Re: [PATCH 1/2] git-commit: Disallow unchanged tree in non-merge mode

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, 13 Sep 2007, Linus Torvalds wrote:
> 
> Ok, I'm downloading those tar-balls to reproduce and hopefully see what's 
> going on, but I'm not going to be able to get at it today.

Ok, I'm seeing it, and there are serious problems in the diffcore-rename 
code.

The problems include things like actual overflow in 31 bits (a signed 
integer) from the multiplication of "num_create * num_src".

But you'll almost certainly never even get there, because you'd long since 
have given up on the O(n^2) behaviour of the "cheap tests", that aren't 
really cheap enough, if only because even just comparing the 20-byte SHA1 
hashes will basically take forever since they will always miss in the 
cache for lots of names.

So yes, we really *have* to have a rename limit, even if it's only to 
avoid the technical bug of overflowing the multiplication.

Testing this actually showed another bug too: "git diff" would actually 
never call "diff_setup_done() at all, if "diffopt.output_format" had been 
set explicitly to something. So doing a "git diff --stat" would totally 
ignore all the sanity checks (and, what caused me to find it, the 
initialization of "diffopt.rename_limit") that diff_setup_done() is 
supposed to do!

So I'm going to send out two patches - one to fix the "diff_setup_done()" 
bug, and one that replaces the default rename_limit with something saner. 
Both seem to be real bugs.

			Linus