archive/tar: add FileInfoNames interface

An optional interface FileInfoNames has been added.

If the parameter fi of FileInfoHeader implements the interface
the Gname/Uname of the return value Header
are provided by the method of the interface.

Also added testing.

Fixes #50102

Change-Id: I47976e238eb20ed43113b060e4f83a14ae49493e
GitHub-Last-Rev: a213613c79
GitHub-Pull-Request: golang/go#65273
Reviewed-on: https://go-review.googlesource.com/c/go/+/558355
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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() (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() (string, error) #50102

doc/next/6-stdlib/99-minor/archive/tar/50102.md

@@ -0,0 +1,3 @@
+If the argument to [`FileInfoHeader`](/archive/tar#FileInfoHeader) implements the new [`FileInfoNames`](/archive/tar#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 Uname/Gname lookup.

src/archive/tar/common.go

@@ -612,7 +612,7 @@ 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 sysStat func(fi fs.FileInfo, h *Header, doNameLookups bool) error
 
 const (
 	// Mode constants from the USTAR spec:
@@ -639,6 +639,10 @@ 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]
+// Header.Gname and Header.Uname
+// 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 +715,36 @@ func FileInfoHeader(fi fs.FileInfo, link string) (*Header, error) {
 			}
 		}
 	}
+	var doNameLookups = true
+	if iface, ok := fi.(FileInfoNames); ok {
+		doNameLookups = false
+		var err error
+		h.Gname, err = iface.Gname()
+		if err != nil {
+			return nil, err
+		}
+		h.Uname, err = iface.Uname()
+		if err != nil {
+			return nil, err
+		}
+	}
 	if sysStat != nil {
-		return h, sysStat(fi, h)
+		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.
+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
 // data section even if a size is specified.
 func isHeaderOnlyType(flag byte) bool {

src/archive/tar/stat_unix.go

@@ -23,14 +23,14 @@ func init() {
 // The downside is that renaming uname or gname by the OS never takes effect.
 var userMap, groupMap sync.Map // map[int]string
 
-func statUnix(fi fs.FileInfo, h *Header) error {
+func statUnix(fi fs.FileInfo, h *Header, doNameLookups bool) error {
 	sys, ok := fi.Sys().(*syscall.Stat_t)
 	if !ok {
 		return nil
 	}
 	h.Uid = int(sys.Uid)
 	h.Gid = int(sys.Gid)
-
+	if doNameLookups {
 		// Best effort at populating Uname and Gname.
 		// The os/user functions may fail for any number of reasons
 		// (not implemented on that platform, cgo not enabled, etc).
@@ -46,7 +46,7 @@ func statUnix(fi fs.FileInfo, h *Header) error {
 			h.Gname = g.Name
 			groupMap.Store(h.Gid, h.Gname)
 		}
-
+	}
 	h.AccessTime = statAtime(sys)
 	h.ChangeTime = statCtime(sys)
 

src/archive/tar/tar_test.go

@@ -848,3 +848,53 @@ func Benchmark(b *testing.B) {
 	})
 
 }
+
+var _ fileInfoNames = fileInfoNames{}
+
+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() (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" {
+		t.Fatalf("header.Uname: got %s, want %s", header.Uname, "Uname")
+	}
+	if header.Gname != "Gname" {
+		t.Fatalf("header.Gname: got %s, want %s", header.Gname, "Gname")
+	}
+}