Sync directory to get durable writes in local backend

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)