lib/model: Handle filename conflicts on Windows.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3810
LGTM: calmh

lib/osutil/name_conflict_windows.go

@@ -0,0 +1,48 @@
+// 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/.
+
+// +build windows
+
+package osutil
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"syscall"
+)
+
+// CheckNameConflict returns true if every path component of name up to and
+// including filepath.Join(base, name) doesn't conflict with any existing
+// files or folders with different names. Base and name must both be clean and
+// name must be relative to base.
+func CheckNameConflict(base, name string) bool {
+	// Conflicts can be caused by different casing (e.g. foo and FOO) or
+	// by the use of short names (e.g. foo.barbaz and FOO~1.BAR).
+	path := base
+	parts := strings.Split(name, string(os.PathSeparator))
+	for _, part := range parts {
+		path = filepath.Join(path, part)
+		pathp, err := syscall.UTF16PtrFromString(path)
+		if err != nil {
+			return false
+		}
+		var data syscall.Win32finddata
+		handle, err := syscall.FindFirstFile(pathp, &data)
+		if err == syscall.ERROR_FILE_NOT_FOUND {
+			return true
+		}
+		if err != nil {
+			return false
+		}
+		syscall.FindClose(handle)
+		fileName := syscall.UTF16ToString(data.FileName[:])
+		if part != fileName {
+			return false
+		}
+	}
+	return true
+}