code: func (fms *fileMetadataStore) getOS(layer ChainID) (string, error) {
  	contentBytes, err := ioutil.ReadFile(fms.getLayerFilename(layer, "os"))
  	if err != nil {
  		// For backwards compatibility, the os file may not exist. Default to "windows" if missing.
  		if os.IsNotExist(err) {
  			return "windows", nil
  		}
  		return "", err
  	}
  	content := strings.TrimSpace(string(contentBytes))
  
  	if content != "windows" && content != "linux" {
  		return "", fmt.Errorf("invalid operating system value: %s", content)
  	}
  
  	return content, nil
  }
docs: // getOS reads the "os" file from the layer filestore