lib/fs: The interface and basicfs

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3748

lib/fs/basicfs_symlink_windows.go

@@ -0,0 +1,195 @@
+// 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]))
+}