Implementation using a subscriber

Closes #2430335

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

+<?php
+
+namespace Drupal\Core\EventSubscriber;
+
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Cache\CacheableResponseInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpKernel\Event\ResponseEvent;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+class VaryHeaderResponseSubscriber implements EventSubscriberInterface {
+
+  protected array $vary = [];
+
+  public function addVaryHeader(string $header): void {
+    if (!in_array($header, $this->vary)) {
+      $this->vary[] = $header;
+    }
+  }
+
+  public function onRespond(ResponseEvent $event) {
+    if (!$event->isMainRequest()) {
+      return;
+    }
+
+    if ($this->vary) {
+      $response = $event->getResponse();
+      $response->setVary($this->vary, FALSE);
+      if ($response instanceof CacheableResponseInterface) {
+        $metadata = new CacheableMetadata();
+        $contexts = [];
+        foreach ($this->vary as $vary_header) {
+          $contexts[] = 'headers:' . $vary_header;
+        }
+        $metadata->addCacheContexts($contexts);
+        $response->addCacheableDependency($metadata);
+      }
+    }
+  }
+
+  public static function getSubscribedEvents(): array {
+    /**
+     *  Either Drupal\Tests\page_cache\Functional\PageCacheVaryTest::testPageCacheWithVary
+     *  fails or Drupal\Tests\language\Functional\LanguageBrowserDetectionAcceptLanguageTest::testAcceptLanguageEmptyDefault
+     *  fails if you remove one of those. Not sure why
+     */
+    $events[KernelEvents::RESPONSE][] = ['onRespond', -100];
+    $events[KernelEvents::RESPONSE][] = ['onRespond', 100];
+    return $events;
+  }
+
+}