Re: [PATCH v2] diff: Fix modified lines stats with --stat and --numstat

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> Summarizing the above, I think the second best fix is this (which
> means that the posted patch is the third best):
>
> 	/*
> 	 * diff_fill_oid_info() marked one/two->oid with null_oid
> 	 * for a path whose oid is not available.  Disable early
> 	 * return optimization for them.
> 	 */
> 	if (oideq(&one->oid, &null_oid) || oideq(&two->oid, &null_oid))
> 		same_contents = 0; /* could be different */
> 	else if (oideq(&one->oid, &two->oid))
> 		same_contents = 1; /* definitely the same */
> 	else
> 		same_contents = 0; /* definitely different */

A tangent.

There is this code in diff.c::fill_metainfo() that is used to
populate the "index" header element of "diff --patch" output:

	if (one && two && !oideq(&one->oid, &two->oid)) {
		const unsigned hexsz = the_hash_algo->hexsz;
		int abbrev = o->abbrev ? o->abbrev : DEFAULT_ABBREV;

		if (o->flags.full_index)
			abbrev = hexsz;

		if (o->flags.binary) {
			mmfile_t mf;
			if ((!fill_mmfile(o->repo, &mf, one) &&
			     diff_filespec_is_binary(o->repo, one)) ||
			    (!fill_mmfile(o->repo, &mf, two) &&
			     diff_filespec_is_binary(o->repo, two)))
				abbrev = hexsz;
		}
		strbuf_addf(msg, "%s%sindex %s..%s", line_prefix, set,
			    diff_abbrev_oid(&one->oid, abbrev),
			    diff_abbrev_oid(&two->oid, abbrev));
		if (one->mode == two->mode)
			strbuf_addf(msg, " %06o", one->mode);
		strbuf_addf(msg, "%s\n", reset);
	}

Currently it is OK because there can only be one side that
diff_fill_oid_info() would mark as "oid unavailable" (e.g. reading
standard input stream).  If a new feature is introducing a situation
where both ends have null_oid, which was so far been impossible,
we'd probably need to factor out the condition used in the above
into a helper function, e.g.

    static int cannot_be_the_same(struct diff_filespec *one, struct diff_filespec *two)
    {
	if ((oideq(&one->oid, &null_oid) || oideq(&two->oid, &null_oid))
		return 1;
	else if (oideq(&one->oid, &two->oid))
		return 0;
	else
		return 1;
    }

and rewrite the conditional in fill_metainfo() to

	if (one && two && cannot_be_the_same(one, two)) {
		...

The "second best fix" could then become a single liner:

	same_contents = !cannot_be_the_same(one, two);

using the helper.