class MaintenanceModeSubscriber

1. 8.x drupal/core/lib/Drupal/Core/EventSubscriber/MaintenanceModeSubscriber.php \Drupal\Core\EventSubscriber\MaintenanceModeSubscriber
2. 8.x drupal/core/modules/user/lib/Drupal/user/EventSubscriber/MaintenanceModeSubscriber.php \Drupal\user\EventSubscriber\MaintenanceModeSubscriber
3. 8.x drupal/core/modules/system/tests/modules/menu_test/lib/Drupal/menu_test/EventSubscriber/MaintenanceModeSubscriber.php \Drupal\menu_test\EventSubscriber\MaintenanceModeSubscriber

menu_test.services.yml in drupal/core/modules/system/tests/modules/menu_test/menu_test.services.yml

drupal/core/modules/system/tests/modules/menu_test/menu_test.services.yml

menu_test_maintenance_mode_subscriber in drupal/core/modules/system/tests/modules/menu_test/menu_test.services.yml

Drupal\menu_test\EventSubscriber\MaintenanceModeSubscriber

class MaintenanceModeSubscriber implements EventSubscriberInterface {

  /**
   * Set the page online if called from a certain path.
   *
   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
   *   The event to process.
   */
  public function onKernelRequestMaintenance(GetResponseEvent $event) {
    $request = $event
      ->getRequest();

    // Allow access to menu_login_callback even if in maintenance mode.
    if ($request->attributes
      ->get('_maintenance') == MENU_SITE_OFFLINE && $request->attributes
      ->get('system_path') == 'menu_login_callback') {
      $request->attributes
        ->set('_maintenance', MENU_SITE_ONLINE);
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events[KernelEvents::REQUEST][] = array(
      'onKernelRequestMaintenance',
      35,
    );
    return $events;
  }

}