lib/model: Add fsync of files and directories, option to disable (fixes #3711)

lib/osutil/sync.go

@@ -0,0 +1,37 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package osutil
+
+import (
+	"os"
+	"runtime"
+)
+
+func SyncFile(path string) error {
+	flag := 0
+	if runtime.GOOS == "windows" {
+		flag = os.O_WRONLY
+	}
+	fd, err := os.OpenFile(path, flag, 0)
+	if err != nil {
+		return err
+	}
+	defer fd.Close()
+	// MacOS and Windows do not flush the disk cache
+	if err := fd.Sync(); err != nil {
+		return err
+	}
+	return nil
+}
+
+func SyncDir(path string) error {
+	if runtime.GOOS == "windows" {
+		// not supported by Windows
+		return nil
+	}
+	return SyncFile(path)
+}