Cache drupal private images

Question

Is there a way to cache drupal private images in client browser? I have a website with private profile images that are used many times and I want to cache them even for a very few time. How can I do that?

Thanks

Answer 1

I finally found how to handle it : there is a hook_file_download where i can add and set header for the file :

function mymodule_file_download($uri){


// Getting file info
  $info = image_get_info($uri);

  // For images files
  if(isset($info['mime_type']) && strpos($info['mime_type'], 'image/') === 0){
    // cache max-age : 1 day 
    // expire-date : now + 86400 s
    // etag using a hash of the file size variable
    return array('Cache-Control' => 'max-age=86400, private',
                 'Expires' => gmdate('D, d M Y H:i:s', time() + 86400) .' GMT',
                 'Last-Modified' => gmdate('D, d M Y H:i:s', time() - 86400) .' GMT',
                 'ETag' => strtr(md5($info['file_size']), 0, 10));
  }
}

Answer 2

From https://drupal.org/node/1844126#comment-8215137, a version that does not grant access to all files:

/**
 * Implements hook_file_download().
 */
function mymodule_system_file_download($uri) {
  if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
    $ifs = strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']);
    $file = db_select('file_managed', 'fm')
      ->fields('fm')
      ->condition("uri", $uri)
      ->execute()
      ->fetchAssoc();
    if (!empty($file)) {
      $modified = $file['timestamp'];
      if ($modified < $ifs) {
        header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');
        exit();
      }
    }
  }
  // Don't return headers to avoid granting access to all files.
  $max_age = variable_get('page_cache_maximum_age', 0);
  drupal_add_http_header('Cache-Control', 'private, max-age=' . $max_age);
  drupal_add_http_header('Last-Modified', gmdate(DATE_RFC1123, REQUEST_TIME));
  drupal_add_http_header('ETag', strtr(md5($uri), 0, 10));
}