bug #43998 [HttpKernel] [HttpCache] Don't throw on 304 Not Modified (aleho)

This PR was merged into the 4.4 branch.

Discussion
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[HttpKernel] [HttpCache] Don't throw on 304 Not Modified

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fixes  #43997
| License       | MIT
| Doc PR        | ~

If the response cache keeps a `Last-Modified` header clients will request with `If-Modified-Since`.
The surrogate will not handle a `304 Not Modified` correctly, resulting in a 500 and a failed request.

This fixes that request / response cycle, as observed in testing PR #42355.

Commits
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d27f02a1a1 [HttpKernel] [HttpCache] Don't throw on 304 Not Modified

Author: fabpot

HttpCache/AbstractSurrogate.php

@@ -95,7 +95,7 @@ public function handle(HttpCache $cache, $uri, $alt, $ignoreErrors)
         try {
             $response = $cache->handle($subRequest, HttpKernelInterface::SUB_REQUEST, true);
 
-            if (!$response->isSuccessful()) {
+            if (!$response->isSuccessful() && Response::HTTP_NOT_MODIFIED !== $response->getStatusCode()) {
                 throw new \RuntimeException(sprintf('Error when rendering "%s" (Status code is %d).', $subRequest->getUri(), $response->getStatusCode()));
             }
 

Tests/HttpCache/EsiTest.php

@@ -221,6 +221,15 @@ public function testHandleWhenResponseIsNot200AndAltIsPresent()
         $this->assertEquals('bar', $esi->handle($cache, '/', '/alt', false));
     }
 
+    public function testHandleWhenResponseIsNotModified()
+    {
+        $esi = new Esi();
+        $response = new Response('');
+        $response->setStatusCode(304);
+        $cache = $this->getCache(Request::create('/'), $response);
+        $this->assertEquals('', $esi->handle($cache, '/', '/alt', true));
+    }
+
     protected function getCache($request, $response)
     {
         $cache = $this->getMockBuilder(HttpCache::class)->setMethods(['getRequest', 'handle'])->disableOriginalConstructor()->getMock();