbd: always query export list in fixed new style protocol

From: Eric Blake <eblake@redhat.com>
To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: Alex Bligh <alex@alex.org.uk>,
	"nbd-general@lists.sourceforge.net"
	<nbd-general@lists.sourceforge.net>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"Daniel P. Berrange" <berrange@redhat.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [Nbd] [PULL 23/28] nbd: always query export list in fixed new style protocol
Date: Tue, 17 May 2016 10:58:08 -0600	[thread overview]
Message-ID: <573B4DA0.502@redhat.com> (raw)
In-Reply-To: <20160517163926.GA1683@redhat.com>

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On 05/17/2016 10:39 AM, Richard W.M. Jones wrote:
> 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.

Will adding the empty string break qemu 2.5 connections?  qemu 2.6  is
picky in that if the name it is requesting is not in the advertised
list, it gives up; but if there is NO advertised list, it assumes the
name will just work.  Since nbdkit has the latter behavior, I'm thinking
that nbdkit returning NBD_REP_ERR_UNSUP instead of NBD_REP_ACK is a
smarter course of action.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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