code: def close(self):
          
          with self._close_lock:
              sfd = self._sfd
              if sfd >= 0:
                  self._sfd = -1
                  self._signals = frozenset()
                  close(sfd)
docs: Close the internal signalfd file descriptor if it isn't closed
  
          :raises OSError:
              If the underlying ``close(2)`` fails. The error message matches
              those found in the manual page.