docs: def wait_callback(connection):
      """callback function suitable for ``psycopg2.set_wait_callback``
  
      pass this function to ``psycopg2.extensions.set_wait_callack`` to force any
      blocking operations from psycopg2 to only block the current coroutine,
      rather than the entire thread or process
  
      to undo the change and return to normal blocking operation, use
      `psycopg2.extensions.set_wait_callback(None)``
      """
      while 1:
          state = connection.poll()
  
          if state == extensions.POLL_OK:
              break
          elif state == extensions.POLL_READ:
              descriptor.wait_fds([(connection.fileno(), 1)])
          elif state == extensions.POLL_WRITE:
              descriptor.wait_fds([(connection.fileno(), 2)])
          else:
              raise psycopg2.OperationalError("Bad poll result: %r" % state)