Re: Delitifier broken (Re: diff-core segfault)

On Mon, 12 Dec 2005, Junio C Hamano wrote:

Junio C Hamano [off-list ref] writes:

quoted

Darrin Thompson [off-list ref] writes:

quoted

Could someone confirm that this exists on more recent git heads and fix
if needed?

(1) Yup.  I can reproduce it.
(2) Will look into it when able.

This is not just "diff".  Our deltify code is half-broken, and
in the worst case this can corrupt our packs if an empty blob is
involved.

I would say involving an empty blob with deltas _is_ the bug in the 
first place.  Please don't let that happen.

Especially with pack files, an empty blob can be represented with a 
_single_ byte.  A delta must always be against something else and simply 
storing the reference for the object the delta is against will always 
use at least 20 bytes even for empty ones.

The problem is if from_size or to_size is empty, it does not
produce any.

        if (!from_size || !to_size || delta_prepare(from_buf, from_size, &bdf))
                return NULL;
	

I think either we need to make the users more careful or fix
deltifier to produce trivial delta.  I'd vote for the latter;
let me rig up something.

If my opinion is still of any weight I'd strongly vote for the former.  
A delta against an empty object, or a delta that produces an empty 
object simply makes no sense since it is always suboptimal compared to
storing the non deltified object (or finding another object to deltify 
against).  Allowing empty deltas only paper over another more 
fundamental bug IMHO.


Nicolas