esm: fix erroneous re-initialization of ESMLoader

lib/internal/process/esm_loader.js

@@ -40,14 +40,20 @@ async function importModuleDynamicallyCallback(wrap, specifier, assertions) {
 };
 
 const esmLoader = new ESMLoader();
-
 exports.esmLoader = esmLoader;
 
+// Module.runMain() causes loadESM() to re-run (which it should do); however, this should NOT cause
+// ESM to be re-initialised; doing so causes duplicate custom loaders to be added to the public
+// esmLoader.
+let isESMInitialized = false;
+
 /**
  * Causes side-effects: user-defined loader hooks are added to esmLoader.
  * @returns {void}
  */
 async function initializeLoader() {
+  if (isESMInitialized) { return; }
+
   const { getOptionValue } = require('internal/options');
   const customLoaders = getOptionValue('--experimental-loader');
 
@@ -75,6 +81,8 @@ async function initializeLoader() {
   // Hooks must then be added to external/public loader
   // (so they're triggered in userland)
   await esmLoader.addCustomLoaders(keyedExportsList);
+
+  isESMInitialized = true;
 }
 
 exports.loadESM = async function loadESM(callback) {

test/es-module/test-esm-initialization.mjs

@@ -0,0 +1,31 @@
+import '../common/index.mjs';
+import fixtures from '../common/fixtures.js';
+import assert from 'node:assert';
+import { spawnSync } from 'node:child_process';
+import { fileURLToPath } from 'node:url';
+
+
+{ // Verify unadulterated source is loaded when there are no loaders
+  const { status, stderr, stdout } = spawnSync(
+    process.execPath,
+    [
+      '--loader',
+      fixtures.fileURL('es-module-loaders', 'loader-resolve-passthru.mjs'),
+      '--no-warnings',
+      fileURLToPath(fixtures.fileURL('es-modules', 'runmain.mjs')),
+    ],
+    { encoding: 'utf8' },
+  );
+
+  const resolveHookRunCount = [...(stdout.matchAll(/resolve passthru/g) ?? new Array() )]
+    .length - 1; // less 1 because the first is the needle
+
+  assert.strictEqual(stderr, '');
+  /**
+   * resolveHookRunCount = 2:
+   * 1. fixtures/…/runmain.mjs
+   * 2. node:module (imported by fixtures/…/runmain.mjs)
+   */
+  assert.strictEqual(resolveHookRunCount, 2);
+  assert.strictEqual(status, 0);
+}

test/fixtures/es-modules/runmain.mjs

@@ -0,0 +1,7 @@
+import { runMain } from 'node:module';
+
+try { await import.meta.resolve('doesnt-matter.mjs') } catch {}
+
+runMain();
+
+try { await import.meta.resolve('doesnt-matter.mjs') } catch {}