LibJS: Implement await properly for async functions

Fixes #20275

```
Summary:
    Diff Tests:
        +4 ✅    -4 ❌   

Diff Tests:
    test/built-ins/Array/fromAsync/non-iterable-input-with-thenable
    -async-mapped-awaits-callback-result-once.js ❌ -> ✅
    test/language/expressions/await/async-await-interleaved.js ❌ -> ✅
    test/language/expressions/await/await-awaits-thenables-that-
    throw.js ❌ -> ✅
    test/language/expressions/await/await-awaits-thenables.js ❌ -> ✅
```

Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -1582,12 +1582,39 @@ Bytecode::CodeGenerationErrorOr<void> CallExpression::generate_bytecode(Bytecode
     return {};
 }
 
+static void generate_await(Bytecode::Generator& generator, Bytecode::Register received_completion_register, Bytecode::Register received_completion_type_register, Bytecode::Register received_completion_value_register, Bytecode::IdentifierTableIndex type_identifier, Bytecode::IdentifierTableIndex value_identifier);
+
+// https://tc39.es/ecma262/#sec-return-statement-runtime-semantics-evaluation
 Bytecode::CodeGenerationErrorOr<void> ReturnStatement::generate_bytecode(Bytecode::Generator& generator) const
 {
-    if (m_argument)
+    if (m_argument) {
+        //  ReturnStatement : return Expression ;
+        //     1. Let exprRef be ? Evaluation of Expression.
+        //     2. Let exprValue be ? GetValue(exprRef).
         TRY(m_argument->generate_bytecode(generator));
-    else
+
+        //     3. If GetGeneratorKind() is async, set exprValue to ? Await(exprValue).
+        // Spec Issue?: The spec doesn't seem to do implicit await on explicit return for async functions, but does for
+        //              async generators. However, the major engines do so, and this is observable via constructor lookups
+        //              on Promise objects and custom thenables.
+        //              See: https://tc39.es/ecma262/#sec-asyncblockstart
+        //              c. Assert: If we return here, the async function either threw an exception or performed an implicit or explicit return; all awaiting is done.
+        if (generator.is_in_async_function()) {
+            auto received_completion_register = generator.allocate_register();
+            auto received_completion_type_register = generator.allocate_register();
+            auto received_completion_value_register = generator.allocate_register();
+
+            auto type_identifier = generator.intern_identifier("type");
+            auto value_identifier = generator.intern_identifier("value");
+            generate_await(generator, received_completion_register, received_completion_type_register, received_completion_value_register, type_identifier, value_identifier);
+        }
+
+        //     4. Return Completion Record { [[Type]]: return, [[Value]]: exprValue, [[Target]]: empty }.
+    } else {
+        //  ReturnStatement : return ;
+        //    1. Return Completion Record { [[Type]]: return, [[Value]]: undefined, [[Target]]: empty }.
         generator.emit<Bytecode::Op::LoadImmediate>(js_undefined());
+    }
 
     if (generator.is_in_generator_or_async_function()) {
         generator.perform_needed_unwinds<Bytecode::Op::Yield>();
@@ -1613,8 +1640,6 @@ static void get_received_completion_type_and_value(Bytecode::Generator& generato
     generator.emit<Bytecode::Op::Store>(received_completion_value_register);
 }
 
-static void generate_await(Bytecode::Generator& generator, Bytecode::Register received_completion_register, Bytecode::Register received_completion_type_register, Bytecode::Register received_completion_value_register, Bytecode::IdentifierTableIndex type_identifier, Bytecode::IdentifierTableIndex value_identifier);
-
 enum class AwaitBeforeYield {
     No,
     Yes,

Userland/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.cpp

@@ -9,6 +9,7 @@
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/NativeFunction.h>
 #include <LibJS/Runtime/PromiseCapability.h>
+#include <LibJS/Runtime/PromiseConstructor.h>
 #include <LibJS/Runtime/VM.h>
 
 namespace JS {
@@ -21,7 +22,7 @@ ThrowCompletionOr<Value> AsyncFunctionDriverWrapper::create(Realm& realm, Genera
     auto wrapper = MUST_OR_THROW_OOM(realm.heap().allocate<AsyncFunctionDriverWrapper>(realm, realm, *generator_object, *top_level_promise));
     // Prime the generator:
     // This runs until the first `await value;`
-    wrapper->continue_async_execution(realm.vm(), js_undefined(), true);
+    wrapper->continue_async_execution(realm.vm(), js_undefined(), true, IsInitialExecution::Yes);
 
     return top_level_promise;
 }
@@ -29,39 +30,102 @@ ThrowCompletionOr<Value> AsyncFunctionDriverWrapper::create(Realm& realm, Genera
 AsyncFunctionDriverWrapper::AsyncFunctionDriverWrapper(Realm& realm, NonnullGCPtr<GeneratorObject> generator_object, NonnullGCPtr<Promise> top_level_promise)
     : Promise(realm.intrinsics().promise_prototype())
     , m_generator_object(generator_object)
-    , m_on_fulfillment(*NativeFunction::create(realm, "async.on_fulfillment"sv, [this](VM& vm) -> ThrowCompletionOr<Value> {
-        auto arg = vm.argument(0);
-        if (m_expect_promise) {
-            continue_async_execution(vm, arg, true);
-            m_expect_promise = false;
-            return js_undefined();
-        }
-        return arg;
-    }))
-    , m_on_rejection(*NativeFunction::create(realm, "async.on_rejection"sv, [this](VM& vm) -> ThrowCompletionOr<Value> {
-        auto arg = vm.argument(0);
-        if (m_expect_promise) {
-            continue_async_execution(vm, arg, false);
-            m_expect_promise = false;
-            return js_undefined();
-        }
-        return throw_completion(arg);
-    }))
     , m_top_level_promise(top_level_promise)
     , m_self_handle(make_handle(*this))
 {
 }
 
-void AsyncFunctionDriverWrapper::continue_async_execution(VM& vm, Value value, bool is_successful)
+// 27.7.5.3 Await ( value ), https://tc39.es/ecma262/#await
+ThrowCompletionOr<void> AsyncFunctionDriverWrapper::await(JS::Value value)
+{
+    auto& vm = this->vm();
+    auto& realm = *vm.current_realm();
+
+    // 1. Let asyncContext be the running execution context.
+    m_suspended_execution_context = vm.running_execution_context().copy();
+
+    // 2. Let promise be ? PromiseResolve(%Promise%, value).
+    auto* promise_object = TRY(promise_resolve(vm, realm.intrinsics().promise_constructor(), value));
+
+    // 3. Let fulfilledClosure be a new Abstract Closure with parameters (v) that captures asyncContext and performs the
+    //    following steps when called:
+    auto fulfilled_closure = [this](VM& vm) -> ThrowCompletionOr<Value> {
+        auto value = vm.argument(0);
+
+        // a. Let prevContext be the running execution context.
+        auto& prev_context = vm.running_execution_context();
+
+        // FIXME: b. Suspend prevContext.
+
+        // c. Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
+        TRY(vm.push_execution_context(m_suspended_execution_context.value(), {}));
+
+        // d. Resume the suspended evaluation of asyncContext using NormalCompletion(v) as the result of the operation that
+        //    suspended it.
+        continue_async_execution(vm, value, true);
+
+        // e. Assert: When we reach this step, asyncContext has already been removed from the execution context stack and
+        //    prevContext is the currently running execution context.
+        VERIFY(&vm.running_execution_context() == &prev_context);
+
+        // f. Return undefined.
+        return js_undefined();
+    };
+
+    // 4. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 1, "", « »).
+    auto on_fulfilled = NativeFunction::create(realm, move(fulfilled_closure), 1, "");
+
+    // 5. Let rejectedClosure be a new Abstract Closure with parameters (reason) that captures asyncContext and performs the
+    //    following steps when called:
+    auto rejected_closure = [this](VM& vm) -> ThrowCompletionOr<Value> {
+        auto reason = vm.argument(0);
+
+        // a. Let prevContext be the running execution context.
+        auto& prev_context = vm.running_execution_context();
+
+        // FIXME: b. Suspend prevContext.
+
+        // c. Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
+        TRY(vm.push_execution_context(m_suspended_execution_context.value(), {}));
+
+        // d. Resume the suspended evaluation of asyncContext using ThrowCompletion(reason) as the result of the operation that
+        //    suspended it.
+        continue_async_execution(vm, reason, false);
+
+        // e. Assert: When we reach this step, asyncContext has already been removed from the execution context stack and
+        //    prevContext is the currently running execution context.
+        VERIFY(&vm.running_execution_context() == &prev_context);
+
+        // f. Return undefined.
+        return js_undefined();
+    };
+
+    // 6. Let onRejected be CreateBuiltinFunction(rejectedClosure, 1, "", « »).
+    auto on_rejected = NativeFunction::create(realm, move(rejected_closure), 1, "");
+
+    // 7. Perform PerformPromiseThen(promise, onFulfilled, onRejected).
+    m_current_promise = verify_cast<Promise>(promise_object);
+    m_current_promise->perform_then(on_fulfilled, on_rejected, {});
+
+    // 8. Remove asyncContext from the execution context stack and restore the execution context that is at the top of the
+    //    execution context stack as the running execution context.
+    // NOTE: This is done later on for us in continue_async_execution.
+
+    // NOTE: None of these are necessary. 10-12 are handled by step d of the above lambdas.
+    // 9. Let callerContext be the running execution context.
+    // 10. Resume callerContext passing empty. If asyncContext is ever resumed again, let completion be the Completion Record with which it is resumed.
+    // 11. Assert: If control reaches here, then asyncContext is the running execution context again.
+    // 12. Return completion.
+    return {};
+}
+
+void AsyncFunctionDriverWrapper::continue_async_execution(VM& vm, Value value, bool is_successful, IsInitialExecution is_initial_execution)
 {
     auto generator_result = is_successful
         ? m_generator_object->resume(vm, value, {})
         : m_generator_object->resume_abrupt(vm, throw_completion(value), {});
 
     auto result = [&, this]() -> ThrowCompletionOr<void> {
-        // This loop is for the trivial case of awaiting a non-Promise value,
-        // and pseudo promises, that are actually resolved in a synchronous manner
-        // It's either this, a goto, or a needles indirection
         while (true) {
             if (generator_result.is_throw_completion())
                 return generator_result.throw_completion();
@@ -95,39 +159,12 @@ void AsyncFunctionDriverWrapper::continue_async_execution(VM& vm, Value value, b
                 return {};
             }
 
-            if (!promise_value.is_object() || !is<Promise>(promise_value.as_object())) {
-                // We hit the trivial case of `await value`, where value is not a
-                // Promise, so we can just continue the execution
-                generator_result = m_generator_object->resume(vm, promise_value, {});
-                continue;
-            }
-
             // We hit `await Promise`
-            m_current_promise = static_cast<Promise*>(&promise_value.as_object());
-            // FIXME: We need to be a bit explicit here,
-            //        because for non async promises we arrive late to register us as handlers,
-            //        so we need to just pretend we are early and do the main logic ourselves,
-            //        Boon: This allows us to short-circuit to immediately continuing the execution
-            // FIXME: This then causes a warning to be printed to the console, that we supposedly did not handle the promise
-            if (m_current_promise->state() == Promise::State::Fulfilled) {
-                generator_result = m_generator_object->resume(vm, m_current_promise->result(), {});
-                continue;
-            }
-            if (m_current_promise->state() == Promise::State::Rejected) {
-                generator_result = m_generator_object->resume_abrupt(vm, throw_completion(m_current_promise->result()), {});
+            auto await_result = this->await(promise_value);
+            if (await_result.is_throw_completion()) {
+                generator_result = m_generator_object->resume_abrupt(vm, await_result.release_error(), {});
                 continue;
             }
-            // Due to the nature of promise capabilities we might get called on either one path,
-            // so we use a flag to make sure only accept one call
-            // FIXME: There might be a cleaner way to accomplish this
-            m_expect_promise = true;
-            auto promise_capability = PromiseCapability::create(vm, *m_current_promise,
-                m_on_fulfillment,
-                m_on_rejection);
-            m_current_promise->perform_then(
-                m_on_fulfillment,
-                m_on_rejection,
-                promise_capability);
             return {};
         }
     }();
@@ -138,17 +175,21 @@ void AsyncFunctionDriverWrapper::continue_async_execution(VM& vm, Value value, b
         // We should not execute anymore, so we are safe to allow our selfs to be GC'd
         m_self_handle = {};
     }
+
+    // For the initial execution, the execution context will be popped for us later on by ECMAScriptFunctionObject.
+    if (is_initial_execution == IsInitialExecution::No)
+        vm.pop_execution_context();
 }
 
 void AsyncFunctionDriverWrapper::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
     visitor.visit(m_generator_object);
-    visitor.visit(m_on_fulfillment);
-    visitor.visit(m_on_rejection);
     visitor.visit(m_top_level_promise);
     if (m_current_promise)
         visitor.visit(m_current_promise);
+    if (m_suspended_execution_context.has_value())
+        m_suspended_execution_context->visit_edges(visitor);
 }
 
 }

Userland/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.h

@@ -18,23 +18,27 @@ class AsyncFunctionDriverWrapper final : public Promise {
     JS_OBJECT(AsyncFunctionDriverWrapper, Promise);
 
 public:
+    enum class IsInitialExecution {
+        No,
+        Yes,
+    };
+
     static ThrowCompletionOr<Value> create(Realm&, GeneratorObject*);
 
     virtual ~AsyncFunctionDriverWrapper() override = default;
     void visit_edges(Cell::Visitor&) override;
 
-    void continue_async_execution(VM&, Value, bool is_successful);
+    void continue_async_execution(VM&, Value, bool is_successful, IsInitialExecution is_initial_execution = IsInitialExecution::No);
 
 private:
     AsyncFunctionDriverWrapper(Realm&, NonnullGCPtr<GeneratorObject>, NonnullGCPtr<Promise> top_level_promise);
+    ThrowCompletionOr<void> await(Value);
 
-    bool m_expect_promise { false };
     NonnullGCPtr<GeneratorObject> m_generator_object;
-    NonnullGCPtr<NativeFunction> m_on_fulfillment;
-    NonnullGCPtr<NativeFunction> m_on_rejection;
     NonnullGCPtr<Promise> m_top_level_promise;
     GCPtr<Promise> m_current_promise { nullptr };
     Handle<AsyncFunctionDriverWrapper> m_self_handle;
+    Optional<ExecutionContext> m_suspended_execution_context;
 };
 
 }

Userland/Libraries/LibJS/Tests/builtins/AsyncGenerator/AsyncGenerator.prototype.next.js

@@ -7,7 +7,7 @@ describe("correct behaviour", () => {
         yield b + 1;
         yield Promise.resolve(b + 2);
         yield* [Promise.resolve(b + 3), Promise.resolve(b + 4), Promise.resolve(b + 5)];
-        return b + 6;
+        return Promise.resolve(b + 6);
     }
 
     test("length is 1", () => {
@@ -64,7 +64,7 @@ describe("correct behaviour", () => {
         expect(seventhRunResult).toBe(9);
     });
 
-    test("can return a value", () => {
+    test("can return a value and return implicitly awaits", () => {
         const eighthRunResult = runGenerator("bad7", false);
         expect(eighthRunResult.value).toBe(10);
         expect(eighthRunResult.done).toBeTrue();

Userland/Libraries/LibJS/Tests/syntax/async-await.js

@@ -202,7 +202,7 @@ describe("await cannot be used in class static init blocks", () => {
 });
 
 describe("await thenables", () => {
-    test.xfail("async returning a thanable variable without fulfilling", () => {
+    test("async returning a thanable variable without fulfilling", () => {
         let isCalled = false;
         const obj = {
             then() {
@@ -216,7 +216,7 @@ describe("await thenables", () => {
         expect(isCalled).toBe(true);
     });
 
-    test.xfail("async returning a thanable variable that fulfills", () => {
+    test("async returning a thanable variable that fulfills", () => {
         let isCalled = false;
         const obj = {
             then(fulfill) {
@@ -231,7 +231,7 @@ describe("await thenables", () => {
         expect(isCalled).toBe(true);
     });
 
-    test.xfail("async returning a thenable directly without fulfilling", () => {
+    test("async returning a thenable directly without fulfilling", () => {
         let isCalled = false;
         const f = async () => ({
             then() {
@@ -243,7 +243,7 @@ describe("await thenables", () => {
         expect(isCalled).toBe(true);
     });
 
-    test.xfail("async returning a thenable directly that fulfills", () => {
+    test("async returning a thenable directly that fulfills", () => {
         let isCalled = false;
         const f = async () => ({
             then(fulfill) {
@@ -256,3 +256,39 @@ describe("await thenables", () => {
         expect(isCalled).toBe(true);
     });
 });
+
+describe("await observably looks up constructor of Promise objects", () => {
+    let calls = 0;
+    function makeConstructorObservable(promise) {
+        Object.defineProperty(promise, "constructor", {
+            get() {
+                calls++;
+                return Promise;
+            },
+        });
+        return promise;
+    }
+
+    async function test() {
+        await makeConstructorObservable(Promise.resolve(1));
+        await makeConstructorObservable(
+            new Promise(resolve => {
+                resolve();
+            })
+        );
+        await makeConstructorObservable(new Boolean(true));
+        await makeConstructorObservable({});
+        await makeConstructorObservable(new Number(2));
+        try {
+            await makeConstructorObservable(Promise.reject(3));
+        } catch {}
+        try {
+            return makeConstructorObservable(Promise.reject(1));
+        } catch {
+            return 2;
+        }
+    }
+    test();
+    runQueuedPromiseJobs();
+    expect(calls).toBe(4);
+});