function drupal_serve_page_from_cache

Sets HTTP headers in preparation for a cached page response.

The headers allow as much as possible in proxies and browsers without any particular knowledge about the pages. Modules can override these headers using drupal_add_http_header().

If the request is conditional (using If-Modified-Since and If-None-Match), and the conditions match those currently in the cache, a 304 Not Modified response is sent.

File

includes/bootstrap.inc, line 1606

Code

function drupal_serve_page_from_cache(stdClass $cache) {
    // Negotiate whether to use compression.
    $page_compression = !empty($cache->data['page_compressed']) && !empty($cache->data['body']);
    $return_compressed = $page_compression && isset($_SERVER['HTTP_ACCEPT_ENCODING']) && strpos($_SERVER['HTTP_ACCEPT_ENCODING'], 'gzip') !== FALSE;
    // Get headers set in hook_boot(). Keys are lower-case.
    $hook_boot_headers = drupal_get_http_header();
    // Headers generated in this function, that may be replaced or unset using
    // drupal_add_http_headers(). Keys are mixed-case.
    $default_headers = array();
    foreach ($cache->data['headers'] as $name => $value) {
        // In the case of a 304 response, certain headers must be sent, and the
        // remaining may not (see RFC 2616, section 10.3.5). Do not override
        // headers set in hook_boot().
        $name_lower = strtolower($name);
        if (in_array($name_lower, array(
            'content-location',
            'expires',
            'cache-control',
            'vary',
        )) && !isset($hook_boot_headers[$name_lower])) {
            drupal_add_http_header($name, $value);
            unset($cache->data['headers'][$name]);
        }
    }
    // If the client sent a session cookie, a cached copy will only be served
    // to that one particular client due to Vary: Cookie. Thus, do not set
    // max-age > 0, allowing the page to be cached by external proxies, when a
    // session cookie is present unless the Vary header has been replaced or
    // unset in hook_boot().
    $max_age = !isset($_COOKIE[session_name()]) || isset($hook_boot_headers['vary']) ? variable_get('page_cache_maximum_age', 0) : 0;
    $default_headers['Cache-Control'] = 'public, max-age=' . $max_age;
    // Entity tag should change if the output changes.
    $etag = '"' . $cache->created . '-' . intval($return_compressed) . '"';
    header('Etag: ' . $etag);
    // See if the client has provided the required HTTP headers.
    $if_modified_since = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) : FALSE;
    $if_none_match = isset($_SERVER['HTTP_IF_NONE_MATCH']) ? stripslashes($_SERVER['HTTP_IF_NONE_MATCH']) : FALSE;
    if ($if_modified_since && $if_none_match && $if_none_match == $etag && $if_modified_since == $cache->created) {
        // if-modified-since must match
        header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');
        drupal_send_headers($default_headers);
        return;
    }
    // Send the remaining headers.
    foreach ($cache->data['headers'] as $name => $value) {
        drupal_add_http_header($name, $value);
    }
    $default_headers['Last-Modified'] = gmdate(DATE_RFC7231, $cache->created);
    // HTTP/1.0 proxies does not support the Vary header, so prevent any caching
    // by sending an Expires date in the past. HTTP/1.1 clients ignores the
    // Expires header if a Cache-Control: max-age= directive is specified (see RFC
    // 2616, section 14.9.3).
    $default_headers['Expires'] = 'Sun, 19 Nov 1978 05:00:00 GMT';
    drupal_send_headers($default_headers);
    // Allow HTTP proxies to cache pages for anonymous users without a session
    // cookie. The Vary header is used to indicates the set of request-header
    // fields that fully determines whether a cache is permitted to use the
    // response to reply to a subsequent request for a given URL without
    // revalidation. If a Vary header has been set in hook_boot(), it is assumed
    // that the module knows how to cache the page.
    if (!isset($hook_boot_headers['vary']) && !variable_get('omit_vary_cookie')) {
        header('Vary: Cookie');
    }
    if ($page_compression) {
        header('Vary: Accept-Encoding', FALSE);
        // If page_compression is enabled, the cache contains gzipped data.
        if ($return_compressed) {
            // $cache->data['body'] is already gzip'ed, so make sure
            // zlib.output_compression does not compress it once more.
            ini_set('zlib.output_compression', '0');
            header('Content-Encoding: gzip');
        }
        else {
            // The client does not support compression, so unzip the data in the
            // cache. Strip the gzip header and run uncompress.
            $cache->data['body'] = gzinflate(substr(substr($cache->data['body'], 10), 0, -8));
        }
    }
    // Print the page.
    print $cache->data['body'];
}