local: mark repository files as read-only

This is intended to prevent accidental modifications of data files.
Marking the files as read-only was accidentally removed in #1258.

internal/backend/local/local.go

@@ -130,9 +130,10 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 		return errors.Wrap(err, "Close")
 	}
 
+	// try to mark file as read-only to avoid accidential modifications
 	// ignore if the operation fails as some filesystems don't allow the chmod call
 	// e.g. exfat and network file systems with certain mount options
-	err = setNewFileMode(filename, backend.Modes.File)
+	err = setFileReadonly(filename, backend.Modes.File)
 	if err != nil && !os.IsPermission(err) {
 		return errors.Wrap(err, "Chmod")
 	}

internal/backend/local/local_unix.go

@@ -9,6 +9,6 @@ import (
 )
 
 // set file to readonly
-func setNewFileMode(f string, mode os.FileMode) error {
-	return fs.Chmod(f, mode)
+func setFileReadonly(f string, mode os.FileMode) error {
+	return fs.Chmod(f, mode&^0222)
 }

internal/backend/local/local_windows.go

@@ -7,6 +7,6 @@ import (
 // We don't modify read-only on windows,
 // since it will make us unable to delete the file,
 // and this isn't common practice on this platform.
-func setNewFileMode(f string, mode os.FileMode) error {
+func setFileReadonly(f string, mode os.FileMode) error {
 	return nil
 }