code: def sync_close(self):
          
          if self._closed:
              return
          while self._free:
              conn = self._free.popleft()
              if not conn.closed:
                  # pragma: no cover
                  conn.sync_close()
          for conn in self._used:
              if not conn.closed:
                  # pragma: no cover
                  conn.sync_close()
              self._terminated.add(conn)
          self._used.clear()
          self._closed = True
docs: 同步关闭