code: def get_available_urls(self, urls):
      
      baseurl_to_urls = {self._baseurl(url): url for url in urls}
      pingtimes = self._pinger.pings(list(baseurl_to_urls.keys()))  # List of pairs (host, time in ms).
      self._log.debug('Artifact cache server ping times: {}'
                      .format(', '.join(['{}: {:.6f} secs'.format(*p) for p in pingtimes])))
  
      sorted_pingtimes = sorted(pingtimes, key=lambda x: x[1])
      available_urls = [baseurl_to_urls[baseurl] for baseurl, pingtime in sorted_pingtimes
                        if pingtime < Pinger.UNREACHABLE]
      self._log.debug('Available cache servers: {0}'.format(available_urls))
  
      return available_urls
docs: Return reachable urls sorted by their ping times.