code: def _mainthread_poll_readable(self):
          
          events = self._recv_selector.select(self.block_time)
          for key, mask in events:
              if mask == selectors.EVENT_READ:
                  self._recv_selector.unregister(key.fileobj)
  
                  self._threads_limiter.start_thread(target=self._subthread_handle_readable,
                                                     args=(key.fileobj,))
docs: Searches for readable client sockets. These sockets are then put in a subthread
          to be handled by _handle_readable