[Web] Improve Godot shutdown and cleanup.

platform/web/js/engine/config.js

@@ -275,7 +275,7 @@ const InternalConfig = function (initConfig) { // eslint-disable-line no-unused-
 			'print': this.onPrint,
 			'printErr': this.onPrintError,
 			'thisProgram': this.executable,
-			'noExitRuntime': true,
+			'noExitRuntime': false,
 			'dynamicLibraries': [`${loadPath}.side.wasm`],
 			'instantiateWasm': function (imports, onSuccess) {
 				function done(result) {

platform/web/web_main.cpp

@@ -41,30 +41,25 @@
 
 static OS_Web *os = nullptr;
 static uint64_t target_ticks = 0;
+static bool main_started = false;
+static bool shutdown_complete = false;
 
 void exit_callback() {
-	emscripten_cancel_main_loop(); // After this, we can exit!
-	Main::cleanup();
+	if (!shutdown_complete) {
+		return; // Still waiting.
+	}
+	if (main_started) {
+		Main::cleanup();
+		main_started = false;
+	}
 	int exit_code = OS_Web::get_singleton()->get_exit_code();
 	memdelete(os);
 	os = nullptr;
-	emscripten_force_exit(exit_code); // No matter that we call cancel_main_loop, regular "exit" will not work, forcing.
+	emscripten_force_exit(exit_code); // Exit runtime.
 }
 
 void cleanup_after_sync() {
-	emscripten_set_main_loop(exit_callback, -1, false);
-}
-
-void early_cleanup() {
-	emscripten_cancel_main_loop(); // After this, we can exit!
-	int exit_code = OS_Web::get_singleton()->get_exit_code();
-	memdelete(os);
-	os = nullptr;
-	emscripten_force_exit(exit_code); // No matter that we call cancel_main_loop, regular "exit" will not work, forcing.
-}
-
-void early_cleanup_sync() {
-	emscripten_set_main_loop(early_cleanup, -1, false);
+	shutdown_complete = true;
 }
 
 void main_loop_callback() {
@@ -87,7 +82,8 @@ void main_loop_callback() {
 		target_ticks += (uint64_t)(1000000 / max_fps);
 	}
 	if (os->main_loop_iterate()) {
-		emscripten_cancel_main_loop(); // Cancel current loop and wait for cleanup_after_sync.
+		emscripten_cancel_main_loop(); // Cancel current loop and set the cleanup one.
+		emscripten_set_main_loop(exit_callback, -1, false);
 		godot_js_os_finish_async(cleanup_after_sync);
 	}
 }
@@ -109,10 +105,14 @@ extern EMSCRIPTEN_KEEPALIVE int godot_web_main(int argc, char *argv[]) {
 		}
 		os->set_exit_code(exit_code);
 		// Will only exit after sync.
-		godot_js_os_finish_async(early_cleanup_sync);
+		emscripten_set_main_loop(exit_callback, -1, false);
+		godot_js_os_finish_async(cleanup_after_sync);
 		return exit_code;
 	}
 
+	os->set_exit_code(0);
+	main_started = true;
+
 	// Ease up compatibility.
 	ResourceLoader::set_abort_on_missing_resources(false);