loader: use default loader as cascaded loader in the in loader worker

lib/internal/main/worker_thread.js

@@ -133,7 +133,10 @@ port.on('message', (message) => {
     if (manifestSrc) {
       require('internal/process/policy').setup(manifestSrc, manifestURL);
     }
-    setupUserModules();
+    const isLoaderWorker =
+      doEval === 'internal' &&
+      filename === require('internal/modules/esm/utils').loaderWorkerId;
+    setupUserModules(isLoaderWorker);
 
     if (!hasStdin)
       process.stdin.push(null);

lib/internal/modules/esm/hooks.js

@@ -52,6 +52,7 @@ const {
 } = require('internal/modules/esm/resolve');
 const {
   getDefaultConditions,
+  loaderWorkerId,
 } = require('internal/modules/esm/utils');
 const { deserializeError } = require('internal/error_serdes');
 const {
@@ -494,7 +495,7 @@ class HooksProxy {
     const lock = new SharedArrayBuffer(SHARED_MEMORY_BYTE_LENGTH);
     this.#lock = new Int32Array(lock);
 
-    this.#worker = new InternalWorker('internal/modules/esm/worker', {
+    this.#worker = new InternalWorker(loaderWorkerId, {
       stderr: false,
       stdin: false,
       stdout: false,

lib/internal/modules/esm/loader.js

@@ -411,7 +411,10 @@ let emittedExperimentalWarning = false;
  * @returns {DefaultModuleLoader | CustomizedModuleLoader}
  */
 function createModuleLoader(useCustomLoadersIfPresent = true) {
-  if (useCustomLoadersIfPresent) {
+  if (useCustomLoadersIfPresent &&
+      // Don't spawn a new worker if we're already in a worker thread created by instantiating CustomizedModuleLoader;
+      // doing so would cause an infinite loop.
+      !require('internal/modules/esm/utils').isLoaderWorker()) {
     const userLoaderPaths = getOptionValue('--experimental-loader');
     if (userLoaderPaths.length > 0) {
       if (!emittedExperimentalWarning) {

lib/internal/modules/esm/utils.js

@@ -91,14 +91,22 @@ async function importModuleDynamicallyCallback(wrap, specifier, assertions) {
   throw new ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING();
 }
 
-function initializeESM() {
+// This is configured during pre-execution. Specifically it's set to true for
+// the loader worker in internal/main/worker_thread.js.
+let _isLoaderWorker = false;
+function initializeESM(isLoaderWorker = false) {
+  _isLoaderWorker = isLoaderWorker;
   initializeDefaultConditions();
   // Setup per-isolate callbacks that locate data or callbacks that we keep
   // track of for different ESM modules.
   setInitializeImportMetaObjectCallback(initializeImportMetaObject);
   setImportModuleDynamicallyCallback(importModuleDynamicallyCallback);
 }
 
+function isLoaderWorker() {
+  return _isLoaderWorker;
+}
+
 async function initializeHooks() {
   const customLoaderPaths = getOptionValue('--experimental-loader');
 
@@ -165,4 +173,6 @@ module.exports = {
   initializeHooks,
   getDefaultConditions,
   getConditionsSet,
+  loaderWorkerId: 'internal/modules/esm/worker',
+  isLoaderWorker,
 };

lib/internal/modules/esm/worker.js

@@ -29,7 +29,7 @@ const { receiveMessageOnPort } = require('internal/worker/io');
 const {
   WORKER_TO_MAIN_THREAD_NOTIFICATION,
 } = require('internal/modules/esm/shared_constants');
-const { initializeESM, initializeHooks } = require('internal/modules/esm/utils');
+const { initializeHooks } = require('internal/modules/esm/utils');
 
 
 function transferArrayBuffer(hasError, source) {
@@ -67,7 +67,6 @@ async function customizedModuleWorker(lock, syncCommPort, errorHandler) {
   let hasInitializationError = false;
 
   try {
-    initializeESM();
     const initResult = await initializeHooks();
     hooks = initResult.hooks;
     preloadScripts = initResult.preloadScripts;

lib/internal/process/pre_execution.js

@@ -119,9 +119,9 @@ function prepareExecution(options) {
   }
 }
 
-function setupUserModules() {
+function setupUserModules(isLoaderWorker = false) {
   initializeCJSLoader();
-  initializeESMLoader();
+  initializeESMLoader(isLoaderWorker);
   const CJSLoader = require('internal/modules/cjs/loader');
   assert(!CJSLoader.hasLoadedAnyUserCJSModule);
   loadPreloadModules();
@@ -600,9 +600,9 @@ function initializeCJSLoader() {
   initializeCJS();
 }
 
-function initializeESMLoader() {
+function initializeESMLoader(isLoaderWorker) {
   const { initializeESM } = require('internal/modules/esm/utils');
-  initializeESM();
+  initializeESM(isLoaderWorker);
 
   // Patch the vm module when --experimental-vm-modules is on.
   // Please update the comments in vm.js when this block changes.

test/es-module/test-loaders-workers-spawned.mjs

@@ -0,0 +1,77 @@
+import { spawnPromisified } from '../common/index.mjs';
+import * as fixtures from '../common/fixtures.mjs';
+import assert from 'node:assert';
+import { execPath } from 'node:process';
+import { describe, it } from 'node:test';
+
+describe('Worker threads do not spawn infinitely', { concurrency: true }, () => {
+  it('should not trigger an infinite loop when using a loader exports no recognized hooks', async () => {
+    const { code, signal, stdout, stderr } = await spawnPromisified(execPath, [
+      '--no-warnings',
+      '--experimental-loader',
+      fixtures.fileURL('empty.js'),
+      '--eval',
+      'setTimeout(() => console.log("hello"),99)',
+    ]);
+
+    assert.strictEqual(stderr, '');
+    assert.match(stdout, /^hello\r?\n$/);
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+  });
+
+  it('should support a CommonJS entry point and a loader that imports a CommonJS module', async () => {
+    const { code, signal, stdout, stderr } = await spawnPromisified(execPath, [
+      '--no-warnings',
+      '--experimental-loader',
+      fixtures.fileURL('es-module-loaders/loader-with-dep.mjs'),
+      fixtures.path('print-delayed.js'),
+    ]);
+
+    assert.strictEqual(stderr, '');
+    assert.match(stdout, /^delayed\r?\n$/);
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+  });
+
+  it('should support --require and --import along with using a loader written in CJS and CJS entry point', async () => {
+    const { code, signal, stdout, stderr } = await spawnPromisified(execPath, [
+      '--no-warnings',
+      '--eval',
+      'setTimeout(() => console.log("D"),99)',
+      '--import',
+      fixtures.fileURL('printC.js'),
+      '--experimental-loader',
+      fixtures.fileURL('printB.js'),
+      '--require',
+      fixtures.path('printA.js'),
+    ]);
+
+    assert.strictEqual(stderr, '');
+    // The worker code should always run before the --import, but the console.log might arrive late.
+    assert.match(stdout, /^A\r?\nA\r?\n(B\r?\nC|C\r?\nB)\r?\nD\r?\n$/);
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+  });
+
+  it('should support --require and --import along with using a loader written in ESM and ESM entry point', async () => {
+    const { code, signal, stdout, stderr } = await spawnPromisified(execPath, [
+      '--no-warnings',
+      '--require',
+      fixtures.path('printA.js'),
+      '--experimental-loader',
+      'data:text/javascript,console.log("B")',
+      '--import',
+      fixtures.fileURL('printC.js'),
+      '--input-type=module',
+      '--eval',
+      'setTimeout(() => console.log("D"),99)',
+    ]);
+
+    assert.strictEqual(stderr, '');
+    // The worker code should always run before the --import, but the console.log might arrive late.
+    assert.match(stdout, /^A\r?\nA\r?\n(B\r?\nC|C\r?\nB)\r?\nD\r?\n$/);
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+  });
+});

test/fixtures/print-delayed.js

@@ -0,0 +1,3 @@
+setTimeout(() => {
+  console.log('delayed');
+}, 100);