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

changelog/unreleased/issue-4103

@@ -0,0 +1,12 @@
+Bugfix: fix restic dump of tar file with 32-bit binary
+
+In restic up to 0.14.0, the restic dump from a 32-bit binary of a
+snapshot of standard input that was created in Windows has as a
+result a tar file whose content has a negative uid and gid. As a
+result, gnu tar exits with failure status whenever it tries to
+access such a tar file. With this fix, the tar file that is now
+dumped from a 32-bit binary has content with non-negative uid and
+gid.
+
+https://github.com/restic/restic/issues/4103
+https://github.com/restic/restic/pull/4104

internal/dump/tar.go

@@ -3,6 +3,7 @@ package dump
 import (
 	"archive/tar"
 	"context"
+	"math"
 	"os"
 	"path/filepath"
 	"strings"
@@ -38,6 +39,14 @@ const (
 	cISVTX = 0o1000 // Save text (sticky bit)
 )
 
+// substitute a uid or gid of -1 (which is converted to 2^32 - 1) with zero
+func tarId(id uint32) int {
+	if id == math.MaxUint32 {
+		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:        tarId(node.UID),
-		Gid:        int(node.GID),
+		Gid:        tarId(node.GID),
 		Uname:      node.User,
 		Gname:      node.Group,
 		ModTime:    node.ModTime,