code: def shutdown(self, callback=None):
          
          if self._state == self.S_UNWRAPPED:
              raise RuntimeError('no security layer present')
          self._state = self.S_SHUTDOWN
          self._shutdown_cb = callback
          ssldata, appdata = self.feed_ssldata(b'')
          assert appdata == [] or appdata == [b'']
          return ssldata
docs: Start the SSL shutdown sequence. Return a list of ssldata.
  
          The optional *callback* argument can be used to install a callback that
          will be called when the shutdown is complete. The callback will be
          called without arguments.