bug symfony#44395 [HttpKernel] fix sending Vary: Accept-Language when appropriate (nicolas-grekas)

This PR was merged into the 5.4 branch.

Discussion
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[HttpKernel] fix sending Vary: Accept-Language when appropriate

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

That's something I figured our while reviewing symfony#44386 and that we missed in symfony#43108:
the `Vary` header should be sent when we use `Accept-Language`, not when we send `Content-Language`.
/cc @chalasr

Commits
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afab34d [HttpKernel] fix sending Vary: Accept-Language when appropriate

Author: fabpot

src/Symfony/Component/HttpKernel/EventListener/LocaleListener.php

@@ -70,6 +70,7 @@ private function setLocale(Request $request)
             $request->setLocale($locale);
         } elseif ($this->useAcceptLanguageHeader && $this->enabledLocales && ($preferredLanguage = $request->getPreferredLanguage($this->enabledLocales))) {
             $request->setLocale($preferredLanguage);
+            $request->attributes->set('_vary_by_language', true);
         }
     }
 

src/Symfony/Component/HttpKernel/EventListener/ResponseListener.php

@@ -50,6 +50,9 @@ public function onKernelResponse(ResponseEvent $event)
 
         if ($this->addContentLanguageHeader && !$response->isInformational() && !$response->isEmpty() && !$response->headers->has('Content-Language')) {
             $response->headers->set('Content-Language', $event->getRequest()->getLocale());
+        }
+
+        if ($event->getRequest()->attributes->get('_vary_by_language')) {
             $response->setVary('Accept-Language', false);
         }