code: func builtinCurrentUser(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	data := ctx.GetSessionVars()
  	if data == nil {
  		return d, errors.Errorf("Missing session variable when evalue builtin")
  	}
  
  	d.SetString(data.User)
  	return d, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_current-user
  // TODO: The value of CURRENT_USER() can differ from the value of USER(). We will finish this after we support grant tables.