Add a MaintenanceModeSubscriber.

core/lib/Drupal/Core/DrupalKernel.php

@@ -23,6 +23,7 @@
 use Drupal\Core\EventSubscriber\AccessSubscriber;
 use Drupal\Core\EventSubscriber\PathSubscriber;
 use Drupal\Core\EventSubscriber\LegacyControllerSubscriber;
+use Drupal\Core\EventSubscriber\MaintenanceModeSubscriber;
 
 use Exception;
 
@@ -70,6 +71,7 @@ public function __construct(EventDispatcherInterface $dispatcher, ControllerReso
       // replacement system.
       $this->dispatcher->addSubscriber(new ViewSubscriber($negotiation));
       $this->dispatcher->addSubscriber(new AccessSubscriber());
+      $this->dispatcher->addSubscriber(new MaintenanceModeSubscriber());
       $this->dispatcher->addSubscriber(new PathSubscriber());
       $this->dispatcher->addSubscriber(new LegacyControllerSubscriber());
 

core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php

+<?php
+
+namespace Drupal\Core\EventSubscriber;
+
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\KernelEvents;
+use Symfony\Component\HttpKernel\Event\GetResponseEvent;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * @file
+ * Definition of Drupal\Core\EventSubscriber\MaintenanceModeSubscriber.
+ */
+
+/**
+ * Maintenance mode subscriber for controller requests.
+ */
+class MaintenanceModeSubscriber implements EventSubscriberInterface {
+
+  /**
+   * Response with the maintenance page when the site is offline.
+   *
+   * @param GetResponseEvent $event
+   *   The Event to process.
+   */
+  public function onKernelRequestMaintenanceModeCheck(GetResponseEvent $event) {
+    // Check if the site is offline.
+    $page_callback_result = _menu_site_is_offline() ? MENU_SITE_OFFLINE : MENU_SITE_ONLINE;
+
+    // Allow other modules to change the site status but not the path because
+    // that would not change the global variable. hook_url_inbound_alter() can
+    // be used to change the path. Code later will not use the $read_only_path
+    // variable.
+    $read_only_path = !empty($path) ? $path : $_GET['q'];
+    drupal_alter('menu_site_status', $page_callback_result, $read_only_path);
+
+    // Only continue if the site is online.
+    if ($page_callback_result != MENU_SITE_ONLINE) {
+      // Deliver the 503 page.
+      drupal_maintenance_theme();
+      drupal_set_title(t('Site under maintenance'));
+      $content = theme('maintenance_page', array('content' => filter_xss_admin(variable_get('maintenance_mode_message', t('@site is currently under maintenance. We should be back shortly. Thank you for your patience.', array('@site' => variable_get('site_name', 'Drupal')))))));
+      $response = new Response('Service unavailable', 503);
+      $response->setContent($content);
+      $event->setResponse($response);
+    }
+  }
+
+  /**
+   * Registers the methods in this class that should be listeners.
+   *
+   * @return array
+   *   An array of event listener definitions.
+   */
+  static function getSubscribedEvents() {
+    $events[KernelEvents::REQUEST][] = array('onKernelRequestMaintenanceModeCheck', 40);
+    return $events;
+  }
+}