code: func (f *Fluent) appendBuffer(msg *msgToSend) error {
  	select {
  	case f.pending <- msg:
  	default:
  		return fmt.Errorf("fluent#appendBuffer: Buffer full, limit %v", f.Config.BufferLimit)
  	}
  	return nil
  }
docs: // appendBuffer appends data to buffer with lock.