packfile: extract logic to count number of objects

In a subsequent commit we're about to introduce a new
`odb_source_count_objects()` function so that we can make the logic
pluggable. Prepare for this change by extracting the logic that we have
to count packed objects into a standalone function.

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

Author: pks-t

packfile.c

@@ -1101,6 +1101,36 @@ struct packfile_list_entry *packfile_store_get_packs(struct packfile_store *stor
 	return store->packs.head;
 }
 
+int packfile_store_count_objects(struct packfile_store *store,
+				 unsigned long *out)
+{
+	struct packfile_list_entry *e;
+	struct multi_pack_index *m;
+	unsigned long count = 0;
+	int ret;
+
+	m = get_multi_pack_index(store->source);
+	if (m)
+		count += m->num_objects + m->num_objects_in_base;
+
+	for (e = packfile_store_get_packs(store); e; e = e->next) {
+		if (e->pack->multi_pack_index)
+			continue;
+		if (open_pack_index(e->pack)) {
+			ret = -1;
+			goto out;
+		}
+
+		count += e->pack->num_objects;
+	}
+
+	*out = count;
+	ret = 0;
+
+out:
+	return ret;
+}
+
 /*
  * Give a fast, rough count of the number of objects in the repository. This
  * ignores loose objects completely. If you have a lot of them, then either
@@ -1113,21 +1143,16 @@ unsigned long repo_approximate_object_count(struct repository *r)
 	if (!r->objects->approximate_object_count_valid) {
 		struct odb_source *source;
 		unsigned long count = 0;
-		struct packed_git *p;
 
 		odb_prepare_alternates(r->objects);
-
 		for (source = r->objects->sources; source; source = source->next) {
-			struct multi_pack_index *m = get_multi_pack_index(source);
-			if (m)
-				count += m->num_objects + m->num_objects_in_base;
-		}
+			struct odb_source_files *files = odb_source_files_downcast(source);
+			unsigned long c;
 
-		repo_for_each_pack(r, p) {
-			if (p->multi_pack_index || open_pack_index(p))
-				continue;
-			count += p->num_objects;
+			if (!packfile_store_count_objects(files->packed, &c))
+				count += c;
 		}
+
 		r->objects->approximate_object_count = count;
 		r->objects->approximate_object_count_valid = 1;
 	}

packfile.h

@@ -268,6 +268,15 @@ enum kept_pack_type {
 	KEPT_PACK_IN_CORE = (1 << 1),
 };
 
+/*
+ * Count the number objects contained in the given packfile store. If
+ * successful, the number of objects will be written to the `out` pointer.
+ *
+ * Return 0 on success, a negative error code otherwise.
+ */
+int packfile_store_count_objects(struct packfile_store *store,
+				 unsigned long *out);
+
 /*
  * Retrieve the cache of kept packs from the given packfile store. Accepts a
  * combination of `kept_pack_type` flags. The cache is computed on demand and