cgfsng: set errno to ENOENT on get_hierarchy()

src/lxc/cgroups/cgfsng.c

@@ -179,15 +179,19 @@ static void must_append_controller(char **klist, char **nlist, char ***clist,
 /* Given a handler's cgroup data, return the struct hierarchy for the controller
  * @c, or NULL if there is none.
  */
-struct hierarchy *get_hierarchy(struct cgroup_ops *ops, const char *c)
+struct hierarchy *get_hierarchy(struct cgroup_ops *ops, const char *controller)
 {
 	int i;
 
-	if (!ops->hierarchies)
+	errno = ENOENT;
+
+	if (!ops->hierarchies) {
+		TRACE("There are no useable cgroup controllers");
 		return NULL;
+	}
 
 	for (i = 0; ops->hierarchies[i]; i++) {
-		if (!c) {
+		if (!controller) {
 			/* This is the empty unified hierarchy. */
 			if (ops->hierarchies[i]->controllers &&
 			    !ops->hierarchies[i]->controllers[0])
@@ -196,10 +200,15 @@ struct hierarchy *get_hierarchy(struct cgroup_ops *ops, const char *c)
 			continue;
 		}
 
-		if (string_in_list(ops->hierarchies[i]->controllers, c))
+		if (string_in_list(ops->hierarchies[i]->controllers, controller))
 			return ops->hierarchies[i];
 	}
 
+	if (controller)
+		WARN("There is no useable %s controller", controller);
+	else
+		WARN("There is no empty unified cgroup hierarchy");
+
 	return NULL;
 }
 

src/lxc/cgroups/cgroup.h

@@ -99,6 +99,7 @@ struct cgroup_ops {
 	 *   hierarchy wins.
 	 */
 	struct hierarchy **hierarchies;
+	/* Pointer to the unified hierarchy. Do not free! */
 	struct hierarchy *unified;
 
 	/*

src/lxc/commands.c

@@ -654,7 +654,7 @@ static int lxc_cmd_stop_callback(int fd, struct lxc_cmd_req *req,
 		 * lxc_unfreeze() would do another cmd (GET_CGROUP) which would
 		 * deadlock us.
 		 */
-		if (cgroup_ops->get_cgroup(cgroup_ops, "freezer") == NULL)
+		if (!cgroup_ops->get_cgroup(cgroup_ops, "freezer"))
 			return 0;
 
 		if (cgroup_ops->unfreeze(cgroup_ops))