code: func (p *Pod) GetAppTreeStoreID(app types.ACName) (string, error) {
  	path, err := filepath.Rel("/", common.AppTreeStoreIDPath("", app))
  	if err != nil {
  		return "", err
  	}
  	treeStoreID, err := p.readFile(path)
  	if err != nil {
  		// When not using overlayfs, apps don't have a treeStoreID file. In
  		// other cases we've got a problem.
  		if !(os.IsNotExist(err) && !p.UsesOverlay()) {
  			return "", errwrap.Wrap(fmt.Errorf("no treeStoreID found for app %s", app), err)
  		}
  	}
  	return string(treeStoreID), nil
  }
docs: // GetAppTreeStoreID returns the treeStoreID of the provided app.
  // TODO(yifan): Maybe make this unexported.