- Patch #836630 by catch: module_implements() caching is broken.

includes/module.inc

@@ -586,6 +586,9 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
   }
 
   if (!isset($implementations[$hook])) {
+    // The hook is not cached, so ensure that whether or not it has
+    // implementations, that the cache is updated at the end of the request.
+    $implementations['#write_cache'] = TRUE;
     $hook_info = module_hook_info();
     $implementations[$hook] = array();
     $list = module_list(FALSE, FALSE, $sort);
@@ -593,8 +596,6 @@ function module_implements($hook, $sort = FALSE, $reset = FALSE) {
       $include_file = isset($hook_info[$hook]['group']) && module_load_include('inc', $module, $module . '.' . $hook_info[$hook]['group']);
       if (module_hook($module, $hook)) {
         $implementations[$hook][$module] = $include_file ? $hook_info[$hook]['group'] : FALSE;
-        // We added something to the cache, so write it when we are done.
-        $implementations['#write_cache'] = TRUE;
       }
     }
     // Allow modules to change the weight of specific implementations but avoid