ref-filter.c: use peeled tag for '*' format fields

In most builtins ('rev-parse <revision>^{}', 'show-ref --dereference'),
"dereferencing" a tag refers to a recursive peel of the tag object. Unlike
these cases, the dereferencing prefix ('*') in 'for-each-ref' format
specifiers triggers only a single, non-recursive dereference of a given tag
object. For most annotated tags, a single dereference is all that is needed
to access the tag's associated commit or tree; "recursive" and
"non-recursive" dereferencing are functionally equivalent in these cases.
However, nested tags (annotated tags whose target is another annotated tag)
dereferenced once return another tag, where a recursive dereference would
return the commit or tree.

Currently, if a user wants to filter & format refs and include information
about a recursively-dereferenced tag, they can do so with something like
'cat-file --batch-check':

    git for-each-ref --format="%(objectname)^{} %(refname)" <pattern> |
        git cat-file --batch-check="%(objectname) %(rest)"

But the combination of commands is inefficient. So, to improve the
performance of this use case and align the defererencing behavior of
'for-each-ref' with that of other commands, update the ref formatting code
to use the peeled tag (from 'peel_iterated_oid()') to populate '*' fields
rather than the tag's immediate target object (from 'get_tagged_oid()').

Additionally, add a test to 't6300-for-each-ref' to verify new nested tag
behavior and update 't6302-for-each-ref-filter.sh' to print the correct
value for nested dereferenced fields.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 9 deletions

diff --git a/ref-filter.c b/ref-filter.c
index ca4ebed..b1093ae 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -2424,17 +2424,12 @@ static int populate_value(struct ref_array_item *ref, struct strbuf *err)
 		return 0;
 
 	/*
-	 * If it is a tag object, see if we use a value that derefs
-	 * the object, and if we do grab the object it refers to.
+	 * If it is a tag object, see if we use the peeled value. If we do,
+	 * grab the peeled OID.
 	 */
-	oi_deref.oid = *get_tagged_oid((struct tag *)obj);
+	if (need_tagged && peel_iterated_oid(&obj->oid, &oi_deref.oid))
+		die("bad tag");
 
-	/*
-	 * NEEDSWORK: This derefs tag only once, which
-	 * is good to deal with chains of trust, but
-	 * is not consistent with what deref_tag() does
-	 * which peels the onion to the core.
-	 */
 	return get_object(ref, 1, &obj, &oi_deref, err);
 }