Re: [PATCHv2] tag: add --points-at list option

[Tom Grennan <tmgrennan@gmail.com>]

On Tue, Feb 07, 2012 at 04:30:12PM -0500, Jeff King wrote:
>On Tue, Feb 07, 2012 at 12:20:44PM -0800, Junio C Hamano wrote:
>
>> Jeff King <peff@peff.net> writes:
>> 
>> >> I think the following would show the pointed at tag too.
>> >>   $ git tag my-v1.7.9 v1.7.9
>> >>   $ ./git-tag -l --points-at v1.7.9
>> >>   my-v1.7.9
>> >>   v1.7.9
>> >> 
>> >> vs.
>> >> 
>> >>   $ ./git-tag -l --points-at v1.7.9
>> >>   my-v1.7.9
>> >> 
>> >> I found that I had to filter matching refnames.
>> >
>> > Ah, so you are trying _not_ to show lightweight tags (I thought you
>> > meant you also wanted to show them)? But I still don't see why the code
>> > I posted before wouldn't work in that case. The "object" field of v1.7.9
>> > is not the sha1 of the v1.7.9 tag object, but rather some commit, so it
>> > would not match.
>> 
>> I think he is trying to avoid saying "v1.7.9 points at itself", and wants
>> to know not just the value of $(rev-parse v1.7.9) but the refname.
>
>Hmm. I read his example again, and now I'm even more confused.
>
>If I give an object name to --points-at, should or should not a
>lightweight tag pointing to that object be found?
>
>If not, then I don't see how "git tag --points-at v1.7.9" would find
>v1.7.9. Because we would use get_sha1 to parse "v1.7.9", returning the
>sha1 of the tag object. And then when trying to match, we would look at
>each tag object, find its "object" line, and compare that. In the case
>of considering whether to show the v1.7.9 tag, we would be comparing the
>sha1 of the commit that it points to to the actual tag sha1 itself, and
>not match.
>
>But in that case, nor would we match "my-v1.7.9" above, as it is a
>lightweight tag that also points to v1.7.9's tag object.
>
>If we _do_ want to match lightweight tags, then in the matching phase we
>look for both the sha1 contained in the tag ref, as well as the sha1 of
>the thing the tag points to (_if_ it is a tag object). In that case, we
>would find both v1.7.9 and my-v1.7.9.
>
>So I am not sure which is preferable. But I don't see how you could or
>would want to distinguish the two tags above. They are functionally
>identical, in that they are both refs pointing to the exact same tag
>object. If the example had started with "git tag -s my-v1.7.9 v1.7.9"
>then it would make more sense to me.

v1 and v2 wouldn't list lightweight tags of the points-at objects.
Both versions behave like this:
  $ git tag my-lw-v1.7.9 v1.7.9
  $ git tag my-a-v1.7.9 v1.7.9
  $ git tag my-s-v1.7.9 v1.7.9
  $ git tag -l --points-at v1.7.9
  my-a-v1.7.9
  my-s-v1.7.9

While addressing Junio's comments I realized that by first matching the
sha's and not refnames like the following will show LW tags too.
So, v3 will act like this:

  $ git tag my-lw-v1.7.9 v1.7.9
  $ git tag my-a-v1.7.9 v1.7.9
  $ git tag my-s-v1.7.9 v1.7.9
  $ git tag -l --points-at v1.7.9
  my-lw-v1.7.9
  my-a-v1.7.9
  my-s-v1.7.9

Note, w/o strcmp(pa->refname, refname), this shows the points-at too:

  $ git tag my-lw-v1.7.9 v1.7.9
  $ git tag my-a-v1.7.9 v1.7.9
  $ git tag my-s-v1.7.9 v1.7.9
  $ git tag -l --points-at v1.7.9
  my-lw-v1.7.9
  my-a-v1.7.9
  my-s-v1.7.9
  v1.7.9

Which I don't think we'd want.

static struct points_at *match_points_at(struct points_at *points_at,
					 const char *refname,
					 const unsigned char *sha1)
{
	struct object *obj;
	struct points_at *pa;
	const unsigned char *tagged_sha1;

	/* First look for lightweight tags - those with matching sha's
	 * but different names */
	for (pa = points_at; pa; pa = pa->next)
		if (!hashcmp(pa->sha1, sha1) && strcmp(pa->refname, refname))
			return pa;
	obj = parse_object(sha1);
	if (!obj || obj->type != OBJ_TAG)
		return 0;
	tagged_sha1 = ((struct tag *)obj)->tagged->sha1;
	while (points_at && hashcmp(points_at->sha1, tagged_sha1))
		points_at = points_at->next;
	return points_at;
}

-- 
TomG