code: func appendDefaultMountOptions(mountOptions []string, fsGroup *int64) []string {
  	fileModeFlag := false
  	dirModeFlag := false
  	versFlag := false
  	gidFlag := false
  
  	for _, mountOption := range mountOptions {
  		if strings.HasPrefix(mountOption, fileMode) {
  			fileModeFlag = true
  		}
  		if strings.HasPrefix(mountOption, dirMode) {
  			dirModeFlag = true
  		}
  		if strings.HasPrefix(mountOption, vers) {
  			versFlag = true
  		}
  		if strings.HasPrefix(mountOption, gid) {
  			gidFlag = true
  		}
  	}
  
  	allMountOptions := mountOptions
  	if !fileModeFlag {
  		allMountOptions = append(allMountOptions, fmt.Sprintf("%s=%s", fileMode, defaultFileMode))
  	}
  
  	if !dirModeFlag {
  		allMountOptions = append(allMountOptions, fmt.Sprintf("%s=%s", dirMode, defaultDirMode))
  	}
  
  	if !versFlag {
  		allMountOptions = append(allMountOptions, fmt.Sprintf("%s=%s", vers, defaultVers))
  	}
  
  	if !gidFlag && fsGroup != nil {
  		allMountOptions = append(allMountOptions, fmt.Sprintf("%s=%d", gid, *fsGroup))
  	}
  	return allMountOptions
  }
docs: // check whether mountOptions contain file_mode, dir_mode, vers, gid, if not, append default mode