esm: do not call getSource when format is commonjs

lib/internal/modules/esm/load.js

@@ -132,20 +132,21 @@ async function defaultLoad(url, context = kEmptyObject) {
   if (urlInstance.protocol === 'node:') {
     source = null;
     format ??= 'builtin';
-  } else {
-    let contextToPass = context;
+  } else if (format !== 'commonjs' || defaultType === 'module') {
     if (source == null) {
       ({ responseURL, source } = await getSource(urlInstance, context));
-      contextToPass = { __proto__: context, source };
+      context = { __proto__: context, source };
     }
 
-    // Now that we have the source for the module, run `defaultGetFormat` again in case we detect ESM syntax.
-    format ??= await defaultGetFormat(urlInstance, contextToPass);
+    if (format == null) {
+      // Now that we have the source for the module, run `defaultGetFormat` to detect its format.
+      format = await defaultGetFormat(urlInstance, context);
 
-    if (format === 'commonjs' && contextToPass !== context && defaultType !== 'module') {
-      // For backward compatibility reasons, we need to discard the source in
-      // order for the CJS loader to re-fetch it.
-      source = null;
+      if (format === 'commonjs') {
+        // For backward compatibility reasons, we need to discard the source in
+        // order for the CJS loader to re-fetch it.
+        source = null;
+      }
     }
   }
 

test/es-module/test-esm-dynamic-import-commonjs.js

@@ -0,0 +1,19 @@
+'use strict';
+
+const common = require('../common');
+const fixtures = require('../common/fixtures');
+const assert = require('node:assert');
+
+(async () => {
+
+  // Make sure that the CommonJS module lexer has been initialized.
+  // See https://github.com/nodejs/node/blob/v21.1.0/lib/internal/modules/esm/translators.js#L61-L81.
+  await import(fixtures.fileURL('empty.js'));
+
+  let tickDuringCJSImport = false;
+  process.nextTick(() => { tickDuringCJSImport = true; });
+  await import(fixtures.fileURL('empty.cjs'));
+
+  assert(!tickDuringCJSImport);
+
+})().then(common.mustCall());

test/es-module/test-esm-dynamic-import-commonjs.mjs

@@ -0,0 +1,9 @@
+import '../common/index.mjs';
+import fixtures from '../common/fixtures.js';
+import assert from 'node:assert';
+
+let tickDuringCJSImport = false;
+process.nextTick(() => { tickDuringCJSImport = true; });
+await import(fixtures.fileURL('empty.cjs'));
+
+assert(!tickDuringCJSImport);

test/es-module/test-esm-loader-with-source.mjs

@@ -0,0 +1,10 @@
+// Flags: --experimental-loader ./test/fixtures/es-module-loaders/preset-cjs-source.mjs
+import '../common/index.mjs';
+import * as fixtures from '../common/fixtures.mjs';
+import assert from 'assert';
+
+const { default: existingFileSource } = await import(fixtures.fileURL('es-modules', 'cjs-file.cjs'));
+const { default: noSuchFileSource } = await import(new URL('./no-such-file.cjs', import.meta.url));
+
+assert.strictEqual(existingFileSource, 'no .cjs file was read to get this source');
+assert.strictEqual(noSuchFileSource, 'no .cjs file was read to get this source');

test/fixtures/es-module-loaders/preset-cjs-source.mjs

@@ -0,0 +1,21 @@
+export function resolve(specifier, context, next) {
+  if (specifier.endsWith('/no-such-file.cjs')) {
+    // Shortcut to avoid ERR_MODULE_NOT_FOUND for non-existing file, but keep the url for the load hook.
+    return {
+      shortCircuit: true,
+      url: specifier,
+    };
+  }
+  return next(specifier);
+}
+
+export function load(href, context, next) {
+  if (href.endsWith('.cjs')) {
+    return {
+      format: 'commonjs',
+      shortCircuit: true,
+      source: 'module.exports = "no .cjs file was read to get this source";',
+    };
+  }
+  return next(href);
+}