odb: introduce odb_prepare()

Introduce `odb_prepare()` as a simple wrapper to prepare alternates and
then prepare each individual source. Adapt git-grep(1) to use it.

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

Author: pks-t

builtin/grep.c

@@ -1356,13 +1356,8 @@ int cmd_grep(int argc,
 		if (recurse_submodules)
 			repo_read_gitmodules(the_repository, 1);
 
-		if (startup_info->have_repository) {
-			struct odb_source *source;
-
-			odb_prepare_alternates(the_repository->objects);
-			for (source = the_repository->objects->sources; source; source = source->next)
-				odb_source_prepare(source, 0);
-		}
+		if (startup_info->have_repository)
+			odb_prepare(the_repository->objects, 0);
 
 		start_threads(&opt);
 	} else {

odb.c

@@ -1070,7 +1070,7 @@ void odb_free(struct object_database *o)
 	free(o);
 }
 
-void odb_reprepare(struct object_database *o)
+void odb_prepare(struct object_database *o, enum odb_prepare_flags flags)
 {
 	struct odb_source *source;
 
@@ -1082,13 +1082,19 @@ void odb_reprepare(struct object_database *o)
 	 * the linked list, so existing odbs will continue to exist for
 	 * the lifetime of the process.
 	 */
-	o->loaded_alternates = 0;
-	odb_prepare_alternates(o);
+	if (flags & ODB_PREPARE_FLUSH_CACHES) {
+		o->loaded_alternates = 0;
+		o->object_count_valid = 0;
+	}
 
+	odb_prepare_alternates(o);
 	for (source = o->sources; source; source = source->next)
-		odb_source_prepare(source, ODB_PREPARE_FLUSH_CACHES);
-
-	o->object_count_valid = 0;
+		odb_source_prepare(source, flags);
 
 	obj_read_unlock();
 }
+
+void odb_reprepare(struct object_database *o)
+{
+	odb_prepare(o, ODB_PREPARE_FLUSH_CACHES);
+}

odb.h

@@ -133,9 +133,13 @@ enum odb_prepare_flags {
 };
 
 /*
- * Clear caches, reload alternates and then reload object sources so that new
- * objects may become accessible.
+ * Prepare the object database for use. Calling this function is generally not
+ * needed, but can be useful in case the caller wants to pre-open individual
+ * sources.
  */
+void odb_prepare(struct object_database *o, enum odb_prepare_flags flags);
+
+/* Equivalent to `odb_prepare(o, ODB_PREPARE_FLUSH_CACHES)`. */
 void odb_reprepare(struct object_database *o);
 
 /*