code: func getBootstrapVersion(s Session) (int64, error) {
  	sVal, isNull, err := getTiDBVar(s, tidbServerVersionVar)
  	if err != nil {
  		return 0, errors.Trace(err)
  	}
  	if isNull {
  		return 0, nil
  	}
  	return strconv.ParseInt(sVal, 10, 64)
  }
docs: // getBootstrapVersion gets bootstrap version from mysql.tidb table;