Merge pull request #4104 from philaris/fix_max_uint32_uid_gid_to_zero

in tar dump, convert uid, gid of value -1 to zero

changelog/unreleased/issue-4103

@@ -0,0 +1,9 @@
+Bugfix: fix restic dump of tar file with 32-bit binary
+
+When using a 32-bit build of restic, the `restic dump` command could in some
+cases create tar files containing negative uid and gid. These files cannot be
+read by gnu tar. This corner case especially applies to backups from stdin on Windows.
+We have changed the dump command to create valid tar files in this case.
+
+https://github.com/restic/restic/issues/4103
+https://github.com/restic/restic/pull/4104

internal/dump/tar.go

@@ -38,6 +38,15 @@ const (
 	cISVTX = 0o1000 // Save text (sticky bit)
 )
 
+// in a 32-bit build of restic:
+// substitute a uid or gid of -1 (which was converted to 2^32 - 1) with 0
+func tarIdentifier(id uint32) int {
+	if int(id) == -1 {
+		return 0
+	}
+	return int(id)
+}
+
 func (d *Dumper) dumpNodeTar(ctx context.Context, node *restic.Node, w *tar.Writer) error {
 	relPath, err := filepath.Rel("/", node.Path)
 	if err != nil {
@@ -48,8 +57,8 @@ func (d *Dumper) dumpNodeTar(ctx context.Context, node *restic.Node, w *tar.Writ
 		Name:       filepath.ToSlash(relPath),
 		Size:       int64(node.Size),
 		Mode:       int64(node.Mode.Perm()), // cIS* constants are added later
-		Uid:        int(node.UID),
+		Uid:        tarIdentifier(node.UID),
-		Gid:        int(node.GID),
+		Gid:        tarIdentifier(node.GID),
 		Uname:      node.User,
 		Gname:      node.Group,
 		ModTime:    node.ModTime,