code: func (b *builtinSHA1Sig) evalString(row chunk.Row) (string, bool, error) {
  	str, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  	hasher := sha1.New()
  	_, err = hasher.Write([]byte(str))
  	if err != nil {
  		return "", true, err
  	}
  	return fmt.Sprintf("%x", hasher.Sum(nil)), false, nil
  }
docs: // evalString evals SHA1(str).
  // See https://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html#function_sha1
  // The value is returned as a string of 40 hexadecimal digits, or NULL if the argument was NULL.