Re: [PATCH] pack-objects: re-validate data we copy from elsewhere.

[Linus Torvalds <torvalds@osdl.org>]

On Sun, 3 Sep 2006, Linus Torvalds wrote:
> 
> Ok. Is it less painful if it just checks the zlib CRC (and that the SHA1 
> _exists_ for a delta - although I guess we check that indirectly by just 
> accepting the delta in the first place)? That combination should still be 
> a fairly strong check, of course.

Thinking some more about it, doing the stupid "apply full delta chain and 
check the final SHA1" is horribly inefficient, because if you have a repo 
that packs well, you'd expect to have a lot of things with a 10-deep delta 
chain.

And doing it the silly way means that you'll do each object independently, 
ie for a 10-deep chain you'd unpack the base object ten times, and apply 
the first delta 9 times, the second one 8 times etc etc. And each time 
you'd deflate everything, since we don't keep a cache of actual object 
contents.

So I'd expect that with full SHA1 checking, you'd end up doing ~45 
deflates for the ten-object chain, instead of doing just 10.

So it should hopefully be _much_ cheaper to just check the zlib CRC, not 
because the "apply delta" and "calculate sha1" are necessarily all that 
expensive, but because the unoptimized chain-unpacking is doing so much 
unnecessary work.

			Linus

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