code: func (s *FilesystemStore) save(session *Session) error {
  	encoded, err := securecookie.EncodeMulti(session.Name(), session.Values,
  		s.Codecs...)
  	if err != nil {
  		return err
  	}
  	filename := filepath.Join(s.path, "session_"+session.ID)
  	fileMutex.Lock()
  	defer fileMutex.Unlock()
  	return ioutil.WriteFile(filename, []byte(encoded), 0600)
  }
docs: // save writes encoded session.Values to a file.