code: func (c *channelPool) Close(conn interface{}) error {
  	if conn == nil {
  		return errors.New("connection is nil. rejecting")
  	}
  	c.mu.Lock()
  	defer c.mu.Unlock()
  	if c.close == nil {
  		return nil
  	}
  	return c.close(conn)
  }
docs: // Close 关闭单条连接