load_ref_decorations(): fix decoration with tags

Commit 88473c8bae ("load_ref_decorations(): avoid parsing non-tag
objects", 2021-06-22) introduced a shortcut to `add_ref_decoration()`:
Rather than calling `parse_object()`, we go for `oid_object_info()` and
then `lookup_object_by_type()` using the type just discovered. As
detailed in the commit message, this provides a significant time saving.

Unfortunately, it also changes the behavior: We lose all annotated tags
from the decoration.

The reason this happens is in the loop where we try to peel the tags, we
won't necessarily have parsed that first object. If we haven't, its
`tagged` field will be NULL, so we won't actually add a decoration for
the pointed-to object.

Make sure to parse the tag object at the top of the peeling loop. This
effectively restores the pre-88473c8bae parsing -- but only of tags,
allowing us to keep most of the possible speedup from 88473c8bae.

On my big ~220k ref test case (where it's mostly non-tags), the
timings [using "git log -1 --decorate"] are:

  - before either patch: 2.945s
  - with my broken patch: 0.707s
  - with [this patch]: 0.788s

The simplest way to do this is to just conditionally parse before the
loop:

  if (obj->type == OBJ_TAG)
          parse_object(&obj->oid);

But we can observe that our tag-peeling loop needs to peel already, to
examine recursive tags-of-tags. So instead of introducing a new call to
parse_object(), we can simply move the parsing higher in the loop:
instead of parsing the new object before we loop, parse each tag object
before we look at its "tagged" field.

This has another beneficial side effect: if a tag points at a commit (or
other non-tag type), we do not bother to parse the commit at all now.
And we know it is a commit without calling oid_object_info(), because
parsing the surrounding tag object will have created the correct in-core
object based on the "type" field of the tag.

Our test coverage for --decorate was obviously not good, since we missed
this quite-basic regression. The new tests covers an annotated tag
(showing the fix), but also that we correctly show annotations for
lightweight tags and double-annotated tag-of-tags.

Reported-by: Martin Ågren <martin.agren@gmail.com>
Helped-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/log-tree.c b/log-tree.c
index 4f69ed1..6dc4412 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -174,11 +174,11 @@ static int add_ref_decoration(const char *refname, const struct object_id *oid,
 
 	add_name_decoration(deco_type, refname, obj);
 	while (obj->type == OBJ_TAG) {
+		if (!obj->parsed)
+			parse_object(the_repository, &obj->oid);
 		obj = ((struct tag *)obj)->tagged;
 		if (!obj)
 			break;
-		if (!obj->parsed)
-			parse_object(the_repository, &obj->oid);
 		add_name_decoration(DECORATION_REF_TAG, refname, obj);
 	}
 	return 0;