Re: [PATCH 1/2] diff: Fix modified lines stats with --stat and --numstat

[Taylor Blau <me@ttaylorr.com>]

On Fri, Sep 18, 2020 at 07:32:55AM -0400, Thomas Guyot-Sionnest wrote:
> In builtin_diffstat(), when both files are coming from "stdin" (which
> could be better described as the file's content being written directly
> into the file object), oideq() compares two null hashes and ignores the
> actual differences for the statistics.
>
> This patch checks if is_stdin flag is set on both sides and compare
> contents directly.
>
> Signed-off-by: Thomas Guyot-Sionnest <tguyot@gmail.com>
> ---
>  diff.c                | 5 ++++-
>  t/t3206-range-diff.sh | 8 ++++----
>  2 files changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/diff.c b/diff.c
> index a5114fa864..2995527896 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -3681,7 +3681,10 @@ static void builtin_diffstat(const char *name_a, const char *name_b,
>  		return;
>  	}
>
> -	same_contents = oideq(&one->oid, &two->oid);
> +	if (one->is_stdin && two->is_stdin)
> +		same_contents = !strcmp(one->data, two->data);

Hmm. A couple of thoughts here:

  - strcmp seems like a slow-down here, since we'll have to go through
    at worst the smaller of one->data and two->data to compare each of
    them.

  - strcmp is likely not the right way to do that, since we could be
    diffing binary content, in which case we'd want to continue over
    NULs and instead stop at a fixed length (the minimum of the length
    of one->data and two->data, specifically). I'd have expected memcmp
    here instead.

  - Why do we have to do this at all all the way up in
    'builtin_diffstat'? I would expect these to contain the right
    OIDs by the time they are given back to us from the call to
    'diff_fill_oid_info' in 'run_diffstat'.

So, my last point is the most important of the three. I'd expect
something more along the lines of:

  1. diff_fill_oid_info resolve the link to the pipe, and
  2. index_path handles the resolved fd.

...but it looks like that is already what it's doing? I'm confused why
this doesn't work as-is.

> +	else
> +		same_contents = oideq(&one->oid, &two->oid);
>
>  	if (diff_filespec_is_binary(o->repo, one) ||
>  	    diff_filespec_is_binary(o->repo, two)) {
> diff --git a/t/t3206-range-diff.sh b/t/t3206-range-diff.sh
> index e024cff65c..4715e75b68 100755
> --- a/t/t3206-range-diff.sh
> +++ b/t/t3206-range-diff.sh
> @@ -258,11 +258,11 @@ test_expect_success 'changed commit with --stat diff option' '
>  	     a => b | 0
>  	     1 file changed, 0 insertions(+), 0 deletions(-)
>  	3:  $(test_oid t3) ! 3:  $(test_oid c3) s/11/B/
> -	     a => b | 0
> -	     1 file changed, 0 insertions(+), 0 deletions(-)
> +	     a => b | 2 +-
> +	     1 file changed, 1 insertion(+), 1 deletion(-)
>  	4:  $(test_oid t4) ! 4:  $(test_oid c4) s/12/B/
> -	     a => b | 0
> -	     1 file changed, 0 insertions(+), 0 deletions(-)
> +	     a => b | 2 +-
> +	     1 file changed, 1 insertion(+), 1 deletion(-)

The tests definitely demonstrate that the old behavior was wrong,
though...

Thanks,
Taylor