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

[Wouter Verhelst <w@uter.be>]

Hi Eric,

On Fri, Nov 03, 2017 at 05:45:07PM -0500, Eric Blake wrote:
> 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.

I'd like to take a step back first and wonder what a zero-sized read
would actually mean. "Please read no data at this offset". What good
does that do? I can't see anything useful coming out of that.

If we can come up with some useful semantics that we could apply to a
zero-sized read, then I guess it makes sense to worry about handling it.
In the absense of that though, I think it is more appropriate to just
document that a client should not send it, but that a server's behaviour
upon such a command is not defined (other than that it should not cause
the server to quit, through a crash or otherwise).

In fact, the current spec, as you point out, already implicitly forbids
(with MUST) nonzero writes by requiring a data chunk to have nonzero
payload (and that can't happen when you have a zero-sized read). It
might make sense to make that explicit.

-- 
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