/**
  * Delete a list of thumbnails visible at urls
  * @param string $dir base dir of the files.
  * @param array $files of strings: relative filenames (to $dir)
  */
 protected function purgeThumbList($dir, $files)
 {
     $fileListDebug = strtr(var_export($files, true), array("\n" => ''));
     wfDebug(__METHOD__ . ": {$fileListDebug}\n");
     $purgeList = array();
     foreach ($files as $file) {
         # Check that the base file name is part of the thumb name
         # This is a basic sanity check to avoid erasing unrelated directories
         if (strpos($file, $this->getName()) !== false || strpos($file, "-thumbnail") !== false) {
             $purgeList[] = "{$dir}/{$file}";
         }
     }
     # Delete the thumbnails
     $this->repo->quickPurgeBatch($purgeList);
     # Clear out the thumbnail directory if empty
     $this->repo->quickCleanDir($dir);
 }