If the argument to [FileInfoHeader] implements the new [FileInfoNames]
interface, then the interface methods will be used to set the Uname/Gname
of the file header. This allows applications to override the system-dependent

			return nil, err
		}
		h.Uname, err = iface.Uname()
		if err != nil {
			return nil, err
		}
	}
	if sysStat != nil {
		return h, sysStat(fi, h, doNameLookups)
	}
	return h, nil
}

// FileInfoNames extends [fs.FileInfo].
// Passing an instance of this to [FileInfoHeader] permits the caller
// to avoid a system-dependent name lookup by specifying the Uname and Gname directly.

pkg archive/tar, type FileInfoNames interface { Gname, IsDir, ModTime, Mode, Name, Size, Sys, Uname } #50102
pkg archive/tar, type FileInfoNames interface, Gname() (string, error) #50102
pkg archive/tar, type FileInfoNames interface, IsDir() bool #50102
pkg archive/tar, type FileInfoNames interface, ModTime() time.Time #50102
pkg archive/tar, type FileInfoNames interface, Mode() fs.FileMode #50102
pkg archive/tar, type FileInfoNames interface, Name() string #50102

	tr := NewReader(&buf)

	entries, err := fsys.ReadDir(".")
	if err != nil {
		t.Fatal(err)
	}

	var curfname string
	for _, entry := range entries {
		curfname = entry.Name()
		if entry.IsDir() {
			curfname += "/"
			continue
		}
		hdr, err := tr.Next()
		if err == io.EOF {
			break // End of archive
		}
		if err != nil {
			t.Fatal(err)

	return nil
}

func (f *fileInfoNames) Uname() (string, error) {
	return "Uname", nil
}

func (f *fileInfoNames) Gname() (string, error) {
	return "Gname", nil
}

func TestFileInfoHeaderUseFileInfoNames(t *testing.T) {
	info := &fileInfoNames{}
	header, err := FileInfoHeader(info, "")
	if err != nil {
		t.Fatal(err)
	}
	if header.Uname != "Uname" {

		if v == "" {
			continue // Keep the original USTAR value
		}
		var id64 int64
		switch k {
		case paxPath:
			hdr.Name = v
		case paxLinkpath:
			hdr.Linkname = v
		case paxUname:
			hdr.Uname = v
		case paxGname:
			hdr.Gname = v
		case paxUid:
			id64, err = strconv.ParseInt(v, 10, 64)
			hdr.Uid = int(id64) // Integer overflow possible
		case paxGid:
			id64, err = strconv.ParseInt(v, 10, 64)

		// (not implemented on that platform, cgo not enabled, etc).
		if u, ok := userMap.Load(h.Uid); ok {
			h.Uname = u.(string)
		} else if u, err := user.LookupId(strconv.Itoa(h.Uid)); err == nil {
			h.Uname = u.Username
			userMap.Store(h.Uid, h.Uname)
		}
		if g, ok := groupMap.Load(h.Gid); ok {
			h.Gname = g.(string)
		} else if g, err := user.LookupGroupId(strconv.Itoa(h.Gid)); err == nil {
			h.Gname = g.Name