[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 12:21:11PM +0000, Richard W.M. Jones wrote:
> On Sat, Mar 23, 2019 at 06:57:14AM -0500, Eric Blake wrote:
> > 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 ...

OK I fixed this bug (in nbdkit).

The block-status branch *works* with qemu 2.12.0, so another option
here is to do nothing and wait until that work is finished and merged.

NB During this I discovered another (but minor) bug in qemu.  If you
feed qemu a long series of block status replies then it eventually
closes the connection.  However it does not print an error message.

Rich.

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