Sync directory to get durable writes in local backend

internal/backend/local/local.go

@@ -142,7 +142,9 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 	}
 
 	// Ignore error if filesystem does not support fsync.
-	if err = f.Sync(); err != nil && !errors.Is(err, syscall.ENOTSUP) {
+	err = f.Sync()
+	syncNotSup := errors.Is(err, syscall.ENOTSUP)
+	if err != nil && !syncNotSup {
 		return errors.WithStack(err)
 	}
 
@@ -154,6 +156,14 @@ func (b *Local) Save(ctx context.Context, h restic.Handle, rd restic.RewindReade
 		return errors.WithStack(err)
 	}
 
+	// Now sync the directory to commit the Rename.
+	if !syncNotSup {
+		err = fsyncDir(dir)
+		if err != nil {
+			return errors.WithStack(err)
+		}
+	}
+
 	// 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

internal/backend/local/local_unix.go

@@ -3,11 +3,33 @@
 package local
 
 import (
+	"errors"
 	"os"
+	"syscall"
 
 	"github.com/restic/restic/internal/fs"
 )
 
+// fsyncDir flushes changes to the directory dir.
+func fsyncDir(dir string) error {
+	d, err := os.Open(dir)
+	if err != nil {
+		return err
+	}
+
+	err = d.Sync()
+	if errors.Is(err, syscall.ENOTSUP) {
+		err = nil
+	}
+
+	cerr := d.Close()
+	if err == nil {
+		err = cerr
+	}
+
+	return err
+}
+
 // set file to readonly
 func setFileReadonly(f string, mode os.FileMode) error {
 	return fs.Chmod(f, mode&^0222)

internal/backend/local/local_windows.go

@@ -4,6 +4,9 @@ import (
 	"os"
 )
 
+// Can't explicitly flush directory changes on Windows.
+func fsyncDir(dir string) error { return nil }
+
 // 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.