syncthing/lib/fs/lstat_windows.go

// Copyright (C) 2015 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 https://mozilla.org/MPL/2.0/.

// +build windows

package fs

import (
	"fmt"
	"os"
	"syscall"
	"unsafe"

	"golang.org/x/sys/windows"
)

func isDirectoryJunction(path string) (bool, error) {
	namep, err := syscall.UTF16PtrFromString(path)
	if err != nil {
		return false, fmt.Errorf("syscall.UTF16PtrFromString failed with: %s", err)
	}
	attrs := uint32(syscall.FILE_FLAG_BACKUP_SEMANTICS | syscall.FILE_FLAG_OPEN_REPARSE_POINT)
	h, err := syscall.CreateFile(namep, 0, 0, nil, syscall.OPEN_EXISTING, attrs, 0)
	if err != nil {
		return false, fmt.Errorf("syscall.CreateFile failed with: %s", err)
	}
	defer syscall.CloseHandle(h)

	//https://docs.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-file_attribute_tag_info
	const fileAttributeTagInfo = 9
	type FILE_ATTRIBUTE_TAG_INFO struct {
		FileAttributes uint32
		ReparseTag     uint32
	}

	var ti FILE_ATTRIBUTE_TAG_INFO
	err = windows.GetFileInformationByHandleEx(windows.Handle(h), fileAttributeTagInfo, (*byte)(unsafe.Pointer(&ti)), uint32(unsafe.Sizeof(ti)))
	if err != nil {
		if errno, ok := err.(syscall.Errno); ok && errno == windows.ERROR_INVALID_PARAMETER {
			// It appears calling GetFileInformationByHandleEx with
			// FILE_ATTRIBUTE_TAG_INFO fails on FAT file system with
			// ERROR_INVALID_PARAMETER. Clear ti.ReparseTag in that
			// instance to indicate no symlinks are possible.
			ti.ReparseTag = 0
		} else {
			return false, fmt.Errorf("windows.GetFileInformationByHandleEx failed with: %s", err)
		}
	}
	return ti.ReparseTag == windows.IO_REPARSE_TAG_MOUNT_POINT, nil
}

type dirJunctFileInfo struct {
	os.FileInfo
}

func (fi *dirJunctFileInfo) Mode() os.FileMode {
	return fi.FileInfo.Mode() ^ os.ModeSymlink | os.ModeDir
}

func (fi *dirJunctFileInfo) IsDir() bool {
	return true
}

func underlyingLstat(name string) (os.FileInfo, error) {
	var fi, err = os.Lstat(name)

	// NTFS directory junctions are treated as ordinary directories,
	// see https://forum.syncthing.net/t/option-to-follow-directory-junctions-symbolic-links/14750
	if err == nil && fi.Mode()&os.ModeSymlink != 0 {
		var isJunct bool
		isJunct, err = isDirectoryJunction(name)
		if err == nil && isJunct {
			return &dirJunctFileInfo{fi}, nil
		}
	}
	return fi, err
}