[Libguestfs] nbdkit & qemu 2.12: qemu-img: Protocol error: simple reply when structured reply chunk was expected

[Richard W.M. Jones]

On Sat, Mar 23, 2019 at 06:57:14AM -0500, Eric Blake wrote:
> On 3/23/19 6:42 AM, Richard W.M. Jones wrote:
> > (b) Try and get it fixed in RHEL.  I filed a BZ already but I guess it
> > won't be fixed any time soon: https://bugzilla.redhat.com/1692018
> 
> Several RHEL 7 bugs against NBD are pending - there's hope for RHEL 7.7,
> but you are also right that guessing timelines is hard.

Yes, my worry is that if we miss 7.7 then this bug will exist forever
since as far as I know we (Red Hat) are not planning any further qemu
improvements in 7.8 and above.

> > (c) Add a workaround in nbdkit.
> > 
> 
> And what workaround would that be? Looking at the qemu patch, the
> problem is that qemu asked for "base:allocation" and nbdkit replied with
> 0 contexts. The workaround would either be to reply with
> NBD_REP_ERR_UNSUP (the behavior before NBD_OPT_SET_META_CONTEXT was
> added), or to implement "base:allocation" (even if we implement the
> poor-man's version that always reports that the entire image is data,
> for all requests, without any feedback from the plugins).  The former
> feels odd, but the latter seems oddly appealing as progress towards our
> end goal (kind of how our initial implementation of NBD_CMD_WRITE_ZEROES
> was always advertised even before we finished wiring up plug support).

A good point here is what happens with the block-status branch[1].
Let's see:

$ ./nbdkit memory size=64M --run '/home/rjones/d/qemu/qemu-img convert $nbd /var/tmp/out'
qemu-img: Payload too large
nbdkit: memory.1: error: write reply: NBD_CMD_BLOCK_STATUS: Broken pipe

Oh dear.  I believe this is actually a bug in the block status code so
let's see if I can narrow this down first ...

Rich.

[1] https://github.com/rwmjones/nbdkit/tree/block-status

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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