Fix craftInvokerFunction on JSPI (ASYNCIFY=2) (#19281)

* Fix craftInvokerFunction on JSPI (ASYNCIFY=2)

When the function wrapped by craftInvokerFunction doesn't return, rv is
undeclared, which results in an exception being thrown at crafted function
runtime.

Author: mboc-qt

src/embind/embind.js

@@ -1129,7 +1129,7 @@ var LibraryEmbind = {
     invokerFnBody += "return Asyncify.currData ? Asyncify.whenDone().then(onDone) : onDone(" + (returns ? "rv" : "") +");\n"
 #elif ASYNCIFY == 2
     invokerFnBody += "}\n";
-    invokerFnBody += "return " + (isAsync ? "rv.then(onDone)" : "onDone(rv)") + ";";
+    invokerFnBody += "return " + (isAsync ? "rv.then(onDone)" : "onDone(" + (returns ? "rv" : "") + ")") + ";";
 #endif
 
     invokerFnBody += "}\n";

test/embind/embind_jspi_test.cpp

@@ -39,9 +39,27 @@ int stdFunction(const MyClass& target, int i) {
   return i + 1;
 }
 
+EM_ASYNC_JS(void, jsSuspend, (), {
+  await new Promise(resolve => {
+    Module.unsuspendResolve = resolve;
+  });
+});
+
+void suspend() {
+  jsSuspend();
+};
+
+void unsuspend() {
+  EM_ASM({
+    Module.unsuspendResolve();
+  });
+}
+
 EMSCRIPTEN_BINDINGS(xxx) {
   function("voidFunc", &voidFunc, async());
   function("intFunc", &intFunc, async());
+  function("unsuspend", &unsuspend);
+  function("suspend", &suspend, async());
 
   class_<MyClass>("MyClass")
     .constructor<>()
@@ -76,6 +94,9 @@ EM_ASYNC_JS(void, test, (), {
     await check(Module.MyClass.voidClass());
     await check(Module.MyClass.intClass(1), 2);
 
+    setTimeout(Module.unsuspend);
+    await Module.suspend();
+
     console.log('done');
   } catch (e) {
     console.log('Failed: ' + e.stack);