code: func (b *builtinUserSig) evalString(row chunk.Row) (string, bool, error) {
  	data := b.ctx.GetSessionVars()
  	if data == nil || data.User == nil {
  		return "", true, errors.Errorf("Missing session variable when eval builtin")
  	}
  
  	return data.User.String(), false, nil
  }
docs: // evalString evals a builtinUserSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_user