code: func (b *BasePathFs) RealPath(name string) (path string, err error) {
  	if err := validateBasePathName(name); err != nil {
  		return name, err
  	}
  
  	bpath := filepath.Clean(b.path)
  	path = filepath.Clean(filepath.Join(bpath, name))
  	if !strings.HasPrefix(path, bpath) {
  		return name, os.ErrNotExist
  	}
  
  	return path, nil
  }
docs: // on a file outside the base path it returns the given file name and an error,
  // else the given file with the base path prepended