code: func (b *builtinCurrentRoleSig) evalString(row chunk.Row) (string, bool, error) {
  	data := b.ctx.GetSessionVars()
  	if data == nil || data.ActiveRoles == nil {
  		return "", true, errors.Errorf("Missing session variable when eval builtin")
  	}
  	if len(data.ActiveRoles) == 0 {
  		return "", false, nil
  	}
  	res := ""
  	for i, r := range data.ActiveRoles {
  		res += r.String()
  		if i != len(data.ActiveRoles)-1 {
  			res += ","
  		}
  	}
  	return res, false, nil
  }
docs: // evalString evals a builtinCurrentUserSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_current-user