Assert if MODULARIZE factory function is used with new. (#23210)

Once we start using `async` for for this function then `new` stops
working: #23157.

Using `new` in this was was always an antipattern but there was nothing
atopping users from doing this.

ChangeLog.md

@@ -20,6 +20,11 @@ See docs/process.md for more on how version tagging works.
 
 3.1.75 (in development)
 -----------------------
+- When using `-sMODULARIZE` we now assert if the factory function is called with
+  the JS `new` keyword.  e.g. `a = new Module()` rather than `b = Module()`.
+  This paves the way for marking the function as `async` which does not allow
+  `new` to be used.  This usage of `new` here was never documented and is
+  considered and antipattern. (#23210)
 - `PATH.basename()` no longer calls `PATH.normalize()`, so that
   `PATH.basename("a/.")` returns `"."` instead of `"a"` and
   `PATH.basename("a/b/..")` returns `".."` instead of `"a"`. This is in line with

test/test_browser.py

@@ -4795,28 +4795,19 @@ def test_browser_run_from_different_directory(self):
 
   # Similar to `test_browser_run_from_different_directory`, but asynchronous because of `-sMODULARIZE`
   def test_browser_run_from_different_directory_async(self):
-    for args, creations in [
-      (['-sMODULARIZE'], [
-        'Module();',    # documented way for using modularize
-        'new Module();' # not documented as working, but we support it
-       ]),
-    ]:
-      print(args)
-      # compile the code with the modularize feature and the preload-file option enabled
-      self.compile_btest('browser_test_hello_world.c', ['-o', 'test.js', '-O3'] + args)
-      ensure_dir('subdir')
-      shutil.move('test.js', Path('subdir/test.js'))
-      shutil.move('test.wasm', Path('subdir/test.wasm'))
-      for creation in creations:
-        print(creation)
-        # Make sure JS is loaded from subdirectory
-        create_file('test-subdir.html', '''
-          <script src="subdir/test.js"></script>
-          <script>
-            %s
-          </script>
-        ''' % creation)
-        self.run_browser('test-subdir.html', '/report_result?0')
+    # compile the code with the modularize feature and the preload-file option enabled
+    self.compile_btest('browser_test_hello_world.c', ['-o', 'test.js', '-O3', '-sMODULARIZE'])
+    ensure_dir('subdir')
+    shutil.move('test.js', Path('subdir/test.js'))
+    shutil.move('test.wasm', Path('subdir/test.wasm'))
+    # Make sure JS is loaded from subdirectory
+    create_file('test-subdir.html', '''
+      <script src="subdir/test.js"></script>
+      <script>
+        Module();
+      </script>
+    ''')
+    self.run_browser('test-subdir.html', '/report_result?0')
 
   # Similar to `test_browser_run_from_different_directory`, but
   # also also we eval the initial code, so currentScript is not present. That prevents us

test/test_other.py

@@ -6774,6 +6774,12 @@ def test_modularize_strict(self):
     output = self.run_js('run.js')
     self.assertEqual(output, 'hello, world!\n')
 
+  def test_modularize_new_misuse(self):
+    self.run_process([EMCC, test_file('hello_world.c'), '-sMODULARIZE', '-sEXPORT_NAME=Foo'])
+    create_file('run.js', 'var m = require("./a.out.js"); new m();')
+    err = self.run_js('run.js', assert_returncode=NON_ZERO)
+    self.assertContained('Error: Foo() should not be called with `new Foo()`', err)
+
   @parameterized({
     '': ([],),
     'export_name': (['-sEXPORT_NAME=Foo'],),

tools/link.py

@@ -2452,6 +2452,19 @@ def modularize():
         'wrapper_function': wrapper_function,
       }
 
+  if settings.ASSERTIONS and settings.MODULARIZE != 'instance':
+    src += '''\
+(() => {
+  // Create a small, never-async wrapper around %(EXPORT_NAME)s which
+  // checks for callers incorrectly using it with `new`.
+  var real_%(EXPORT_NAME)s = %(EXPORT_NAME)s;
+  %(EXPORT_NAME)s = function(arg) {
+    if (new.target) throw new Error("%(EXPORT_NAME)s() should not be called with `new %(EXPORT_NAME)s()`");
+    return real_%(EXPORT_NAME)s(arg);
+  }
+})();
+''' % {'EXPORT_NAME': settings.EXPORT_NAME}
+
   # Given the async nature of how the Module function and Module object
   # come into existence in AudioWorkletGlobalScope, store the Module
   # function under a different variable name so that AudioWorkletGlobalScope