lib/config, lib/fs: Make junction behaviour configurable (ref #6606) (#6907)

lib/fs/basicfs_lstat_windows.go

@@ -0,0 +1,82 @@
+// 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 {
+	// Simulate a directory and not a symlink; also set the execute
+	// bits so the directory can be traversed Unix-side.
+	return fi.FileInfo.Mode() ^ os.ModeSymlink | os.ModeDir | 0111
+}
+
+func (fi *dirJunctFileInfo) IsDir() bool {
+	return true
+}
+
+func (f *BasicFilesystem) underlyingLstat(name string) (os.FileInfo, error) {
+	var fi, err = os.Lstat(name)
+
+	// NTFS directory junctions can be treated as ordinary directories,
+	// see https://forum.syncthing.net/t/option-to-follow-directory-junctions-symbolic-links/14750
+	if err == nil && f.junctionsAsDirs && fi.Mode()&os.ModeSymlink != 0 {
+		var isJunct bool
+		isJunct, err = isDirectoryJunction(name)
+		if err == nil && isJunct {
+			return &dirJunctFileInfo{fi}, nil
+		}
+	}
+	return fi, err
+}