code: def clean(cls, pid):
          
          with cls._lock:
              try:
                  cls._ensure_pool_exists(pid)
              except KeyError:
                  LOGGER.debug('Pool clean invoked against missing pool %s', pid)
                  return
              cls._pools[pid].clean()
              cls._maybe_remove_pool(pid)
docs: Clean the specified pool, removing any closed connections or
          stale locks.
  
          :param str pid: The pool id to clean