working mock test

doc/api/esm.md

@@ -790,6 +790,9 @@ its own `require` using  `module.createRequire()`.
 
 ```js
 /**
+ * @param {{
+     port: MessagePort,
+   }} utilities Things that preload code might find useful
  * @returns {string} Code to run before application startup
  */
 export function globalPreload() {
@@ -806,6 +809,35 @@ const require = createRequire(cwd() + '/<preload>');
 }
 ```
 
+In order to allow communication between the application and the loader, another
+argument is provided to the preload code: `port`. This is available as a
+parameter to the loader hook and inside of the source text returned by the hook.
+Some care must be taken in order to properly call [`port.ref()`][] and
+[`port.unref()`][] to prevent a process from being in a state where it won't
+close normally.
+
+```js
+/**
+ * This example has the application context send a message to the loader
+ * and sends the message back to the application context
+ * @param {{
+     port: MessagePort,
+   }} utilities Things that preload code might find useful
+ * @returns {string} Code to run before application startup
+ */
+export function globalPreload({ port }) {
+  port.onmessage = (evt) => {
+    port.postMessage(evt.data);
+  };
+  return `\
+    port.postMessage('console.log("I went to the Loader and back");');
+    port.onmessage = (evt) => {
+      eval(evt.data);
+    };
+  `;
+}
+```
+
 ### Examples
 
 The various loader hooks can be used together to accomplish wide-ranging
@@ -1377,6 +1409,8 @@ success!
 [`module.createRequire()`]: module.md#modulecreaterequirefilename
 [`module.syncBuiltinESMExports()`]: module.md#modulesyncbuiltinesmexports
 [`package.json`]: packages.md#nodejs-packagejson-field-definitions
+[`port.ref()`]: https://nodejs.org/dist/latest-v17.x/docs/api/worker_threads.html#portref
+[`port.unref()`]: https://nodejs.org/dist/latest-v17.x/docs/api/worker_threads.html#portunref
 [`process.dlopen`]: process.md#processdlopenmodule-filename-flags
 [`string`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String
 [`util.TextDecoder`]: util.md#class-utiltextdecoder

lib/internal/modules/esm/initialize_import_meta.js

@@ -0,0 +1,32 @@
+'use strict';
+
+const { getOptionValue } = require('internal/options');
+const experimentalImportMetaResolve =
+getOptionValue('--experimental-import-meta-resolve');
+const { PromisePrototypeThen, PromiseReject } = primordials;
+const asyncESM = require('internal/process/esm_loader');
+
+function createImportMetaResolve(defaultParentUrl) {
+  return async function resolve(specifier, parentUrl = defaultParentUrl) {
+    return PromisePrototypeThen(
+      asyncESM.esmLoader.resolve(specifier, parentUrl),
+      ({ url }) => url,
+      (error) => (
+        error.code === 'ERR_UNSUPPORTED_DIR_IMPORT' ?
+          error.url : PromiseReject(error))
+    );
+  };
+}
+
+function initializeImportMeta(meta, context) {
+  const url = context.url;
+
+  // Alphabetical
+  if (experimentalImportMetaResolve)
+    meta.resolve = createImportMetaResolve(url);
+  meta.url = url;
+}
+
+module.exports = {
+  initializeImportMeta
+};

lib/internal/modules/esm/loader.js

@@ -18,6 +18,7 @@ const {
   SafeWeakMap,
   globalThis,
 } = primordials;
+const { MessageChannel } = require('internal/worker/io');
 
 const {
   ERR_INVALID_ARG_TYPE,
@@ -39,6 +40,9 @@ const {
   defaultResolve,
   DEFAULT_CONDITIONS,
 } = require('internal/modules/esm/resolve');
+const {
+  initializeImportMeta
+} = require('internal/modules/esm/initialize_import_meta');
 const { defaultLoad } = require('internal/modules/esm/load');
 const { translators } = require(
   'internal/modules/esm/translators');
@@ -76,6 +80,8 @@ class ESMLoader {
     defaultResolve,
   ];
 
+  #importMetaInitializer = initializeImportMeta;
+
   /**
    * Map of already-loaded CJS modules to use
    */
@@ -359,7 +365,18 @@ class ESMLoader {
     if (!count) return;
 
     for (let i = 0; i < count; i++) {
-      const preload = this.#globalPreloaders[i]();
+      const channel = new MessageChannel();
+      const {
+        port1: insidePreload,
+        port2: insideLoader,
+      } = channel;
+
+      insidePreload.unref();
+      insideLoader.unref();
+
+      const preload = this.#globalPreloaders[i]({
+        port: insideLoader
+      });
 
       if (preload == null) return;
 
@@ -373,22 +390,60 @@ class ESMLoader {
       const { compileFunction } = require('vm');
       const preloadInit = compileFunction(
         preload,
-        ['getBuiltin'],
+        ['getBuiltin', 'port', 'setImportMetaCallback'],
         {
           filename: '<preload>',
         }
       );
       const { NativeModule } = require('internal/bootstrap/loaders');
-
-      FunctionPrototypeCall(preloadInit, globalThis, (builtinName) => {
-        if (NativeModule.canBeRequiredByUsers(builtinName)) {
-          return require(builtinName);
+      // We only allow replacing the importMetaInitializer during preload,
+      // after preload is finished, we disable the ability to replace it
+      //
+      // This exposes accidentally setting the initializer too late by
+      // throwing an error.
+      let finished = false;
+      let replacedImportMetaInitializer = false;
+      let next = this.#importMetaInitializer;
+      try {
+        // Calls the compiled preload source text gotten from the hook
+        // Since the parameters are named we use positional parameters
+        // see compileFunction above to cross reference the names
+        FunctionPrototypeCall(
+          preloadInit,
+          globalThis,
+          // Param getBuiltin
+          (builtinName) => {
+            if (NativeModule.canBeRequiredByUsers(builtinName)) {
+              return require(builtinName);
+            }
+            throw new ERR_INVALID_ARG_VALUE('builtinName', builtinName);
+          },
+          // Param port
+          insidePreload,
+          // Param setImportMetaCallback
+          (fn) => {
+            if (finished || typeof fn !== 'function') {
+              throw new ERR_INVALID_ARG_TYPE('fn', fn);
+            }
+            replacedImportMetaInitializer = true;
+            const parent = next;
+            next = (meta, context) => {
+              return fn(meta, context, parent);
+            };
+          });
+      } finally {
+        finished = true;
+        if (replacedImportMetaInitializer) {
+          this.#importMetaInitializer = next;
         }
-        throw new ERR_INVALID_ARG_VALUE('builtinName', builtinName);
-      });
+      }
     }
   }
 
+  importMetaInitialize(meta, context) {
+    this.#importMetaInitializer(meta, context);
+  }
+
   /**
    * Resolve the location of the module.
    *

lib/internal/modules/esm/translators.js

@@ -8,8 +8,6 @@ const {
   ObjectGetPrototypeOf,
   ObjectPrototypeHasOwnProperty,
   ObjectKeys,
-  PromisePrototypeThen,
-  PromiseReject,
   SafeArrayIterator,
   SafeMap,
   SafeSet,
@@ -52,9 +50,6 @@ const {
 const { maybeCacheSourceMap } = require('internal/source_map/source_map_cache');
 const moduleWrap = internalBinding('module_wrap');
 const { ModuleWrap } = moduleWrap;
-const { getOptionValue } = require('internal/options');
-const experimentalImportMetaResolve =
-    getOptionValue('--experimental-import-meta-resolve');
 const asyncESM = require('internal/process/esm_loader');
 const { emitWarningSync } = require('internal/process/warning');
 const { TextDecoder } = require('internal/encoding');
@@ -111,25 +106,6 @@ async function importModuleDynamically(specifier, { url }) {
   return asyncESM.esmLoader.import(specifier, url);
 }
 
-function createImportMetaResolve(defaultParentUrl) {
-  return async function resolve(specifier, parentUrl = defaultParentUrl) {
-    return PromisePrototypeThen(
-      asyncESM.esmLoader.resolve(specifier, parentUrl),
-      ({ url }) => url,
-      (error) => (
-        error.code === 'ERR_UNSUPPORTED_DIR_IMPORT' ?
-          error.url : PromiseReject(error))
-    );
-  };
-}
-
-function initializeImportMeta(meta, { url }) {
-  // Alphabetical
-  if (experimentalImportMetaResolve)
-    meta.resolve = createImportMetaResolve(url);
-  meta.url = url;
-}
-
 // Strategy for loading a standard JavaScript module.
 translators.set('module', async function moduleStrategy(url, source, isMain) {
   assertBufferSource(source, true, 'load');
@@ -138,7 +114,9 @@ translators.set('module', async function moduleStrategy(url, source, isMain) {
   debug(`Translating StandardModule ${url}`);
   const module = new ModuleWrap(url, undefined, source, 0, 0);
   moduleWrap.callbackMap.set(module, {
-    initializeImportMeta,
+    initializeImportMeta: (meta, wrap) => this.importMetaInitialize(meta, {
+      url: wrap.url
+    }),
     importModuleDynamically,
   });
   return module;

test/es-module/test-esm-loader-mock.mjs

@@ -0,0 +1,45 @@
+// Flags: --loader ./test/fixtures/es-module-loaders/mock-loader.mjs
+import '../common/index.mjs';
+import assert from 'assert/strict';
+
+// This is provided by test/fixtures/es-module-loaders/mock-loader.mjs
+import mock from 'node:mock';
+
+mock('node:events', {
+  EventEmitter: 'This is mocked!'
+});
+
+// This resolves to node:events
+// It is intercepted by mock-loader and doesn't return the normal value
+assert.deepStrictEqual(await import('events'), Object.defineProperty({
+  __proto__: null,
+  EventEmitter: 'This is mocked!'
+}, Symbol.toStringTag, {
+  enumerable: false,
+  value: 'Module'
+}));
+
+const mutator = mock('node:events', {
+  EventEmitter: 'This is mocked v2!'
+});
+
+// It is intercepted by mock-loader and doesn't return the normal value.
+// This is resolved separately from the import above since the specifiers
+// are different.
+const mockedV2 = await import('node:events');
+assert.deepStrictEqual(mockedV2, Object.defineProperty({
+  __proto__: null,
+  EventEmitter: 'This is mocked v2!'
+}, Symbol.toStringTag, {
+  enumerable: false,
+  value: 'Module'
+}));
+
+mutator.EventEmitter = 'This is mocked v3!';
+assert.deepStrictEqual(mockedV2, Object.defineProperty({
+  __proto__: null,
+  EventEmitter: 'This is mocked v3!'
+}, Symbol.toStringTag, {
+  enumerable: false,
+  value: 'Module'
+}));

test/fixtures/es-module-loaders/loader-side-effect.mjs

@@ -1,5 +1,5 @@
 // Arrow function so it closes over the this-value of the preload scope.
-const globalPreload = () => {
+const globalPreloadSrc = () => {
   /* global getBuiltin */
   const assert = getBuiltin('assert');
   const vm = getBuiltin('vm');
@@ -24,9 +24,9 @@ const implicitGlobalConst = 42 * 42;
   globalThis.explicitGlobalProperty = 42 * 42 * 42;
 }
 
-export function getGlobalPreloadCode() {
+export function globalPreload() {
   return `\
 <!-- assert: inside of script goal -->
-(${globalPreload.toString()})();
+(${globalPreloadSrc.toString()})();
 `;
 }