refs: keep track of unresolved reference value in iterators

Since ref iterators do not hold onto the direct value of a reference
without resolving it, the only way to get ahold of a direct value of a
symbolic ref is to make a separate call to refs_read_symbolic_ref.

To make accessing the direct value of a symbolic ref more efficient,
let's save the direct value of the ref in the iterators for both the
files backend and the reftable backend.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/files-backend.c

@@ -245,9 +245,12 @@ static void loose_fill_ref_dir_regular_file(struct files_ref_store *refs,
 {
 	struct object_id oid;
 	int flag;
+	const char *referent = refs_resolve_ref_unsafe(&refs->base,
+						       refname,
+						       RESOLVE_REF_READING,
+						       &oid, &flag);
 
-	if (!refs_resolve_ref_unsafe(&refs->base, refname, RESOLVE_REF_READING,
-				     &oid, &flag)) {
+	if (!referent) {
 		oidclr(&oid, the_repository->hash_algo);
 		flag |= REF_ISBROKEN;
 	} else if (is_null_oid(&oid)) {
@@ -268,7 +271,11 @@ static void loose_fill_ref_dir_regular_file(struct files_ref_store *refs,
 		oidclr(&oid, the_repository->hash_algo);
 		flag |= REF_BAD_NAME | REF_ISBROKEN;
 	}
-	add_entry_to_dir(dir, create_ref_entry(refname, &oid, flag));
+
+	if (!(flag & REF_ISSYMREF))
+		referent = NULL;
+
+	add_entry_to_dir(dir, create_ref_entry(refname, referent, &oid, flag));
 }
 
 /*
@@ -886,6 +893,8 @@ static int files_ref_iterator_advance(struct ref_iterator *ref_iterator)
 		iter->base.refname = iter->iter0->refname;
 		iter->base.oid = iter->iter0->oid;
 		iter->base.flags = iter->iter0->flags;
+		iter->base.referent = iter->iter0->referent;
+
 		return ITER_OK;
 	}
 

refs/iterator.c

@@ -29,6 +29,7 @@ void base_ref_iterator_init(struct ref_iterator *iter,
 {
 	iter->vtable = vtable;
 	iter->refname = NULL;
+	iter->referent = NULL;
 	iter->oid = NULL;
 	iter->flags = 0;
 }
@@ -199,6 +200,7 @@ static int merge_ref_iterator_advance(struct ref_iterator *ref_iterator)
 		}
 
 		if (selection & ITER_YIELD_CURRENT) {
+			iter->base.referent = (*iter->current)->referent;
 			iter->base.refname = (*iter->current)->refname;
 			iter->base.oid = (*iter->current)->oid;
 			iter->base.flags = (*iter->current)->flags;

refs/ref-cache.c

@@ -34,6 +34,7 @@ struct ref_dir *get_ref_dir(struct ref_entry *entry)
 }
 
 struct ref_entry *create_ref_entry(const char *refname,
+				   const char *referent,
 				   const struct object_id *oid, int flag)
 {
 	struct ref_entry *ref;
@@ -41,6 +42,8 @@ struct ref_entry *create_ref_entry(const char *refname,
 	FLEX_ALLOC_STR(ref, name, refname);
 	oidcpy(&ref->u.value.oid, oid);
 	ref->flag = flag;
+	ref->u.value.referent = xstrdup_or_null(referent);
+
 	return ref;
 }
 
@@ -66,6 +69,7 @@ static void free_ref_entry(struct ref_entry *entry)
 		 */
 		clear_ref_dir(&entry->u.subdir);
 	}
+	free(entry->u.value.referent);
 	free(entry);
 }
 
@@ -431,6 +435,7 @@ static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
 			level->index = -1;
 		} else {
 			iter->base.refname = entry->name;
+			iter->base.referent = entry->u.value.referent;
 			iter->base.oid = &entry->u.value.oid;
 			iter->base.flags = entry->flag;
 			return ITER_OK;

refs/ref-cache.h

@@ -42,6 +42,7 @@ struct ref_value {
 	 * referred to by the last reference in the symlink chain.
 	 */
 	struct object_id oid;
+	char *referent;
 };
 
 /*
@@ -173,6 +174,7 @@ struct ref_entry *create_dir_entry(struct ref_cache *cache,
 				   const char *dirname, size_t len);
 
 struct ref_entry *create_ref_entry(const char *refname,
+				   const char *referent,
 				   const struct object_id *oid, int flag);
 
 /*

refs/refs-internal.h

@@ -299,6 +299,7 @@ enum do_for_each_ref_flags {
 struct ref_iterator {
 	struct ref_iterator_vtable *vtable;
 	const char *refname;
+	const char *referent;
 	const struct object_id *oid;
 	unsigned int flags;
 };

refs/reftable-backend.c

@@ -455,6 +455,7 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 	struct reftable_ref_iterator *iter =
 		(struct reftable_ref_iterator *)ref_iterator;
 	struct reftable_ref_store *refs = iter->refs;
+	const char *referent = NULL;
 
 	while (!iter->err) {
 		int flags = 0;
@@ -494,8 +495,12 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 				the_repository->hash_algo);
 			break;
 		case REFTABLE_REF_SYMREF:
-			if (!refs_resolve_ref_unsafe(&iter->refs->base, iter->ref.refname,
-						     RESOLVE_REF_READING, &iter->oid, &flags))
+			referent = refs_resolve_ref_unsafe(&iter->refs->base,
+								      iter->ref.refname,
+								      RESOLVE_REF_READING,
+								      &iter->oid,
+								      &flags);
+			if (!referent)
 				oidclr(&iter->oid, the_repository->hash_algo);
 			break;
 		default:
@@ -523,6 +528,7 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 				continue;
 
 		iter->base.refname = iter->ref.refname;
+		iter->base.referent = referent;
 		iter->base.oid = &iter->oid;
 		iter->base.flags = flags;