module: fix `--preserve-symlinks-main`

Fixes resolving main module when the `argv[1]` was pointing to a symlink
without its file extension.

PR-URL: https://github.com/nodejs/node/pull/51312
Fixes: https://github.com/nodejs/node/issues/41000
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

lib/internal/modules/cjs/loader.js

@@ -469,15 +469,15 @@ function tryPackage(requestPath, exts, isMain, originalPath) {
 }
 
 /**
- * Check if the file exists and is not a directory if using `--preserve-symlinks` and `isMain` is false, keep symlinks
- * intact, otherwise resolve to the absolute realpath.
+ * Check if the file exists and is not a directory if using `--preserve-symlinks` and `isMain` is false or
+ * `--preserve-symlinks-main` and `isMain` is true , keep symlinks intact, otherwise resolve to the absolute realpath.
  * @param {string} requestPath The path to the file to load.
  * @param {boolean} isMain Whether the file is the main module.
  */
 function tryFile(requestPath, isMain) {
   const rc = _stat(requestPath);
   if (rc !== 0) { return; }
-  if (getOptionValue('--preserve-symlinks') && !isMain) {
+  if (getOptionValue(isMain ? '--preserve-symlinks-main' : '--preserve-symlinks')) {
     return path.resolve(requestPath);
   }
   return toRealPath(requestPath);

test/es-module/test-esm-preserve-symlinks-main.js

@@ -1,12 +1,19 @@
 'use strict';
 
-const common = require('../common');
-const { spawn } = require('child_process');
-const assert = require('assert');
-const path = require('path');
-const fs = require('fs');
-
+const { spawnPromisified, skip } = require('../common');
 const tmpdir = require('../common/tmpdir');
+
+// Invoke the main file via a symlink.  In this case --preserve-symlinks-main
+// dictates that it'll resolve relative imports in the main file relative to
+// the symlink, and not relative to the symlink target; the file structure set
+// up below requires this to not crash when loading ./submodule_link.js
+
+const assert = require('node:assert');
+const fs = require('node:fs');
+const path = require('node:path');
+const { execPath } = require('node:process');
+const { describe, it } = require('node:test');
+
 tmpdir.refresh();
 const tmpDir = tmpdir.path;
 
@@ -14,7 +21,7 @@ fs.mkdirSync(path.join(tmpDir, 'nested'));
 fs.mkdirSync(path.join(tmpDir, 'nested2'));
 
 const entry = path.join(tmpDir, 'nested', 'entry.js');
-const entry_link_absolute_path = path.join(tmpDir, 'link.js');
+const entry_link_absolute_path = path.join(tmpDir, 'index.js');
 const submodule = path.join(tmpDir, 'nested2', 'submodule.js');
 const submodule_link_absolute_path = path.join(tmpDir, 'submodule_link.js');
 
@@ -31,27 +38,39 @@ try {
   fs.symlinkSync(submodule, submodule_link_absolute_path);
 } catch (err) {
   if (err.code !== 'EPERM') throw err;
-  common.skip('insufficient privileges for symlinks');
+  skip('insufficient privileges for symlinks');
 }
 
-function doTest(flags, done) {
-  // Invoke the main file via a symlink.  In this case --preserve-symlinks-main
-  // dictates that it'll resolve relative imports in the main file relative to
-  // the symlink, and not relative to the symlink target; the file structure set
-  // up above requires this to not crash when loading ./submodule_link.js
-  spawn(process.execPath, [
-    '--preserve-symlinks',
-    '--preserve-symlinks-main',
-    entry_link_absolute_path,
-  ], { stdio: 'inherit' })
-    .on('exit', (code) => {
-      assert.strictEqual(code, 0);
-      done();
-    });
-}
+describe('Invoke the main file via a symlink.', { concurrency: true }, () => {
+  it('should resolve relative imports in the main file', async () => {
+    const { code } = await spawnPromisified(execPath, [
+      '--preserve-symlinks',
+      '--preserve-symlinks-main',
+      entry_link_absolute_path,
+    ]);
 
-// First test the commonjs module loader
-doTest([], () => {
-  // Now test the new loader
-  doTest([], () => {});
+    assert.strictEqual(code, 0);
+  });
+
+  it('should resolve relative imports in the main file when file extension is omitted', async () => {
+    const entry_link_absolute_path_without_ext = path.join(tmpDir, 'index');
+
+    const { code } = await spawnPromisified(execPath, [
+      '--preserve-symlinks',
+      '--preserve-symlinks-main',
+      entry_link_absolute_path_without_ext,
+    ]);
+
+    assert.strictEqual(code, 0);
+  });
+
+  it('should resolve relative imports in the main file when filename(index.js) is omitted', async () => {
+    const { code } = await spawnPromisified(execPath, [
+      '--preserve-symlinks',
+      '--preserve-symlinks-main',
+      tmpDir,
+    ]);
+
+    assert.strictEqual(code, 0);
+  });
 });