archive/tar: add FileInfoNames interface

An optional interface FileInfoNames has been added.
If the parameter fi of FileInfoHeader implements the interface
the Gname and Uname of the return value Header are
provided by the method of the interface.

Also added testing.

Fixes #50102

Change-Id: I6fd06c7c9aaf29b22b7384542fe57affed33009a

api/next/50102.txt

@@ -0,0 +1,9 @@
+pkg archive/tar, type FileInfoNames interface { Gname, IsDir, ModTime, Mode, Name, Size, Sys, Uname } #50102
+pkg archive/tar, type FileInfoNames interface, Gname(int) (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
+pkg archive/tar, type FileInfoNames interface, Size() int64 #50102
+pkg archive/tar, type FileInfoNames interface, Sys() interface{} #50102
+pkg archive/tar, type FileInfoNames interface, Uname(int) (string, error) #50102

src/archive/tar/common.go

@@ -614,6 +614,8 @@ func (fi headerFileInfo) String() string {
 // sysStat, if non-nil, populates h from system-dependent fields of fi.
 var sysStat func(fi fs.FileInfo, h *Header) error
 
+var loadUidAndGid func(fi fs.FileInfo, uid, gid *int)
+
 const (
 	// Mode constants from the USTAR spec:
 	// See http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06
@@ -639,6 +641,10 @@ const (
 // Since fs.FileInfo's Name method only returns the base name of
 // the file it describes, it may be necessary to modify Header.Name
 // to provide the full path name of the file.
+//
+// If fi implements [FileInfoNames]
+// the Gname and Uname of the header are
+// provided by the methods of the interface.
 func FileInfoHeader(fi fs.FileInfo, link string) (*Header, error) {
 	if fi == nil {
 		return nil, errors.New("archive/tar: FileInfo is nil")
@@ -711,12 +717,38 @@ func FileInfoHeader(fi fs.FileInfo, link string) (*Header, error) {
 			}
 		}
 	}
+	if iface, ok := fi.(FileInfoNames); ok {
+		var err error
+		if loadUidAndGid != nil {
+			loadUidAndGid(fi, &h.Uid, &h.Gid)
+		}
+		h.Gname, err = iface.Gname(h.Gid)
+		if err != nil {
+			return nil, err
+		}
+		h.Uname, err = iface.Uname(h.Gid)
+		if err != nil {
+			return nil, err
+		}
+		return h, nil
+	}
 	if sysStat != nil {
 		return h, sysStat(fi, h)
 	}
 	return h, nil
 }
 
+// FileInfoNames extends [FileInfo] to translate UID/GID to names.
+// Passing an instance of this to [FileInfoHeader] permits the caller
+// to control UID/GID resolution.
+type FileInfoNames interface {
+	fs.FileInfo
+	// Uname should translate a UID into a user name.
+	Uname(uid int) (string, error)
+	// Gname should translate a GID into a group name.
+	Gname(gid int) (string, error)
+}
+
 // isHeaderOnlyType checks if the given type flag is of the type that has no
 // data section even if a size is specified.
 func isHeaderOnlyType(flag byte) bool {

src/archive/tar/stat_unix.go

@@ -17,6 +17,7 @@ import (
 
 func init() {
 	sysStat = statUnix
+	loadUidAndGid = loadUidAndGidFunc
 }
 
 // userMap and groupMap caches UID and GID lookups for performance reasons.
@@ -99,3 +100,12 @@ func statUnix(fi fs.FileInfo, h *Header) error {
 	}
 	return nil
 }
+
+func loadUidAndGidFunc(fi fs.FileInfo, uid, gid *int) {
+	sys, ok := fi.Sys().(*syscall.Stat_t)
+	if !ok {
+		return
+	}
+	*uid = int(sys.Uid)
+	*gid = int(sys.Gid)
+}

src/archive/tar/tar_test.go

@@ -848,3 +848,51 @@ func Benchmark(b *testing.B) {
 	})
 
 }
+
+type fileInfoNames struct{}
+
+func (f *fileInfoNames) Name() string {
+	return "tmp"
+}
+
+func (f *fileInfoNames) Size() int64 {
+	return 0
+}
+
+func (f *fileInfoNames) Mode() fs.FileMode {
+	return 0777
+}
+
+func (f *fileInfoNames) ModTime() time.Time {
+	return time.Time{}
+}
+
+func (f *fileInfoNames) IsDir() bool {
+	return false
+}
+
+func (f *fileInfoNames) Sys() any {
+	return nil
+}
+
+func (f *fileInfoNames) Uname(uid int) (string, error) {
+	return "Uname", nil
+}
+
+func (f *fileInfoNames) Gname(gid int) (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" {
+		t.Fatalf("header.Uname: got %v, want %v", header.Uname, "Uname")
+	}
+	if header.Gname != "Gname" {
+		t.Fatalf("header.Gname: got %v, want %v", header.Gname, "Gname")
+	}
+}