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

[Eric Blake <eblake@redhat.com>]

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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.

>>
>> 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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