syncthing/lib/fs/basicfs_symlink_windows.go

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// Copyright (C) 2014 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/.
// +build windows
package fs
import (
"os"
"path/filepath"
"github.com/syncthing/syncthing/lib/osutil"
"syscall"
"unicode/utf16"
"unsafe"
)
const (
win32FsctlGetReparsePoint = 0x900a8
win32FileFlagOpenReparsePoint = 0x00200000
win32SymbolicLinkFlagDirectory = 0x1
)
var (
modkernel32 = syscall.NewLazyDLL("kernel32.dll")
procDeviceIoControl = modkernel32.NewProc("DeviceIoControl")
procCreateSymbolicLink = modkernel32.NewProc("CreateSymbolicLinkW")
symlinksSupported = false
)
func init() {
defer func() {
if err := recover(); err != nil {
// Ensure that the supported flag is disabled when we hit an
// error, even though it should already be. Also, silently swallow
// the error since it's fine for a system not to support symlinks.
symlinksSupported = false
}
}()
// Needs administrator privileges.
// Let's check that everything works.
// This could be done more officially:
// http://stackoverflow.com/questions/2094663/determine-if-windows-process-has-privilege-to-create-symbolic-link
// But I don't want to define 10 more structs just to look this up.
base := os.TempDir()
path := filepath.Join(base, "symlinktest")
defer os.Remove(path)
err := DefaultFilesystem.CreateSymlink(path, base, LinkTargetDirectory)
if err != nil {
return
}
stat, err := osutil.Lstat(path)
if err != nil || stat.Mode()&os.ModeSymlink == 0 {
return
}
target, tt, err := DefaultFilesystem.ReadSymlink(path)
if err != nil || osutil.NativeFilename(target) != base || tt != LinkTargetDirectory {
return
}
symlinksSupported = true
}
func DisableSymlinks() {
symlinksSupported = false
}
func (BasicFilesystem) SymlinksSupported() bool {
return symlinksSupported
}
func (BasicFilesystem) ReadSymlink(path string) (string, LinkTargetType, error) {
ptr, err := syscall.UTF16PtrFromString(path)
if err != nil {
return "", LinkTargetUnknown, err
}
handle, err := syscall.CreateFile(ptr, 0, syscall.FILE_SHARE_READ|syscall.FILE_SHARE_WRITE|syscall.FILE_SHARE_DELETE, nil, syscall.OPEN_EXISTING, syscall.FILE_FLAG_BACKUP_SEMANTICS|win32FileFlagOpenReparsePoint, 0)
if err != nil || handle == syscall.InvalidHandle {
return "", LinkTargetUnknown, err
}
defer syscall.Close(handle)
var ret uint16
var data reparseData
r1, _, err := syscall.Syscall9(procDeviceIoControl.Addr(), 8, uintptr(handle), win32FsctlGetReparsePoint, 0, 0, uintptr(unsafe.Pointer(&data)), unsafe.Sizeof(data), uintptr(unsafe.Pointer(&ret)), 0, 0)
if r1 == 0 {
return "", LinkTargetUnknown, err
}
tt := LinkTargetUnknown
if attr, err := syscall.GetFileAttributes(ptr); err == nil {
if attr&syscall.FILE_ATTRIBUTE_DIRECTORY != 0 {
tt = LinkTargetDirectory
} else {
tt = LinkTargetFile
}
}
return osutil.NormalizedFilename(data.printName()), tt, nil
}
func (BasicFilesystem) CreateSymlink(path, target string, tt LinkTargetType) error {
srcp, err := syscall.UTF16PtrFromString(path)
if err != nil {
return err
}
trgp, err := syscall.UTF16PtrFromString(osutil.NativeFilename(target))
if err != nil {
return err
}
// Sadly for Windows we need to specify the type of the symlink,
// whether it's a directory symlink or a file symlink.
// If the flags doesn't reveal the target type, try to evaluate it
// ourselves, and worst case default to the symlink pointing to a file.
mode := 0
if tt == LinkTargetUnknown {
path := target
if !filepath.IsAbs(target) {
path = filepath.Join(filepath.Dir(path), target)
}
stat, err := os.Stat(path)
if err == nil && stat.IsDir() {
mode = win32SymbolicLinkFlagDirectory
}
} else if tt == LinkTargetDirectory {
mode = win32SymbolicLinkFlagDirectory
}
r0, _, err := syscall.Syscall(procCreateSymbolicLink.Addr(), 3, uintptr(unsafe.Pointer(srcp)), uintptr(unsafe.Pointer(trgp)), uintptr(mode))
if r0 == 1 {
return nil
}
return err
}
func (fs BasicFilesystem) ChangeSymlinkType(path string, tt LinkTargetType) error {
target, existingTargetType, err := fs.ReadSymlink(path)
if err != nil {
return err
}
// If it's the same type, nothing to do.
if tt == existingTargetType {
return nil
}
// If the actual type is unknown, but the new type is file, nothing to do
if existingTargetType == LinkTargetUnknown && tt != LinkTargetDirectory {
return nil
}
return osutil.InWritableDir(func(path string) error {
// It should be a symlink as well hence no need to change permissions on
// the file.
os.Remove(path)
return fs.CreateSymlink(path, target, tt)
}, path)
}
type reparseData struct {
reparseTag uint32
reparseDataLength uint16
reserved uint16
substitueNameOffset uint16
substitueNameLength uint16
printNameOffset uint16
printNameLength uint16
flags uint32
// substituteName - 264 widechars max = 528 bytes
// printName - 260 widechars max = 520 bytes
// = 1048 bytes total
buffer [1048 / 2]uint16
}
func (r *reparseData) printName() string {
// offset and length are in bytes but we're indexing a []uint16
offset := r.printNameOffset / 2
length := r.printNameLength / 2
return string(utf16.Decode(r.buffer[offset : offset+length]))
}
func (r *reparseData) substituteName() string {
// offset and length are in bytes but we're indexing a []uint16
offset := r.substitueNameOffset / 2
length := r.substitueNameLength / 2
return string(utf16.Decode(r.buffer[offset : offset+length]))
}