refs: expose peeled object ID via the iterator

Both the "files" and "reftable" backend are able to store peeled values
for tags in the respective formats. This allows for a more efficient
lookup of the target object of such a tag without having to manually
peel via the object database.

The infrastructure to access these peeled object IDs is somewhat funky
though. When iterating through objects, we store a pointer reference to
the current iterator in a global variable. The callbacks invoked by that
iterator are then expected to call `peel_iterated_oid()`, which checks
whether the globally-stored iterator's current reference refers to the
one handed into that function. If so, we ask the iterator to peel the
object, otherwise we manually peel the object via the object database.
Depending on global state like this is somewhat weird and also quite
fragile.

Introduce a new `struct reference::peeled_oid` field that can be
populated by the reference backends. This field can be accessed via a
new function `reference_get_peeled_oid()` that either uses that value,
if set, or alternatively peels via the ODB. With this change we don't
have to rely on global state anymore, but make the peeled object ID
available to the callback functions directly.

Adjust trivial callers that already have a `struct reference` available.
Remaining callers will be adjusted in subsequent commits.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/describe.c

@@ -208,7 +208,7 @@ static int get_name(const struct reference *ref, void *cb_data UNUSED)
 	}
 
 	/* Is it annotated? */
-	if (!peel_iterated_oid(the_repository, ref->oid, &peeled)) {
+	if (!reference_get_peeled_oid(the_repository, ref, &peeled)) {
 		is_annotated = !oideq(ref->oid, &peeled);
 	} else {
 		oidcpy(&peeled, ref->oid);

builtin/gc.c

@@ -1109,7 +1109,7 @@ static int dfs_on_ref(const struct reference *ref, void *cb_data)
 	struct commit_list *stack = NULL;
 	struct commit *commit;
 
-	if (!peel_iterated_oid(the_repository, ref->oid, &peeled))
+	if (!reference_get_peeled_oid(the_repository, ref, &peeled))
 		maybe_peeled = &peeled;
 	if (odb_read_object_info(the_repository->objects, maybe_peeled, NULL) != OBJ_COMMIT)
 		return 0;

builtin/pack-objects.c

@@ -838,7 +838,7 @@ static int mark_tagged(const struct reference *ref, void *cb_data UNUSED)
 
 	if (entry)
 		entry->tagged = 1;
-	if (!peel_iterated_oid(the_repository, ref->oid, &peeled)) {
+	if (!reference_get_peeled_oid(the_repository, ref, &peeled)) {
 		entry = packlist_find(&to_pack, &peeled);
 		if (entry)
 			entry->tagged = 1;
@@ -3309,7 +3309,8 @@ static int add_ref_tag(const struct reference *ref, void *cb_data UNUSED)
 {
 	struct object_id peeled;
 
-	if (!peel_iterated_oid(the_repository, ref->oid, &peeled) && obj_is_packed(&peeled))
+	if (!reference_get_peeled_oid(the_repository, ref, &peeled) &&
+	    obj_is_packed(&peeled))
 		add_tag_chain(ref->oid);
 	return 0;
 }
@@ -4537,7 +4538,7 @@ static int mark_bitmap_preferred_tip(const struct reference *ref, void *data UNU
 	struct object_id peeled;
 	struct object *object;
 
-	if (!peel_iterated_oid(the_repository, ref->oid, &peeled))
+	if (!reference_get_peeled_oid(the_repository, ref, &peeled))
 		maybe_peeled = &peeled;
 
 	object = parse_object_or_die(the_repository, maybe_peeled, ref->name);

commit-graph.c

@@ -1857,7 +1857,7 @@ static int add_ref_to_set(const struct reference *ref, void *cb_data)
 	struct object_id peeled;
 	struct refs_cb_data *data = (struct refs_cb_data *)cb_data;
 
-	if (!peel_iterated_oid(data->repo, ref->oid, &peeled))
+	if (!reference_get_peeled_oid(data->repo, ref, &peeled))
 		maybe_peeled = &peeled;
 	if (odb_read_object_info(data->repo->objects, maybe_peeled, NULL) == OBJ_COMMIT)
 		oidset_insert(data->commits, maybe_peeled);

ls-refs.c

@@ -110,7 +110,7 @@ static int send_ref(const struct reference *ref, void *cb_data)
 
 	if (data->peel && ref->oid) {
 		struct object_id peeled;
-		if (!peel_iterated_oid(the_repository, ref->oid, &peeled))
+		if (!reference_get_peeled_oid(the_repository, ref, &peeled))
 			strbuf_addf(&data->buf, " peeled:%s", oid_to_hex(&peeled));
 	}
 

midx-write.c

@@ -709,7 +709,7 @@ static int add_ref_to_pending(const struct reference *ref, void *cb_data)
 		return 0;
 	}
 
-	if (!peel_iterated_oid(revs->repo, ref->oid, &peeled))
+	if (!reference_get_peeled_oid(revs->repo, ref, &peeled))
 		maybe_peeled = &peeled;
 
 	object = parse_object_or_die(revs->repo, maybe_peeled, ref->name);

pseudo-merge.c

@@ -230,7 +230,7 @@ static int find_pseudo_merge_group_for_ref(const struct reference *ref, void *_d
 	uint32_t i;
 	int has_bitmap;
 
-	if (!peel_iterated_oid(the_repository, ref->oid, &peeled))
+	if (!reference_get_peeled_oid(the_repository, ref, &peeled))
 		maybe_peeled = &peeled;
 
 	c = lookup_commit(the_repository, maybe_peeled);

refs.c

@@ -2334,6 +2334,18 @@ int peel_iterated_oid(struct repository *r, const struct object_id *base, struct
 	return peel_object(r, base, peeled) ? -1 : 0;
 }
 
+int reference_get_peeled_oid(struct repository *repo,
+			     const struct reference *ref,
+			     struct object_id *peeled_oid)
+{
+	if (ref->peeled_oid) {
+		oidcpy(peeled_oid, ref->peeled_oid);
+		return 0;
+	}
+
+	return peel_object(repo, ref->oid, peeled_oid) ? -1 : 0;
+}
+
 int refs_update_symref(struct ref_store *refs, const char *ref,
 		       const char *target, const char *logmsg)
 {

refs.h

@@ -371,10 +371,29 @@ struct reference {
 	 */
 	const struct object_id *oid;
 
+	/*
+	 * An optional peeled object ID. This field _may_ be set for tags in
+	 * case the peeled value is present in the backend. Please refer to
+	 * `reference_get_peeled_oid()`.
+	 */
+	const struct object_id *peeled_oid;
+
 	/* A bitfield of `enum reference_status` flags. */
 	unsigned flags;
 };
 
+/*
+ * Peel the tag to a non-tag commit. If present, this uses the peeled object ID
+ * exposed by the reference backend. Otherwise, the object is peeled via the
+ * object database, which is less efficient.
+ *
+ * Return `0` if the reference could be peeled, a negative error code
+ * otherwise.
+ */
+int reference_get_peeled_oid(struct repository *repo,
+			     const struct reference *ref,
+			     struct object_id *peeled_oid);
+
 /*
  * The signature for the callback function for the for_each_*()
  * functions below.  The memory pointed to by the `struct reference`

refs/packed-backend.c

@@ -963,6 +963,7 @@ static int next_record(struct packed_ref_iterator *iter)
 			iter->base.ref.flags &= ~REF_KNOWS_PEELED;
 		} else {
 			iter->base.ref.flags |= REF_KNOWS_PEELED;
+			iter->base.ref.peeled_oid = &iter->peeled;
 		}
 	} else {
 		oidclr(&iter->peeled, iter->repo->hash_algo);

refs/reftable-backend.c

@@ -547,6 +547,7 @@ struct reftable_ref_iterator {
 	struct reftable_iterator iter;
 	struct reftable_ref_record ref;
 	struct object_id oid;
+	struct object_id peeled_oid;
 
 	char *prefix;
 	size_t prefix_len;
@@ -671,6 +672,8 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 		case REFTABLE_REF_VAL2:
 			oidread(&iter->oid, iter->ref.value.val2.value,
 				refs->base.repo->hash_algo);
+			oidread(&iter->peeled_oid, iter->ref.value.val2.target_value,
+				refs->base.repo->hash_algo);
 			break;
 		case REFTABLE_REF_SYMREF:
 			referent = refs_resolve_ref_unsafe(&iter->refs->base,
@@ -708,6 +711,8 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 		iter->base.ref.name = iter->ref.refname;
 		iter->base.ref.target = referent;
 		iter->base.ref.oid = &iter->oid;
+		if (iter->ref.value_type == REFTABLE_REF_VAL2)
+			iter->base.ref.peeled_oid = &iter->peeled_oid;
 		iter->base.ref.flags = flags;
 
 		break;

repack-midx.c

@@ -22,7 +22,7 @@ static int midx_snapshot_ref_one(const struct reference *ref, void *_data)
 	const struct object_id *maybe_peeled = ref->oid;
 	struct object_id peeled;
 
-	if (!peel_iterated_oid(data->repo, ref->oid, &peeled))
+	if (!reference_get_peeled_oid(data->repo, ref, &peeled))
 		maybe_peeled = &peeled;
 
 	if (oidset_insert(&data->seen, maybe_peeled))