func addFS(tw *tar.Writer, fsys fs.FS) error {
	return fs.WalkDir(fsys, ".", func(name string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		info, err := d.Info()
		if err != nil {
			return err
		}
		// TODO(#49580): Handle symlinks when fs.ReadLinkFS is available.
		if !info.Mode().IsRegular() && !info.Mode().IsDir() {
			return fmt.Errorf("tar: cannot add non-regular file")
		}
		h, err := tar.FileInfoHeader(info, "")
		if err != nil {
			return err
		}
		h.Name = name

		if d.IsDir() {
			h.Name += "/" // dir
			fmt.Printf("XXX: DIR: %+v\n", h.Name)
		}

		if err := tw.WriteHeader(h); err != nil {
			return err
		}

		if d.IsDir() {
			return nil // no contents to copy in
		}

		f, err := fsys.Open(name)
		if err != nil {
			return err
		}
		defer f.Close()
		_, err = io.Copy(tw, f)
		return err
	})
}