fix: set modTime to current in snowball if archive shows empty (#16482)

cmd/untar.go

@@ -30,6 +30,7 @@ import (
 	"path"
 	"runtime"
 	"sync"
+	"time"
 
 	"github.com/cosnicolaou/pbzip2"
 	"github.com/klauspost/compress/s2"
@@ -259,6 +260,12 @@ func untar(ctx context.Context, r io.Reader, putObject func(reader io.Reader, in
 			continue
 		}
 
+		// If zero or earlier modtime, set to current.
+		// Otherwise the resulting objects will be invalid.
+		if header.ModTime.UnixNano() <= 0 {
+			header.ModTime = time.Now()
+		}
+
 		// Sync upload.
 		rc := disconnectReader{r: tarReader}
 		if err := putObject(&rc, header.FileInfo(), name); err != nil {