Detect and report late assignment of Module API elements

src/postamble.js

@@ -199,6 +199,9 @@ function run() {
 #endif
 #if expectToReceiveOnModule('onRuntimeInitialized')
     Module['onRuntimeInitialized']?.();
+#if ASSERTIONS
+    consumedModuleProp('onRuntimeInitialized');
+#endif
 #endif
 
 #if HAS_MAIN
@@ -297,6 +300,9 @@ if (Module['preInit']) {
     Module['preInit'].pop()();
   }
 }
+#if ASSERTIONS
+consumedModuleProp('preInit');
+#endif
 #endif
 
 run();

src/preamble.js

@@ -181,6 +181,9 @@ function preRun() {
       addOnPreRun(Module['preRun'].shift());
     }
   }
+#if ASSERTIONS
+  consumedModuleProp('preRun');
+#endif
 #endif
   <<< ATPRERUNS >>>
 }
@@ -276,6 +279,9 @@ function postRun() {
       addOnPostRun(Module['postRun'].shift());
     }
   }
+#if ASSERTIONS
+  consumedModuleProp('postRun');
+#endif
 #endif
 
   <<< ATPOSTRUNS >>>

src/runtime_debug.js

@@ -33,6 +33,18 @@ function legacyModuleProp(prop, newName, incoming=true) {
   }
 }
 
+function consumedModuleProp(prop) {
+  if (!Object.getOwnPropertyDescriptor(Module, prop)) {
+    Object.defineProperty(Module, prop, {
+      configurable: true,
+      set() {
+        abort(`Attempt to set \`Module.${prop}\` after it has already been processed.  This can happen, for example, when code is injected via '--post-js' rather than '--pre-js'`);
+
+      }
+    });
+  }
+}
+
 function ignoredModuleProp(prop) {
   if (Object.getOwnPropertyDescriptor(Module, prop)) {
     abort(`\`Module.${prop}\` was supplied but \`${prop}\` not included in INCOMING_MODULE_JS_API`);

test/other/codesize/test_codesize_hello_O0.gzsize

@@ -1 +1 @@
-7872
+7985

test/other/codesize/test_codesize_hello_O0.jssize

@@ -1 +1 @@
-20962
+21341

test/other/test_unoptimized_code_size.js.size

@@ -1 +1 @@
-51969
+52492

test/other/test_unoptimized_code_size_strict.js.size

@@ -1 +1 @@
-50401
+50780

test/test_other.py

@@ -15607,3 +15607,12 @@ def test_instantiate_wasm(self):
         return {}; // Compiling asynchronously, no exports.
       }''')
     self.do_runf('test_manual_wasm_instantiate.c', emcc_args=['--pre-js=pre.js'])
+
+  def test_late_module_api_assignment(self):
+    # When sync instantiation is used (or when async/await is used in MODULARIZE mode) certain
+    # Module properties cannot be assigned in `--post-js` code because its too late by the time
+    # it runs.
+    for prop in ('onRuntimeInitialized', 'postRun', 'preRun', 'preInit'):
+      create_file('post.js', f'Module.{prop} = () => console.log("will never fire since assigned too late")')
+      expected = f"Aborted(Attempt to set `Module.{prop}` after it has already been processed.  This can happen, for example, when code is injected via '--post-js' rather than '--pre-js')"
+      self.do_runf(test_file('hello_world.c'), expected, emcc_args=['--post-js=post.js', '-sWASM_ASYNC_COMPILATION=0'], assert_returncode=NON_ZERO)