ClientErrorResponseSubscriber.php

<?php

namespace Drupal\Core\EventSubscriber;

use Drupal\Core\Cache\CacheableMetadata;
use Drupal\Core\Cache\CacheableResponseInterface;
use Symfony\Component\HttpKernel\Event\ResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Response subscriber to set the '4xx-response' cache tag on 4xx responses.
 */
class ClientErrorResponseSubscriber implements EventSubscriberInterface {

  /**
   * Sets the '4xx-response' cache tag on 4xx responses.
   *
   * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
   *   The event to process.
   */
  public function onRespond(ResponseEvent $event) {
    if (!$event
      ->isMainRequest()) {
      return;
    }
    $response = $event
      ->getResponse();
    if (!$response instanceof CacheableResponseInterface) {
      return;
    }
    if ($response
      ->isClientError()) {
      $http_4xx_response_cacheability = new CacheableMetadata();
      $http_4xx_response_cacheability
        ->setCacheTags([
        '4xx-response',
      ]);
      $response
        ->addCacheableDependency($http_4xx_response_cacheability);
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() : array {

    // Priority 10, so that it runs before FinishResponseSubscriber, which will
    // expose the cacheability metadata in the form of headers.
    $events[KernelEvents::RESPONSE][] = [
      'onRespond',
      10,
    ];
    return $events;
  }

}