lib/osutil: Replace IsDir with TraversesSymlink (fixes #3839)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3883
LGTM: calmh

lib/osutil/traversessymlink.go

@@ -0,0 +1,76 @@
+// 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/.
+
+package osutil
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// TraversesSymlinkError is an error indicating symlink traversal
+type TraversesSymlinkError struct {
+	path string
+}
+
+func (e TraversesSymlinkError) Error() string {
+	return fmt.Sprintf("traverses symlink: %s", e.path)
+}
+
+// NotADirectoryError is an error indicating an expected path is not a directory
+type NotADirectoryError struct {
+	path string
+}
+
+func (e NotADirectoryError) Error() string {
+	return fmt.Sprintf("not a directory: %s", e.path)
+}
+
+// TraversesSymlink returns an error if base and any path component of name up to and
+// including filepath.Join(base, name) traverses a symlink.
+// Base and name must both be clean and name must be relative to base.
+func TraversesSymlink(base, name string) error {
+	path := base
+	info, err := Lstat(path)
+	if err != nil {
+		return err
+	}
+	if !info.IsDir() {
+		return &NotADirectoryError{
+			path: base,
+		}
+	}
+
+	if name == "." {
+		// The result of calling TraversesSymlink("some/where", filepath.Dir("foo"))
+		return nil
+	}
+
+	parts := strings.Split(name, string(os.PathSeparator))
+	for _, part := range parts {
+		path = filepath.Join(path, part)
+		info, err := Lstat(path)
+		if err != nil {
+			if os.IsNotExist(err) {
+				return nil
+			}
+			return err
+		}
+		if info.Mode()&os.ModeSymlink != 0 {
+			return &TraversesSymlinkError{
+				path: strings.TrimPrefix(path, base),
+			}
+		}
+		if !info.IsDir() {
+			return &NotADirectoryError{
+				path: strings.TrimPrefix(path, base),
+			}
+		}
+	}
+	return nil
+}