Re: [PATCH v4 06/24] nbd/client: Simplify cookie vs. index computation

[Eric Blake]

On Mon, Jun 12, 2023 at 05:27:19PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 08.06.23 16:56, Eric Blake wrote:
> > Our code relies on a sentinel cookie value of zero for deciding when a
> > packet has been handled, as well as relying on array indices between 0
> > and MAX_NBD_REQUESTS-1 for dereferencing purposes.  As long as we can
> > symmetrically convert between two forms, there is no reason to go with
> > the odd choice of using XOR with a random pointer, when we can instead
> > simplify the mappings with a mere offset of 1.
> 
> Should we go further and use (uint64)-1 as a sentinel cookie value, and just 
> use index as a cookie?  Or, using zero cookie in a wire looks too asymmetric?

I thought about that too, but in the end I decided it would require
auditing more lines of code to make sure I was catching all places
where we currently expected a zero sentinel (where some of those uses
are not obvious, because of things like hiding behind g_new0).  And
there is indeed the argument that if data corruption is going to
happen, it's harder to tell if an all-zero field on the wire was
intentional than a non-zero field.

> 
> > 
> > Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

Thanks; for now, I'll just leave this one as-is.


-- 
Eric Blake, Principal Software Engineer
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