LibJS: Perform received abrupt generator completions in the generator

Previously, throw and return completions would not be executed inside
the generator. This is incorrect, as throw and return need to perform
unwinds which can potentially execute more code inside the generator,
such as finally blocks.

This is done by also passing the completion type alongside the passed
in value. The continuation block will immediately extract and type and
value and perform the appropriate operation for the given type.

For normal completions, this is continuing as normal.
For throw completions, it will perform `throw <value>`.
For return completions, it will perform `return <value>`, which is a
`Yield return` in this case due to being inside a generator.

This also refactors GeneratorObject to properly send across the
completion type and value to the generator inside of trying to operate
on the completions itself.

This is a prerequisite for yield*, as it performs special iterator
operations when receiving a throw/return completion and does not
complete the generator like the regular yield would.

There's still more work to be done to make GeneratorObject::execute
be closer to the spec. It's mostly a restructuring of the existing
GeneratorObject::next_impl.

Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -1591,6 +1591,25 @@ Bytecode::CodeGenerationErrorOr<void> YieldExpression::generate_bytecode(Bytecod
 {
     VERIFY(generator.is_in_generator_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");
+
+    auto get_received_completion_type_and_value = [&]() {
+        // The accumulator is set to an object, for example: { "type": 1 (normal), value: 1337 }
+        generator.emit<Bytecode::Op::Store>(received_completion_register);
+
+        generator.emit<Bytecode::Op::GetById>(type_identifier);
+        generator.emit<Bytecode::Op::Store>(received_completion_type_register);
+
+        generator.emit<Bytecode::Op::Load>(received_completion_register);
+        generator.emit<Bytecode::Op::GetById>(value_identifier);
+        generator.emit<Bytecode::Op::Store>(received_completion_value_register);
+    };
+
     if (m_is_yield_from) {
         return Bytecode::CodeGenerationError {
             this,
@@ -1606,6 +1625,41 @@ Bytecode::CodeGenerationErrorOr<void> YieldExpression::generate_bytecode(Bytecod
     auto& continuation_block = generator.make_block();
     generator.emit<Bytecode::Op::Yield>(Bytecode::Label { continuation_block });
     generator.switch_to_basic_block(continuation_block);
+    get_received_completion_type_and_value();
+
+    auto& normal_completion_continuation_block = generator.make_block();
+    auto& throw_completion_continuation_block = generator.make_block();
+
+    generator.emit<Bytecode::Op::LoadImmediate>(Value(to_underlying(Completion::Type::Normal)));
+    generator.emit<Bytecode::Op::StrictlyEquals>(received_completion_type_register);
+    generator.emit<Bytecode::Op::JumpConditional>(
+        Bytecode::Label { normal_completion_continuation_block },
+        Bytecode::Label { throw_completion_continuation_block });
+
+    auto& throw_value_block = generator.make_block();
+    auto& return_value_block = generator.make_block();
+
+    generator.switch_to_basic_block(throw_completion_continuation_block);
+    generator.emit<Bytecode::Op::LoadImmediate>(Value(to_underlying(Completion::Type::Throw)));
+    generator.emit<Bytecode::Op::StrictlyEquals>(received_completion_type_register);
+
+    // If type is not equal to "throw" or "normal", assume it's "return".
+    generator.emit<Bytecode::Op::JumpConditional>(
+        Bytecode::Label { throw_value_block },
+        Bytecode::Label { return_value_block });
+
+    generator.switch_to_basic_block(throw_value_block);
+    generator.emit<Bytecode::Op::Load>(received_completion_value_register);
+    generator.perform_needed_unwinds<Bytecode::Op::Throw>();
+    generator.emit<Bytecode::Op::Throw>();
+
+    generator.switch_to_basic_block(return_value_block);
+    generator.emit<Bytecode::Op::Load>(received_completion_value_register);
+    generator.perform_needed_unwinds<Bytecode::Op::Yield>();
+    generator.emit<Bytecode::Op::Yield>(nullptr);
+
+    generator.switch_to_basic_block(normal_completion_continuation_block);
+    generator.emit<Bytecode::Op::Load>(received_completion_value_register);
     return {};
 }
 

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -29,6 +29,8 @@ CodeGenerationErrorOr<NonnullOwnPtr<Executable>> Generator::generate(ASTNode con
         auto& start_block = generator.make_block();
         generator.emit<Bytecode::Op::Yield>(Label { start_block });
         generator.switch_to_basic_block(start_block);
+        // NOTE: This doesn't have to handle received throw/return completions, as GeneratorObject::resume_abrupt
+        //       will not enter the generator from the SuspendedStart state and immediately completes the generator.
     }
     TRY(node.generate_bytecode(generator));
     if (generator.is_in_generator_or_async_function()) {

Userland/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.cpp

@@ -36,8 +36,8 @@ ThrowCompletionOr<Value> AsyncFunctionDriverWrapper::react_to_async_task_complet
     auto& realm = *vm.current_realm();
 
     auto generator_result = is_successful
-        ? m_generator_object->next_impl(vm, value, {})
-        : m_generator_object->next_impl(vm, {}, value);
+        ? m_generator_object->resume(vm, value, {})
+        : m_generator_object->resume_abrupt(vm, throw_completion(value), {});
 
     if (generator_result.is_throw_completion()) {
         VERIFY(generator_result.throw_completion().type() == Completion::Type::Throw);

Userland/Libraries/LibJS/Runtime/ErrorTypes.h

@@ -36,6 +36,8 @@
     M(DynamicImportNotAllowed, "Dynamic Imports are not allowed")                                                                       \
     M(FinalizationRegistrySameTargetAndValue, "Target and held value must not be the same")                                             \
     M(GetCapabilitiesExecutorCalledMultipleTimes, "GetCapabilitiesExecutor was called multiple times")                                  \
+    M(GeneratorAlreadyExecuting, "Generator is already executing")                                                                      \
+    M(GeneratorBrandMismatch, "Generator brand '{}' does not match generator brand '{}')")                                              \
     M(GlobalEnvironmentAlreadyHasBinding, "Global environment already has binding '{}'")                                                \
     M(IndexOutOfRange, "Index {} is out of range of array length {}")                                                                   \
     M(InOperatorWithObject, "'in' operator must be used on an object")                                                                  \

Userland/Libraries/LibJS/Runtime/GeneratorObject.cpp

@@ -10,6 +10,7 @@
 #include <LibJS/Runtime/GeneratorObject.h>
 #include <LibJS/Runtime/GeneratorPrototype.h>
 #include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/IteratorOperations.h>
 
 namespace JS {
 
@@ -51,67 +52,79 @@ void GeneratorObject::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_previous_value);
 }
 
-ThrowCompletionOr<Value> GeneratorObject::next_impl(VM& vm, Optional<Value> next_argument, Optional<Value> value_to_throw)
+// 27.5.3.2 GeneratorValidate ( generator, generatorBrand ), https://tc39.es/ecma262/#sec-generatorvalidate
+ThrowCompletionOr<GeneratorObject::GeneratorState> GeneratorObject::validate(VM& vm, Optional<String> const& generator_brand)
 {
-    auto& realm = *vm.current_realm();
-    auto bytecode_interpreter = Bytecode::Interpreter::current();
-    VERIFY(bytecode_interpreter);
+    // 1. Perform ? RequireInternalSlot(generator, [[GeneratorState]]).
+    // 2. Perform ? RequireInternalSlot(generator, [[GeneratorBrand]]).
+    // NOTE: Already done by the caller of resume or resume_abrupt, as they wouldn't have a GeneratorObject otherwise.
+
+    // 3. If generator.[[GeneratorBrand]] is not the same value as generatorBrand, throw a TypeError exception.
+    if (m_generator_brand != generator_brand)
+        return vm.throw_completion<TypeError>(ErrorType::GeneratorBrandMismatch, m_generator_brand.value_or("<empty>"), generator_brand.value_or("<empty>"));
+
+    // 4. Assert: generator also has a [[GeneratorContext]] internal slot.
+    // NOTE: Done by already being a GeneratorObject.
+
+    // 5. Let state be generator.[[GeneratorState]].
+    auto state = m_generator_state;
+
+    // 6. If state is executing, throw a TypeError exception.
+    if (state == GeneratorState::Executing)
+        return vm.throw_completion<TypeError>(ErrorType::GeneratorAlreadyExecuting);
+
+    // 7. Return state.
+    return state;
+}
+
+ThrowCompletionOr<Value> GeneratorObject::execute(VM& vm, Completion const& completion)
+{
+    // Loosely based on step 4 of https://tc39.es/ecma262/#sec-generatorstart mixed with https://tc39.es/ecma262/#sec-generatoryield at the end.
+
+    VERIFY(completion.value().has_value());
 
-    auto generated_value = [](Value value) -> ThrowCompletionOr<Value> {
+    auto generated_value = [](Value value) -> Value {
         if (value.is_object())
-            return TRY(value.as_object().get("result"));
+            return value.as_object().get_without_side_effects("result");
         return value.is_empty() ? js_undefined() : value;
     };
 
-    auto generated_continuation = [&](Value value) -> ThrowCompletionOr<Bytecode::BasicBlock const*> {
+    auto generated_continuation = [&](Value value) -> Bytecode::BasicBlock const* {
         if (value.is_object()) {
-            auto number_value = TRY(value.as_object().get("continuation"));
-            return reinterpret_cast<Bytecode::BasicBlock const*>(static_cast<u64>(TRY(number_value.to_double(vm))));
+            auto number_value = value.as_object().get_without_side_effects("continuation");
+            return reinterpret_cast<Bytecode::BasicBlock const*>(static_cast<u64>(number_value.as_double()));
         }
         return nullptr;
     };
 
-    auto previous_generated_value = TRY(generated_value(m_previous_value));
-
-    auto result = Object::create(realm, realm.intrinsics().object_prototype());
-    result->define_direct_property("value", previous_generated_value, default_attributes);
-
-    if (m_done) {
-        result->define_direct_property("done", Value(true), default_attributes);
-        return result;
+    auto& realm = *vm.current_realm();
+    auto* completion_object = Object::create(realm, nullptr);
+    completion_object->define_direct_property(vm.names.type, Value(to_underlying(completion.type())), default_attributes);
+    completion_object->define_direct_property(vm.names.value, completion.value().value(), default_attributes);
+
+    auto* bytecode_interpreter = Bytecode::Interpreter::current();
+    OwnPtr<Bytecode::Interpreter> temp_bc_interpreter;
+    if (!bytecode_interpreter) {
+        temp_bc_interpreter = make<Bytecode::Interpreter>(realm);
+        bytecode_interpreter = temp_bc_interpreter.ptr();
     }
+    VERIFY(bytecode_interpreter);
 
-    // Extract the continuation
-    auto next_block = TRY(generated_continuation(m_previous_value));
+    auto const* next_block = generated_continuation(m_previous_value);
 
-    if (!next_block) {
-        // The generator has terminated, now we can simply return done=true.
-        m_done = true;
-        result->define_direct_property("done", Value(true), default_attributes);
-        return result;
-    }
+    // We should never enter `execute` again after the generator is complete.
+    VERIFY(next_block);
 
-    // Make sure it's an actual block
     VERIFY(!m_generating_function->bytecode_executable()->basic_blocks.find_if([next_block](auto& block) { return block == next_block; }).is_end());
 
-    // Temporarily switch to the captured execution context
-    TRY(vm.push_execution_context(m_execution_context, {}));
-
-    // Pretend that 'yield' returned the passed value, or threw
-    if (value_to_throw.has_value()) {
-        bytecode_interpreter->accumulator() = js_undefined();
-        return throw_completion(value_to_throw.release_value());
-    }
-
     Bytecode::RegisterWindow* frame = nullptr;
     if (m_frame.has_value())
         frame = &m_frame.value();
 
-    auto next_value = next_argument.value_or(js_undefined());
     if (frame)
-        frame->registers[0] = next_value;
+        frame->registers[0] = completion_object;
     else
-        bytecode_interpreter->accumulator() = next_value;
+        bytecode_interpreter->accumulator() = completion_object;
 
     auto next_result = bytecode_interpreter->run_and_return_frame(*m_generating_function->bytecode_executable(), next_block, frame);
 
@@ -120,12 +133,117 @@ ThrowCompletionOr<Value> GeneratorObject::next_impl(VM& vm, Optional<Value> next
     if (!m_frame.has_value())
         m_frame = move(*next_result.frame);
 
-    m_previous_value = TRY(move(next_result.value));
-    m_done = TRY(generated_continuation(m_previous_value)) == nullptr;
+    auto result_value = move(next_result.value);
+    if (result_value.is_throw_completion()) {
+        // Uncaught exceptions disable the generator.
+        m_generator_state = GeneratorState::Completed;
+        return result_value;
+    }
+    m_previous_value = result_value.release_value();
+    bool done = generated_continuation(m_previous_value) == nullptr;
+
+    m_generator_state = done ? GeneratorState::Completed : GeneratorState::SuspendedYield;
+    return create_iterator_result_object(vm, generated_value(m_previous_value), done);
+}
+
+// 27.5.3.3 GeneratorResume ( generator, value, generatorBrand ), https://tc39.es/ecma262/#sec-generatorresume
+ThrowCompletionOr<Value> GeneratorObject::resume(VM& vm, Value value, Optional<String> generator_brand)
+{
+    // 1. Let state be ? GeneratorValidate(generator, generatorBrand).
+    auto state = TRY(validate(vm, generator_brand));
+
+    // 2. If state is completed, return CreateIterResultObject(undefined, true).
+    if (state == GeneratorState::Completed)
+        return create_iterator_result_object(vm, js_undefined(), true);
+
+    // 3. Assert: state is either suspendedStart or suspendedYield.
+    VERIFY(state == GeneratorState::SuspendedStart || state == GeneratorState::SuspendedYield);
+
+    // 4. Let genContext be generator.[[GeneratorContext]].
+    auto& generator_context = m_execution_context;
+
+    // 5. Let methodContext be the running execution context.
+    auto const& method_context = vm.running_execution_context();
+
+    // FIXME: 6. Suspend methodContext.
+
+    // 8. Push genContext onto the execution context stack; genContext is now the running execution context.
+    // NOTE: This is done out of order as to not permanently disable the generator if push_execution_context throws,
+    //       as `resume` will immediately throw when [[GeneratorState]] is "executing", never allowing the state to change.
+    TRY(vm.push_execution_context(generator_context, {}));
+
+    // 7. Set generator.[[GeneratorState]] to executing.
+    m_generator_state = GeneratorState::Executing;
+
+    // 9. Resume the suspended evaluation of genContext using NormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
+    auto result = execute(vm, normal_completion(value));
+
+    // 10. Assert: When we return here, genContext has already been removed from the execution context stack and methodContext is the currently running execution context.
+    VERIFY(&vm.running_execution_context() == &method_context);
+
+    // 11. Return ? result.
+    return result;
+}
+
+// 27.5.3.4 GeneratorResumeAbrupt ( generator, abruptCompletion, generatorBrand ), https://tc39.es/ecma262/#sec-generatorresumeabrupt
+ThrowCompletionOr<Value> GeneratorObject::resume_abrupt(JS::VM& vm, JS::Completion abrupt_completion, Optional<AK::String> generator_brand)
+{
+    // Not part of the spec, but the spec assumes abruptCompletion.[[Value]] is not empty.
+    VERIFY(abrupt_completion.value().has_value());
+
+    // 1. Let state be ? GeneratorValidate(generator, generatorBrand).
+    auto state = TRY(validate(vm, generator_brand));
+
+    // 2. If state is suspendedStart, then
+    if (state == GeneratorState::SuspendedStart) {
+        // a. Set generator.[[GeneratorState]] to completed.
+        m_generator_state = GeneratorState::Completed;
+
+        // b. Once a generator enters the completed state it never leaves it and its associated execution context is never resumed. Any execution state associated with generator can be discarded at this point.
+        // We don't currently discard anything.
+
+        // c. Set state to completed.
+        state = GeneratorState::Completed;
+    }
+
+    // 3. If state is completed, then
+    if (state == GeneratorState::Completed) {
+        // a. If abruptCompletion.[[Type]] is return, then
+        if (abrupt_completion.type() == Completion::Type::Return) {
+            // i. Return CreateIterResultObject(abruptCompletion.[[Value]], true).
+            return create_iterator_result_object(vm, abrupt_completion.value().value(), true);
+        }
+
+        // b. Return ? abruptCompletion.
+        return abrupt_completion;
+    }
+
+    // 4. Assert: state is suspendedYield.
+    VERIFY(state == GeneratorState::SuspendedYield);
+
+    // 5. Let genContext be generator.[[GeneratorContext]].
+    auto& generator_context = m_execution_context;
+
+    // 6. Let methodContext be the running execution context.
+    auto const& method_context = vm.running_execution_context();
+
+    // FIXME: 7. Suspend methodContext.
+
+    // 9. Push genContext onto the execution context stack; genContext is now the running execution context.
+    // NOTE: This is done out of order as to not permanently disable the generator if push_execution_context throws,
+    //       as `resume_abrupt` will immediately throw when [[GeneratorState]] is "executing", never allowing the state to change.
+    TRY(vm.push_execution_context(generator_context, {}));
+
+    // 8. Set generator.[[GeneratorState]] to executing.
+    m_generator_state = GeneratorState::Executing;
+
+    // 10. Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the Completion Record returned by the resumed computation.
+    auto result = execute(vm, abrupt_completion);
 
-    result->define_direct_property("value", TRY(generated_value(m_previous_value)), default_attributes);
-    result->define_direct_property("done", Value(m_done), default_attributes);
+    // 11. Assert: When we return here, genContext has already been removed from the execution context stack and methodContext is the currently running execution context.
+    VERIFY(&vm.running_execution_context() == &method_context);
 
+    // 12. Return ? result.
     return result;
 }
 

Userland/Libraries/LibJS/Runtime/GeneratorObject.h

@@ -21,17 +21,28 @@ public:
     virtual ~GeneratorObject() override = default;
     void visit_edges(Cell::Visitor&) override;
 
-    ThrowCompletionOr<Value> next_impl(VM&, Optional<Value> next_argument, Optional<Value> value_to_throw);
-    void set_done() { m_done = true; }
+    ThrowCompletionOr<Value> resume(VM&, Value value, Optional<String> generator_brand);
+    ThrowCompletionOr<Value> resume_abrupt(VM&, JS::Completion abrupt_completion, Optional<String> generator_brand);
 
 private:
     GeneratorObject(Realm&, Object& prototype, ExecutionContext);
 
+    enum class GeneratorState {
+        SuspendedStart,
+        SuspendedYield,
+        Executing,
+        Completed,
+    };
+
+    ThrowCompletionOr<GeneratorState> validate(VM&, Optional<String> const& generator_brand);
+    ThrowCompletionOr<Value> execute(VM&, JS::Completion const& completion);
+
     ExecutionContext m_execution_context;
     ECMAScriptFunctionObject* m_generating_function { nullptr };
     Value m_previous_value;
     Optional<Bytecode::RegisterWindow> m_frame;
-    bool m_done { false };
+    GeneratorState m_generator_state { GeneratorState::SuspendedStart };
+    Optional<String> m_generator_brand;
 };
 
 }

Userland/Libraries/LibJS/Runtime/GeneratorPrototype.cpp

@@ -30,23 +30,35 @@ void GeneratorPrototype::initialize(Realm& realm)
 // 27.5.1.2 Generator.prototype.next ( value ), https://tc39.es/ecma262/#sec-generator.prototype.next
 JS_DEFINE_NATIVE_FUNCTION(GeneratorPrototype::next)
 {
+    // 1. Return ? GeneratorResume(this value, value, empty).
     auto* generator_object = TRY(typed_this_object(vm));
-    return generator_object->next_impl(vm, vm.argument(0), {});
+    return generator_object->resume(vm, vm.argument(0), {});
 }
 
 // 27.5.1.3 Generator.prototype.next ( value ), https://tc39.es/ecma262/#sec-generator.prototype.return
 JS_DEFINE_NATIVE_FUNCTION(GeneratorPrototype::return_)
 {
+    // 1. Let g be the this value.
     auto* generator_object = TRY(typed_this_object(vm));
-    generator_object->set_done();
-    return generator_object->next_impl(vm, {}, {});
+
+    // 2. Let C be Completion Record { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
+    auto completion = Completion(Completion::Type::Return, vm.argument(0), {});
+
+    // 3. Return ? GeneratorResumeAbrupt(g, C, empty).
+    return generator_object->resume_abrupt(vm, completion, {});
 }
 
 // 27.5.1.4 Generator.prototype.next ( value ), https://tc39.es/ecma262/#sec-generator.prototype.throw
 JS_DEFINE_NATIVE_FUNCTION(GeneratorPrototype::throw_)
 {
+    // 1. Let g be the this value.
     auto* generator_object = TRY(typed_this_object(vm));
-    return generator_object->next_impl(vm, {}, vm.argument(0));
+
+    // 2. Let C be ThrowCompletion(exception).
+    auto completion = throw_completion(vm.argument(0));
+
+    // 3. Return ? GeneratorResumeAbrupt(g, C, empty).
+    return generator_object->resume_abrupt(vm, completion, {});
 }
 
 }