Re: [PATCH 2/2] diff-lib: skip paths outside prefix in oneway_diff()

On Sun, Jul 26, 2026 at 04:47:05AM -0400, Jeff King wrote:

> diff --git a/diff-lib.c b/diff-lib.c
> index 95f920a9a0..9986f5b141 100644
> --- a/diff-lib.c
> +++ b/diff-lib.c
> @@ -528,6 +528,11 @@ static int oneway_diff(const struct cache_entry * const *src,
>  	if (tree == o->df_conflict_entry)
>  		tree = NULL;
>  
> +	if (revs->diffopt.prefix &&
> +	    strncmp((idx ? idx : tree)->name, revs->diffopt.prefix,
> +		    revs->diffopt.prefix_length))
> +		return 0;
> +

BTW, Coverity complains here that "tree" could be NULL (because we set
it that way in the lines above due to a D/F conflict).

I _think_ it is fine. We only look at "tree" if idx is NULL, and I think
idx is only NULL when we have a deletion. So that implies either:

  1. unpack_trees() passed us both entries as NULL, which doesn't make
     sense. There was no entry to delete!

  2. We set tree to NULL due to a D/F conflict. But a conflict with
     what? There is nothing at the path in the index to conflict.

So AFAICT this is OK and it's just a false positive from Coverity
(though an understandable one; the semantics of the relationship between
"idx" and "tree" are not represented in the code).

Possibly adding:

  if (!idx && !tree)
	BUG("oneway diff with no endpoints");

would help static analysis, but I don't know if that makes things more
or less clear to a human.

-Peff