module: skip preserveSymlinks for main

PR-URL: #19388
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/modules/esm/default_resolve.js

@@ -69,7 +69,9 @@ function resolve(specifier, parentURL) {
     throw e;
   }
 
-  if (!preserveSymlinks) {
+  const isMain = parentURL === undefined;
+
+  if (!preserveSymlinks || isMain) {
     const real = realpathSync(getPathFromURL(url), {
       [internalFS.realpathCacheKey]: realpathCache
     });
@@ -83,7 +85,6 @@ function resolve(specifier, parentURL) {
 
   let format = extensionFormatMap[ext];
   if (!format) {
-    const isMain = parentURL === undefined;
     if (isMain)
       format = 'cjs';
     else

test/es-module/test-esm-symlink-main.js

@@ -0,0 +1,25 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const { spawn } = require('child_process');
+const tmpdir = require('../common/tmpdir');
+const fs = require('fs');
+tmpdir.refresh();
+
+const realPath = path.resolve(__dirname, '../fixtures/es-modules/symlink.mjs');
+const symlinkPath = path.resolve(tmpdir.path, 'symlink.js');
+
+try {
+  fs.symlinkSync(realPath, symlinkPath);
+} catch (err) {
+  if (err.code !== 'EPERM') throw err;
+  common.skip('insufficient privileges for symlinks');
+}
+
+spawn(process.execPath,
+      ['--experimental-modules', '--preserve-symlinks', symlinkPath],
+      { stdio: 'inherit' }).on('exit', (code) => {
+  assert.strictEqual(code, 0);
+});

test/fixtures/es-modules/symlink.mjs

@@ -0,0 +1 @@
+export var symlinked = true;