PageCache::lookup

protected PageCache::lookup(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE)

protected function lookup(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE) {
  if ($response = $this->get($request)) {
    $response->headers->set('X-Drupal-Cache', 'HIT');
  }
  else {
    $response = $this->fetch($request, $type, $catch);
  }

  // Only allow caching in the browser and prevent that the response is stored
  // by an external proxy server when the following conditions apply:
  // 1. There is a session cookie on the request.
  // 2. The Vary: Cookie header is on the response.
  // 3. The Cache-Control header does not contain the no-cache directive.
  if ($request->cookies->has(session_name()) && 
    in_array('Cookie', $response->getVary()) && 
    !$response->headers->hasCacheControlDirective('no-cache')) {

    $response->setPrivate();
  }

  // Negotiate whether to use compression.
  if (extension_loaded('zlib') && $response->headers->get('Content-Encoding') === 'gzip') {
    if (strpos($request->headers->get('Accept-Encoding'), 'gzip') !== FALSE) {
      // The response content is already gzip'ed, so make sure
      // zlib.output_compression does not compress it once more.
      ini_set('zlib.output_compression', '0');
    }
    else {
      // The client does not support compression. Decompress the content and
      // remove the Content-Encoding header.
      $content = $response->getContent();
      $content = gzinflate(substr(substr($content, 10), 0, -8));
      $response->setContent($content);
      $response->headers->remove('Content-Encoding');
    }
  }

  // Perform HTTP revalidation.
  // @todo Use Response::isNotModified() as
  //   per https://www.drupal.org/node/2259489.
  $last_modified = $response->getLastModified();
  if ($last_modified) {
    // See if the client has provided the required HTTP headers.
    $if_modified_since = $request->server->has('HTTP_IF_MODIFIED_SINCE') ? strtotime($request->server->get('HTTP_IF_MODIFIED_SINCE')) : FALSE;
    $if_none_match = $request->server->has('HTTP_IF_NONE_MATCH') ? stripslashes($request->server->get('HTTP_IF_NONE_MATCH')) : FALSE;

    if ($if_modified_since && $if_none_match
       && $if_none_match == $response->getEtag() // etag must match
       && $if_modified_since == $last_modified->getTimestamp()) { // if-modified-since must match
      $response->setStatusCode(304);
      $response->setContent(NULL);

      // In the case of a 304 response, certain headers must be sent, and the
      // remaining may not (see RFC 2616, section 10.3.5).
      foreach (array_keys($response->headers->all()) as $name) {
        if (!in_array($name, array('content-location', 'expires', 'cache-control', 'vary'))) {
          $response->headers->remove($name);
        }
      }
    }
  }

  return $response;
}