[Qemu-devel] structured reply behavior for read of 0 bytes

[Eric Blake]

As currently written, structured reply is documented as:

> NBD_REPLY_TYPE_OFFSET_DATA (1)
> 
> This chunk type is in the content chunk category. length MUST be at least 9. 
> It represents the contents of length - 8 bytes of the file, starting at the 
> absolute offset from the start of the export.

which implies that the data size must be non-zero. But clients can
request a read of size 0 (the spec doesn't forbid it, but neither does
it define special semantics for it), and the existing qemu
implementation as of qemu commit f140e300 sends
NBD_REPLY_TYPE_OFFSET_DATA with length of 8 and no data payload if the
client requests a 0-byte read.  Should we specifically allow this
particular answer, or should a 0-length read be answered solely by
NBD_REPLY_TYPE_NONE, meaning that qemu's current behavior needs a tweak?
 Either way, I probably need another tweak to the NBD spec for
structured reads.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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