module: accept Windows relative path

Before this change, require('.\\test.js') failed while
require('./test.js') succeeded. This changes _resolveLookupPaths so
that both ways are accepted on Windows.

Fixes: nodejs#21918

lib/internal/modules/cjs/loader.js

@@ -80,6 +80,8 @@ const {
   CHAR_9,
 } = require('internal/constants');
 
+const isWindows = process.platform === 'win32';
+
 function stat(filename) {
   filename = path.toNamespacedPath(filename);
   const cache = stat.cache;
@@ -310,7 +312,7 @@ Module._findPath = function(request, paths, isMain) {
 // 'node_modules' character codes reversed
 var nmChars = [ 115, 101, 108, 117, 100, 111, 109, 95, 101, 100, 111, 110 ];
 var nmLen = nmChars.length;
-if (process.platform === 'win32') {
+if (isWindows) {
   // 'from' is the __dirname of the module.
   Module._nodeModulePaths = function(from) {
     // guarantee that 'from' is absolute.
@@ -403,11 +405,12 @@ Module._resolveLookupPaths = function(request, parent, newReturn) {
     return (newReturn ? null : [request, []]);
   }
 
-  // Check for relative path
+  // Check for non-relative path
   if (request.length < 2 ||
       request.charCodeAt(0) !== CHAR_DOT ||
       (request.charCodeAt(1) !== CHAR_DOT &&
-       request.charCodeAt(1) !== CHAR_FORWARD_SLASH)) {
+       request.charCodeAt(1) !== CHAR_FORWARD_SLASH &&
+       (!isWindows || request.charCodeAt(1) !== CHAR_BACKWARD_SLASH))) {
     var paths = modulePaths;
     if (parent) {
       if (!parent.paths)
@@ -480,7 +483,9 @@ Module._resolveLookupPaths = function(request, parent, newReturn) {
 
   // make sure require('./path') and require('path') get distinct ids, even
   // when called from the toplevel js file
-  if (parentIdPath === '.' && id.indexOf('/') === -1) {
+  if (parentIdPath === '.' &&
+      id.indexOf('/') === -1 &&
+      (!isWindows || id.indexOf('\\') === -1)) {
     id = './' + id;
   }
 
@@ -746,8 +751,6 @@ Module.runMain = function() {
 };
 
 Module._initPaths = function() {
-  const isWindows = process.platform === 'win32';
-
   var homeDir;
   var nodePath;
   if (isWindows) {

test/parallel/test-module-relative-lookup.js

@@ -1,15 +1,23 @@
 'use strict';
 
-require('../common');
+const common = require('../common');
 const assert = require('assert');
 const _module = require('module'); // avoid collision with global.module
-const lookupResults = _module._resolveLookupPaths('./lodash');
-let paths = lookupResults[1];
 
-// Current directory gets highest priority for local modules
-assert.strictEqual(paths[0], '.');
+function runTest(moduleName) {
+  const lookupResults = _module._resolveLookupPaths(moduleName);
+  let paths = lookupResults[1];
 
-paths = _module._resolveLookupPaths('./lodash', null, true);
+  // Current directory gets highest priority for local modules
+  assert.strictEqual(paths[0], '.');
 
-// Current directory gets highest priority for local modules
-assert.strictEqual(paths && paths[0], '.');
+  paths = _module._resolveLookupPaths(moduleName, null, true);
+
+  // Current directory gets highest priority for local modules
+  assert.strictEqual(paths && paths[0], '.');
+}
+
+runTest('./lodash');
+if (common.isWindows) {
+  runTest('.\\lodash');
+}