Ticket #25723: 25723.2.diff

src/wp-admin/update.php

@@ -252 +252 @@
                 include(ABSPATH . 'wp-admin/admin-footer.php');
 
         } else {
-                do_action('update-custom_' . $action);
+                /**
+                 * Fires when a custom update request is received.
+                 *
+                 * The dynamic portion of the hook name, $action, refers to the action
+                 * provided in the request for wp-admin/update.php. Can be used to
+                 * provide custom update functionality for themes and plugins.
+                 *
+                 * @since 2.8.0
+                 */
+                do_action( "update-custom_{$action}" );
         }
 }