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
			groupMap.Store(h.Gid, h.Gname)
		}
	}
	h.AccessTime = statAtime(sys)

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

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

// Passing an instance of this to [FileInfoHeader] permits the caller
// to avoid a system-dependent name lookup by specifying the Uname and Gname directly.
type FileInfoNames interface {
	fs.FileInfo
	// Uname should give a user name.
	Uname() (string, error)
	// Gname should give a group name.
	Gname() (string, error)
}

// isHeaderOnlyType checks if the given type flag is of the type that has no

	tw := NewWriter(&buf)
	const name = "/foo"
	tw.WriteHeader(&Header{
		Name: name,
	})
	tw.Close()
	tr := NewReader(&buf)
	h, err := tr.Next()
	if err != nil {
		t.Fatalf("tr.Next with tarinsecurepath=1: got err %v, want nil", err)
	}
	if h.Name != name {
		t.Fatalf("tr.Next with tarinsecurepath=1: got name %q, want %q", h.Name, name)
	}

		if err != nil {
			t.Error(err)
			continue
		}
		if strings.Contains(fi.Name(), "/") {
			t.Errorf("FileInfo of %q contains slash: %q", v.h.Name, fi.Name())
		}
		name := path.Base(v.h.Name)
		if fi.IsDir() {
			name += "/"
		}
		if got, want := h2.Name, name; got != want {
			t.Errorf("i=%d: Name: got %v, want %v", i, got, want)
		}
		if got, want := h2.Size, v.h.Size; got != want {

	for _, test := range []struct {
		name string
		h    *Header
	}{{
		name: "name too long",
		h:    &Header{Name: strings.Repeat("a", maxSpecialFileSize)},
	}, {
		name: "linkname too long",
		h:    &Header{Linkname: strings.Repeat("a", maxSpecialFileSize)},
	}, {
		name: "uname too long",
		h:    &Header{Uname: strings.Repeat("a", maxSpecialFileSize)},
	}, {
		name: "gname too long",

func splitUSTARPath(name string) (prefix, suffix string, ok bool) {
	length := len(name)
	if length <= nameSize || !isASCII(name) {
		return "", "", false
	} else if length > prefixSize+1 {
		length = prefixSize + 1
	} else if name[length-1] == '/' {
		length--
	}

	i := strings.LastIndex(name[:length], "/")
	nlen := len(name) - i - 1 // nlen is length of suffix

	default:
		return nil, nil // Not a PAX format GNU sparse file.
	}
	hdr.Format.mayOnlyBe(FormatPAX)

	// Update hdr from GNU sparse PAX headers.
	if name := hdr.PAXRecords[paxGNUSparseName]; name != "" {
		hdr.Name = name
	}
	size := hdr.PAXRecords[paxGNUSparseSize]
	if size == "" {
		size = hdr.PAXRecords[paxGNUSparseRealSize]
	}
	if size != "" {
		n, err := strconv.ParseInt(size, 10, 64)

//	------------------+--------+-----------+----------
//	Name              |   256B | unlimited | unlimited
//	Linkname          |   100B | unlimited | unlimited
//	Size              | uint33 | unlimited |    uint89
//	Mode              | uint21 |    uint21 |    uint57
//	Uid/Gid           | uint21 | unlimited |    uint57
//	Uname/Gname       |    32B | unlimited |       32B
//	ModTime           | uint33 | unlimited |     int89