Subject: Re: Bug: Segmentation fault in git diff

From: Junio C Hamano <[email-masked]>
Date: Wed, 18 Aug 2021 13:30:05 -0700

Đoàn Trần Công Danh  <[email-masked]> writes:

> This diff could fix the issue, and the test suite still passes:
>
> ---- 8< ----
>
> diff --git a/diff-lib.c b/diff-lib.c
> index f9eadc4fc1..8f303958dd 100644
> --- a/diff-lib.c
> +++ b/diff-lib.c
> @@ -192,7 +192,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
>  			 * from the desired stage.
>  			 */
>  			pair = diff_unmerge(&revs->diffopt, ce->name);
> -			if (wt_mode)
> +			if (pair && wt_mode)
>  				pair->two->mode = wt_mode;
>  			if (ce_stage(ce) != diff_unmerged_stage)
>  				continue;
> ---- >8 -----

Looks sensible.  

The bug is the result of a bad interaction between cd676a51 (diff
--relative: output paths as relative to the current subdirectory,
2008-02-12) that decided to return NULL from diff_unmerge for an
irrelevant path outside the --relative area, and 095ce953
(diff-files: show unmerged entries correctly, 2011-04-22) that
assumed diff_unmerge() would always yield a usable unmerged pair.

As the NULL pair would never be added to the diff_queue(), when the
big loop around this area of code finishes and hands the processing
off to diffcore_std(), the mode futzing the original code was
attempting to do would have affected absolutely nothing, so I do not
think there would be any subtle breakage coming from this fix.

When you submit an "git am"-able patch, please have my Acked-by.

Thanks.