#261148 by chx and pwolanin: Allow recovery from a failed menu build.

includes/menu.inc

@@ -387,7 +387,9 @@ function menu_execute_active_handler($path = NULL) {
   if (_menu_site_is_offline()) {
     return MENU_SITE_OFFLINE;
   }
-  if (variable_get('menu_rebuild_needed', FALSE)) {
+  // Rebuild if we know it's needed, or if the menu masks are missing which
+  // occurs rarely, likely due to a race condition of multiple rebuilds.
+  if (variable_get('menu_rebuild_needed', FALSE) || !variable_get('menu_masks', array())) {
     menu_rebuild();
   }
   if ($router_item = menu_get_item($path)) {