LibJS: Add support for async functions

This commit adds support for the most bare bones version of async
functions, support for async generator functions, async arrow functions
and await expressions are TODO.

Userland/Libraries/LibJS/Runtime/ECMAScriptFunctionObject.cpp

@@ -15,11 +15,14 @@
 #include <LibJS/Runtime/Array.h>
 #include <LibJS/Runtime/ECMAScriptFunctionObject.h>
 #include <LibJS/Runtime/Error.h>
+#include <LibJS/Runtime/ExecutionContext.h>
 #include <LibJS/Runtime/FunctionEnvironment.h>
 #include <LibJS/Runtime/GeneratorObject.h>
 #include <LibJS/Runtime/GeneratorObjectPrototype.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/NativeFunction.h>
+#include <LibJS/Runtime/PromiseConstructor.h>
+#include <LibJS/Runtime/PromiseReaction.h>
 #include <LibJS/Runtime/Value.h>
 
 namespace JS {
@@ -34,6 +37,9 @@ ECMAScriptFunctionObject* ECMAScriptFunctionObject::create(GlobalObject& global_
     case FunctionKind::Generator:
         prototype = global_object.generator_function_prototype();
         break;
+    case FunctionKind::Async:
+        prototype = global_object.async_function_prototype();
+        break;
     }
     return global_object.heap().allocate<ECMAScriptFunctionObject>(global_object, move(name), ecmascript_code, move(parameters), m_function_length, parent_scope, private_scope, *prototype, kind, is_strict, might_need_arguments_object, contains_direct_call_to_eval, is_arrow_function);
 }
@@ -96,6 +102,8 @@ void ECMAScriptFunctionObject::initialize(GlobalObject& global_object)
             // prototype is "g1.prototype" in figure-2 (https://tc39.es/ecma262/img/figure-2.png)
             set_prototype(global_object.generator_object_prototype());
             break;
+        case FunctionKind::Async:
+            break;
         }
         define_direct_property(vm.names.prototype, prototype, Attribute::Writable);
     }
@@ -659,6 +667,84 @@ void ECMAScriptFunctionObject::ordinary_call_bind_this(ExecutionContext& callee_
     MUST(verify_cast<FunctionEnvironment>(local_env)->bind_this_value(global_object(), this_value));
 }
 
+// 27.7.5.1 AsyncFunctionStart ( promiseCapability, asyncFunctionBody ), https://tc39.es/ecma262/#sec-async-functions-abstract-operations-async-function-start
+void ECMAScriptFunctionObject::async_function_start(PromiseCapability const& promise_capability)
+{
+    auto& vm = this->vm();
+
+    // 1. Let runningContext be the running execution context.
+    auto& running_context = vm.running_execution_context();
+
+    // 2. Let asyncContext be a copy of runningContext.
+    auto async_context = running_context.copy();
+
+    // 3. NOTE: Copying the execution state is required for AsyncBlockStart to resume its execution. It is ill-defined to resume a currently executing context.
+
+    // 4. Perform ! AsyncBlockStart(promiseCapability, asyncFunctionBody, asyncContext).
+    async_block_start(promise_capability, async_context);
+}
+
+// 27.7.5.2 AsyncBlockStart ( promiseCapability, asyncBody, asyncContext ), https://tc39.es/ecma262/#sec-asyncblockstart
+void ECMAScriptFunctionObject::async_block_start(PromiseCapability const& promise_capability, ExecutionContext& async_context)
+{
+    auto& vm = this->vm();
+
+    // 1. Assert: promiseCapability is a PromiseCapability Record.
+
+    // 2. Let runningContext be the running execution context.
+    auto& running_context = vm.running_execution_context();
+
+    // 3. Set the code evaluation state of asyncContext such that when evaluation is resumed for that execution context the following steps will be performed:
+    auto* execution_steps = NativeFunction::create(global_object(), "", [async_body = m_ecmascript_code, &promise_capability](auto& vm, auto& global_object) -> ThrowCompletionOr<Value> {
+        // a. Let result be the result of evaluating asyncBody.
+        auto result = async_body->execute(vm.interpreter(), global_object);
+
+        // b. Assert: If we return here, the async function either threw an exception or performed an implicit or explicit return; all awaiting is done.
+
+        // c. 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.
+        vm.pop_execution_context();
+
+        // NOTE: Running the AST node should eventually return a completion.
+        // Until it does, we assume "return" and include the undefined fallback from the call site.
+        // d. If result.[[Type]] is normal, then
+        if (false) {
+            // i. Perform ! Call(promiseCapability.[[Resolve]], undefined, « undefined »).
+            MUST(call(global_object, promise_capability.resolve, js_undefined(), js_undefined()));
+        }
+        // e. Else if result.[[Type]] is return, then
+        else if (!vm.exception()) {
+            // i. Perform ! Call(promiseCapability.[[Resolve]], undefined, « result.[[Value]] »).
+            MUST(call(global_object, promise_capability.resolve, js_undefined(), result.value_or(js_undefined())));
+        }
+        // f. Else,
+        else {
+            // i. Assert: result.[[Type]] is throw.
+
+            // ii. Perform ! Call(promiseCapability.[[Reject]], undefined, « result.[[Value]] »).
+            auto reason = vm.exception()->value();
+            vm.clear_exception();
+            vm.stop_unwind();
+            MUST(call(global_object, promise_capability.reject, js_undefined(), reason));
+        }
+        // g. Return.
+        return js_undefined();
+    });
+
+    // 4. Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
+    vm.push_execution_context(async_context, global_object());
+
+    // 5. Resume the suspended evaluation of asyncContext. Let result be the value returned by the resumed computation.
+    auto result = vm.call(*execution_steps, async_context.this_value.is_empty() ? js_undefined() : async_context.this_value);
+
+    // 6. Assert: When we return here, asyncContext has already been removed from the execution context stack and runningContext is the currently running execution context.
+    VERIFY(&vm.running_execution_context() == &running_context);
+
+    // 7. Assert: result is a normal completion with a value of undefined. The possible sources of completion values are Await or, if the async function doesn't await anything, step 3.g above.
+    VERIFY(result.has_value() && result.value().is_undefined());
+
+    // 8. Return.
+}
+
 // 10.2.1.4 OrdinaryCallEvaluateBody ( F, argumentsList ), https://tc39.es/ecma262/#sec-ordinarycallevaluatebody
 Completion ECMAScriptFunctionObject::ordinary_call_evaluate_body()
 {
@@ -666,6 +752,8 @@ Completion ECMAScriptFunctionObject::ordinary_call_evaluate_body()
     auto* bytecode_interpreter = Bytecode::Interpreter::current();
 
     if (bytecode_interpreter) {
+        if (m_kind == FunctionKind::Async)
+            return vm.throw_completion<InternalError>(global_object(), ErrorType::NotImplemented, "Async function execution in Bytecode interpreter");
         // FIXME: pass something to evaluate default arguments with
         TRY(function_declaration_instantiation(nullptr));
         if (!m_bytecode_executable.has_value()) {
@@ -690,8 +778,8 @@ Completion ECMAScriptFunctionObject::ordinary_call_evaluate_body()
 
         return normal_completion(TRY(GeneratorObject::create(global_object(), result, this, vm.running_execution_context().lexical_environment, bytecode_interpreter->snapshot_frame())));
     } else {
-        if (m_kind != FunctionKind::Regular)
-            return vm.throw_completion<InternalError>(global_object(), ErrorType::NotImplemented, "Non regular function execution in AST interpreter");
+        if (m_kind == FunctionKind::Generator)
+            return vm.throw_completion<InternalError>(global_object(), ErrorType::NotImplemented, "Generator function execution in AST interpreter");
         OwnPtr<Interpreter> local_interpreter;
         Interpreter* ast_interpreter = vm.interpreter_if_exists();
 
@@ -702,14 +790,36 @@ Completion ECMAScriptFunctionObject::ordinary_call_evaluate_body()
 
         VM::InterpreterExecutionScope scope(*ast_interpreter);
 
-        TRY(function_declaration_instantiation(ast_interpreter));
+        if (m_kind == FunctionKind::Regular) {
+            TRY(function_declaration_instantiation(ast_interpreter));
 
-        auto result = m_ecmascript_code->execute(*ast_interpreter, global_object());
-        if (auto* exception = vm.exception())
-            return throw_completion(exception->value());
-        // NOTE: Running the AST node should eventually return a completion.
-        // Until it does, we assume "return" and include the undefined fallback from the call site.
-        return { Completion::Type::Return, result.value_or(js_undefined()), {} };
+            auto result = m_ecmascript_code->execute(*ast_interpreter, global_object());
+            if (auto* exception = vm.exception())
+                return throw_completion(exception->value());
+            // NOTE: Running the AST node should eventually return a completion.
+            // Until it does, we assume "return" and include the undefined fallback from the call site.
+            return { Completion::Type::Return, result.value_or(js_undefined()), {} };
+        } else if (m_kind == FunctionKind::Async) {
+            // 1. Let promiseCapability be ! NewPromiseCapability(%Promise%).
+            auto promise_capability = MUST(new_promise_capability(global_object(), global_object().promise_constructor()));
+
+            // 2. Let declResult be FunctionDeclarationInstantiation(functionObject, argumentsList).
+            auto declaration_result = function_declaration_instantiation(ast_interpreter);
+
+            // 3. If declResult is not an abrupt completion, then
+            if (!declaration_result.is_throw_completion() || !declaration_result.throw_completion().is_abrupt()) {
+                // a. Perform ! AsyncFunctionStart(promiseCapability, FunctionBody).
+                async_function_start(promise_capability);
+            }
+            // 4. Else,
+            else {
+                // a. Perform ! Call(promiseCapability.[[Reject]], undefined, « declResult.[[Value]] »).
+                MUST(call(global_object(), promise_capability.reject, js_undefined(), declaration_result.throw_completion().value()));
+            }
+
+            // 5. Return Completion { [[Type]]: return, [[Value]]: promiseCapability.[[Promise]], [[Target]]: empty }.
+            return Completion { Completion::Type::Return, promise_capability.promise, {} };
+        }
     }
     VERIFY_NOT_REACHED();
 }