Bug 1519100 - Implement Top-level await specification; r=jonco

Tested in: https://phabricator.services.mozilla.com/D93530

Differential Revision: https://phabricator.services.mozilla.com/D93524

js/src/builtin/Module.js

@@ -586,12 +586,33 @@ function ModuleEvaluate()
         ThrowInternalError(JSMSG_BAD_MODULE_STATUS);
     }
 
+    // Top-level Await Step 4
+    if (module.status === MODULE_STATUS_EVALUATED) {
+      module = GetAsyncCycleRoot(module);
+    }
+
+    // Top-level Await Step 5
+    if (module.topLevelCapability) {
+      return module.topLevelCapability;
+    }
+
+    let capability = CreateTopLevelCapability(module);
+
     // Step 4
     let stack = [];
 
     // Steps 5-6
     try {
         InnerModuleEvaluation(module, stack, 0);
+        if (!module.asyncEvaluating) {
+          ModuleTopLevelCapabilityResolve(module);
+        }
+        // Steps 7-8
+        assert(module.status === MODULE_STATUS_EVALUATED,
+               "Bad module status after successful evaluation");
+        assert(stack.length === 0,
+               "Stack should be empty after successful evaluation");
+
     } catch (error) {
         for (let i = 0; i < stack.length; i++) {
             let m = stack[i];
@@ -607,17 +628,11 @@ function ModuleEvaluate()
         assert(module.status === MODULE_STATUS_EVALUATED_ERROR,
                "Bad module status after failed evaluation");
 
-        throw error;
+        ModuleTopLevelCapabilityReject(module, error);
     }
 
-    // Steps 7-8
-    assert(module.status === MODULE_STATUS_EVALUATED,
-           "Bad module status after successful evaluation");
-    assert(stack.length === 0,
-           "Stack should be empty after successful evaluation");
-
     // Step 9
-    return undefined;
+    return capability;
 }
 
 // https://tc39.es/ecma262/#sec-innermoduleevaluation
@@ -648,6 +663,7 @@ function InnerModuleEvaluation(module, stack, index)
     // Steps 6-8
     UnsafeSetReservedSlot(module, MODULE_OBJECT_DFS_INDEX_SLOT, index);
     UnsafeSetReservedSlot(module, MODULE_OBJECT_DFS_ANCESTOR_INDEX_SLOT, index);
+    UnsafeSetReservedSlot(module, MODULE_OBJECT_PENDING_ASYNC_DEPENDENCIES_SLOT, 0);
     index++;
 
     // Step 9
@@ -677,11 +693,32 @@ function InnerModuleEvaluation(module, stack, index)
             UnsafeSetReservedSlot(module, MODULE_OBJECT_DFS_ANCESTOR_INDEX_SLOT,
                                   std_Math_min(module.dfsAncestorIndex,
                                                requiredModule.dfsAncestorIndex));
+        } else {
+          requiredModule = GetAsyncCycleRoot(requiredModule);
+          assert(requiredModule.status === MODULE_STATUS_EVALUATED,
+                `Bad module status in InnerModuleEvaluation: ${requiredModule.status}`);
+          if (requiredModule.evaluationError) {
+            throw GetModuleEvaluationError(module);
+          }
+        }
+        if (requiredModule.asyncEvaluating) {
+            UnsafeSetReservedSlot(module,
+                                  MODULE_OBJECT_PENDING_ASYNC_DEPENDENCIES_SLOT,
+                                  module.pendingAsyncDependencies + 1);
+            AppendAsyncParentModule(requiredModule, module);
         }
     }
 
-    // Step 11
-    ExecuteModule(module);
+    if (module.pendingAsyncDependencies > 0) {
+      UnsafeSetReservedSlot(module, MODULE_OBJECT_ASYNC_EVALUATING_SLOT, true);
+    } else {
+      if (module.async) {
+        ExecuteAsyncModule(module);
+      } else {
+        // Step 11
+        ExecuteModule(module);
+      }
+    }
 
     // Step 12
     assert(CountArrayValues(stack, module) === 1,
@@ -703,3 +740,17 @@ function InnerModuleEvaluation(module, stack, index)
     // Step 15
     return index;
 }
+
+// https://tc39.es/proposal-top-level-await/#sec-execute-async-module
+function ExecuteAsyncModule(module) {
+  // Steps 1-3.
+  assert(module.status == MODULE_STATUS_EVALUATING ||
+         module.status == MODULE_STATUS_EVALUATED, "bad status for async module");
+  assert(module.async, "module is not async");
+  UnsafeSetReservedSlot(module, MODULE_OBJECT_ASYNC_EVALUATING_SLOT, true);
+
+  // Step 4-11 done in AsyncAwait opcode
+
+  ExecuteModule(module);
+}
+

js/src/builtin/ModuleObject.cpp

@@ -31,6 +31,7 @@
 
 #include "vm/JSObject-inl.h"
 #include "vm/JSScript-inl.h"
+#include "vm/List-inl.h"
 #include "vm/NativeObject-inl.h"
 
 using namespace js;
@@ -835,8 +836,13 @@ void ModuleObject::initFunctionDeclarations(
   *functionDeclarations() = std::move(decls);
 }
 
-void ModuleObject::initAsyncSlots(JSContext* cx, bool isAsync) {
+bool ModuleObject::initAsyncSlots(JSContext* cx, bool isAsync,
+                                  HandleObject asyncParentModulesList) {
   initReservedSlot(AsyncSlot, BooleanValue(isAsync));
+  initReservedSlot(AsyncEvaluatingSlot, BooleanValue(false));
+  initReservedSlot(AsyncParentModulesSlot,
+                   ObjectValue(*asyncParentModulesList));
+  return true;
 }
 
 void ModuleObject::initScriptSlots(HandleScript script) {
@@ -967,10 +973,73 @@ bool ModuleObject::isAsync() const {
   return getReservedSlot(AsyncSlot).toBoolean();
 }
 
+bool ModuleObject::isAsyncEvaluating() const {
+  return getReservedSlot(AsyncEvaluatingSlot).toBoolean();
+}
+
+void ModuleObject::setAsyncEvaluating(bool isEvaluating) {
+  return setReservedSlot(AsyncEvaluatingSlot, BooleanValue(isEvaluating));
+}
+
+uint32_t ModuleObject::dfsIndex() const {
+  return getReservedSlot(DFSIndexSlot).toInt32();
+}
+
+uint32_t ModuleObject::dfsAncestorIndex() const {
+  return getReservedSlot(DFSAncestorIndexSlot).toInt32();
+}
+
+JSObject* ModuleObject::topLevelCapability() const {
+  return &getReservedSlot(TopLevelCapabilitySlot).toObject();
+}
+
+PromiseObject* ModuleObject::createTopLevelCapability(
+    JSContext* cx, HandleModuleObject module) {
+  MOZ_ASSERT(module->getReservedSlot(TopLevelCapabilitySlot).isUndefined());
+  Rooted<PromiseObject*> resultPromise(cx, CreatePromiseObjectForAsync(cx));
+  if (!resultPromise) {
+    return nullptr;
+  }
+  module->setInitialTopLevelCapability(resultPromise);
+  return resultPromise;
+}
+
+void ModuleObject::setInitialTopLevelCapability(HandleObject promiseObj) {
+  initReservedSlot(TopLevelCapabilitySlot, ObjectValue(*promiseObj));
+}
+
+inline ListObject* ModuleObject::asyncParentModules() const {
+  return &getReservedSlot(AsyncParentModulesSlot).toObject().as<ListObject>();
+}
+
+bool ModuleObject::appendAsyncParentModule(JSContext* cx,
+                                           HandleModuleObject self,
+                                           HandleModuleObject parent) {
+  Rooted<Value> parentValue(cx, ObjectValue(*parent));
+  return self->asyncParentModules()->append(cx, parentValue);
+}
+
+uint32_t ModuleObject::pendingAsyncDependencies() const {
+  return getReservedSlot(PendingAsyncDependenciesSlot).toInt32();
+}
+
+void ModuleObject::setPendingAsyncDependencies(uint32_t newValue) {
+  return setReservedSlot(PendingAsyncDependenciesSlot, NumberValue(newValue));
+}
+
+bool ModuleObject::hasTopLevelCapability() const {
+  return !getReservedSlot(TopLevelCapabilitySlot).isUndefined();
+}
+
 bool ModuleObject::hadEvaluationError() const {
   return status() == MODULE_STATUS_EVALUATED_ERROR;
 }
 
+void ModuleObject::setEvaluationError(HandleValue newValue) {
+  setReservedSlot(StatusSlot, Int32Value(MODULE_STATUS_EVALUATED_ERROR));
+  return setReservedSlot(EvaluationErrorSlot, newValue);
+}
+
 Value ModuleObject::evaluationError() const {
   MOZ_ASSERT(hadEvaluationError());
   return getReservedSlot(EvaluationErrorSlot);
@@ -1053,7 +1122,8 @@ bool ModuleObject::instantiateFunctionDeclarations(JSContext* cx,
 bool ModuleObject::execute(JSContext* cx, HandleModuleObject self,
                            MutableHandleValue rval) {
 #ifdef DEBUG
-  MOZ_ASSERT(self->status() == MODULE_STATUS_EVALUATING);
+  MOZ_ASSERT(self->status() == MODULE_STATUS_EVALUATING ||
+             self->status() == MODULE_STATUS_EVALUATED);
   if (!AssertFrozen(cx, self)) {
     return false;
   }
@@ -1158,6 +1228,13 @@ DEFINE_GETTER_FUNCTIONS(ModuleObject, starExportEntries, StarExportEntriesSlot)
 DEFINE_GETTER_FUNCTIONS(ModuleObject, dfsIndex, DFSIndexSlot)
 DEFINE_GETTER_FUNCTIONS(ModuleObject, dfsAncestorIndex, DFSAncestorIndexSlot)
 DEFINE_GETTER_FUNCTIONS(ModuleObject, async, AsyncSlot)
+DEFINE_GETTER_FUNCTIONS(ModuleObject, topLevelCapability,
+                        TopLevelCapabilitySlot)
+DEFINE_GETTER_FUNCTIONS(ModuleObject, asyncEvaluating, AsyncEvaluatingSlot)
+DEFINE_GETTER_FUNCTIONS(ModuleObject, asyncParentModules,
+                        AsyncParentModulesSlot)
+DEFINE_GETTER_FUNCTIONS(ModuleObject, pendingAsyncDependencies,
+                        PendingAsyncDependenciesSlot)
 
 /* static */
 bool GlobalObject::initModuleProto(JSContext* cx,
@@ -1175,6 +1252,11 @@ bool GlobalObject::initModuleProto(JSContext* cx,
       JS_PSG("dfsIndex", ModuleObject_dfsIndexGetter, 0),
       JS_PSG("dfsAncestorIndex", ModuleObject_dfsAncestorIndexGetter, 0),
       JS_PSG("async", ModuleObject_asyncGetter, 0),
+      JS_PSG("topLevelCapability", ModuleObject_topLevelCapabilityGetter, 0),
+      JS_PSG("asyncEvaluating", ModuleObject_asyncEvaluatingGetter, 0),
+      JS_PSG("asyncParentModules", ModuleObject_asyncParentModulesGetter, 0),
+      JS_PSG("pendingAsyncDependencies",
+             ModuleObject_pendingAsyncDependenciesGetter, 0),
       JS_PS_END};
 
   static const JSFunctionSpec protoFunctions[] = {
@@ -1420,6 +1502,15 @@ bool frontend::StencilModuleMetadata::initModule(
   }
   module->initFunctionDeclarations(std::move(functionDeclsCopy));
 
+  Rooted<ListObject*> asyncParentModulesList(cx, ListObject::create(cx));
+  if (!asyncParentModulesList) {
+    return false;
+  }
+
+  if (!module->initAsyncSlots(cx, isAsync, asyncParentModulesList)) {
+    return false;
+  }
+
   module->initImportExportData(
       requestedModulesObject, importEntriesObject, localExportEntriesObject,
       indirectExportEntriesObject, starExportEntriesObject);
@@ -1845,6 +1936,193 @@ JSObject* js::CallModuleResolveHook(JSContext* cx,
   return result;
 }
 
+// https://tc39.es/proposal-top-level-await/#sec-getasynccycleroot
+ModuleObject* js::GetAsyncCycleRoot(ModuleObject* module) {
+  // Step 1.
+  MOZ_ASSERT(module->status() == MODULE_STATUS_EVALUATED);
+
+  // Step 2.
+  if (module->asyncParentModules()->empty()) {
+    return module;
+  }
+
+  // Step 3.
+  ModuleObject* currentModule = module;
+  while (currentModule->dfsIndex() > currentModule->dfsAncestorIndex()) {
+    MOZ_ASSERT(!currentModule->asyncParentModules()->empty());
+    ModuleObject* nextCycleModule = &currentModule->asyncParentModules()
+                                         ->get(0)
+                                         .toObject()
+                                         .as<ModuleObject>();
+    MOZ_ASSERT(nextCycleModule->dfsAncestorIndex() <=
+               currentModule->dfsAncestorIndex());
+    currentModule = nextCycleModule;
+  }
+
+  // Step 4.
+  MOZ_ASSERT(currentModule->dfsIndex() == currentModule->dfsAncestorIndex());
+
+  // Step 5.
+  return currentModule;
+}
+
+bool js::AsyncModuleExecutionFulfilledHandler(JSContext* cx, unsigned argc,
+                                              Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  JSFunction& func = args.callee().as<JSFunction>();
+
+  Rooted<ModuleObject*> module(
+      cx, &func.getExtendedSlot(FunctionExtended::MODULE_SLOT)
+               .toObject()
+               .as<ModuleObject>());
+  AsyncModuleExecutionFulfilled(cx, module);
+  args.rval().setUndefined();
+  return true;
+}
+
+bool js::AsyncModuleExecutionRejectedHandler(JSContext* cx, unsigned argc,
+                                             Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  JSFunction& func = args.callee().as<JSFunction>();
+  Rooted<ModuleObject*> module(
+      cx, &func.getExtendedSlot(FunctionExtended::MODULE_SLOT)
+               .toObject()
+               .as<ModuleObject>());
+  AsyncModuleExecutionRejected(cx, module, args.get(0));
+  args.rval().setUndefined();
+  return true;
+}
+
+// Top Level Await
+// https://tc39.es/proposal-top-level-await/#sec-asyncmodulexecutionfulfilled
+void js::AsyncModuleExecutionFulfilled(JSContext* cx,
+                                       HandleModuleObject module) {
+  // Step 1.
+  MOZ_ASSERT(module->status() == MODULE_STATUS_EVALUATED);
+
+  // Step 2.
+  if (!module->isAsyncEvaluating()) {
+    MOZ_ASSERT(module->hadEvaluationError());
+    return;
+  }
+
+  // Step 3.
+  MOZ_ASSERT(!module->hadEvaluationError());
+
+  // Step 4.
+  module->setAsyncEvaluating(false);
+
+  // Step 5.
+  uint32_t length = module->asyncParentModules()->length();
+  Rooted<ModuleObject*> m(cx);
+  Rooted<ModuleObject*> cycleRoot(cx);
+  RootedValue result(cx);
+  for (uint32_t i = 0; i < length; i++) {
+    m = &module->asyncParentModules()->get(i).toObject().as<ModuleObject>();
+
+    if (module->dfsIndex() != module->dfsAncestorIndex()) {
+      MOZ_ASSERT(m->dfsAncestorIndex() <= module->dfsAncestorIndex());
+    }
+
+    m->setPendingAsyncDependencies(m->pendingAsyncDependencies() - 1);
+
+    if (m->pendingAsyncDependencies() == 0 && !m->hadEvaluationError()) {
+      MOZ_ASSERT(m->isAsyncEvaluating());
+
+      cycleRoot = GetAsyncCycleRoot(m);
+
+      if (cycleRoot->hadEvaluationError()) {
+        return;
+      }
+
+      if (m->isAsync()) {
+        // Steps for ExecuteAsyncModule
+        MOZ_ASSERT(m->status() == MODULE_STATUS_EVALUATING ||
+                   m->status() == MODULE_STATUS_EVALUATED);
+        MOZ_ASSERT(m->isAsync());
+        MOZ_ASSERT(m->isAsyncEvaluating());
+        ModuleObject::execute(cx, m, &result);
+      } else {
+        if (ModuleObject::execute(cx, m, &result)) {
+          AsyncModuleExecutionFulfilled(cx, m);
+        } else {
+          AsyncModuleExecutionRejected(cx, m, result);
+        }
+      }
+    }
+  }
+
+  // Step 6.
+  if (module->hasTopLevelCapability()) {
+    MOZ_ASSERT(module->dfsIndex() == module->dfsAncestorIndex());
+    ModuleObject::topLevelCapabilityResolve(cx, module);
+  }
+
+  // Return undefined.
+}
+
+// https://tc39.es/proposal-top-level-await/#sec-asyncmodulexecutionrejected
+void js::AsyncModuleExecutionRejected(JSContext* cx, HandleModuleObject module,
+                                      HandleValue error) {
+  // Step 1.
+  MOZ_ASSERT(module->status() == MODULE_STATUS_EVALUATED);
+
+  // Step 2.
+  if (!module->isAsyncEvaluating()) {
+    MOZ_ASSERT(module->hadEvaluationError());
+    return;
+  }
+
+  // Step 3.
+  MOZ_ASSERT(!module->hadEvaluationError());
+
+  // Step 4.
+  module->setEvaluationError(error);
+
+  // Step 5.
+  module->setAsyncEvaluating(false);
+
+  // Step 6.
+  uint32_t length = module->asyncParentModules()->length();
+  Rooted<ModuleObject*> parent(cx);
+  for (uint32_t i = 0; i < length; i++) {
+    parent =
+        &module->asyncParentModules()->get(i).toObject().as<ModuleObject>();
+    if (module->dfsIndex() != module->dfsAncestorIndex()) {
+      MOZ_ASSERT(parent->dfsAncestorIndex() == module->dfsAncestorIndex());
+    }
+    AsyncModuleExecutionRejected(cx, parent, error);
+  }
+
+  // Step 7.
+  if (module->hasTopLevelCapability()) {
+    MOZ_ASSERT(module->dfsIndex() == module->dfsAncestorIndex());
+    ModuleObject::topLevelCapabilityReject(cx, module, error);
+  }
+
+  // Return undefined.
+}
+
+bool ModuleObject::topLevelCapabilityResolve(JSContext* cx,
+                                             HandleModuleObject module) {
+  RootedValue rval(cx);
+  Rooted<PromiseObject*> promise(
+      cx, &module->topLevelCapability()->as<PromiseObject>());
+  return AsyncFunctionReturned(cx, promise, rval);
+}
+
+bool ModuleObject::topLevelCapabilityReject(JSContext* cx,
+                                            HandleModuleObject module,
+                                            HandleValue error) {
+  Rooted<PromiseObject*> promise(
+      cx, &module->topLevelCapability()->as<PromiseObject>());
+  if (!AsyncFunctionThrown(cx, promise, error)) {
+    return false;
+  }
+  MOZ_ASSERT(promise->state() == JS::PromiseState::Rejected);
+  return true;
+}
+
 JSObject* js::StartDynamicModuleImport(JSContext* cx, HandleScript script,
                                        HandleValue specifierArg) {
   RootedObject promiseConstructor(cx, JS::GetPromiseConstructor(cx));
@@ -2230,7 +2508,12 @@ XDRResult js::XDRModuleObject(XDRState<mode>* xdr,
   MOZ_TRY(xdr->codeUint8(&isAsyncModule));
 
   if (mode == XDR_DECODE) {
-    module->initAsyncSlots(cx, isAsyncModule == 1);
+    Rooted<ListObject*> asyncParentModulesList(cx, ListObject::create(cx));
+    if (!asyncParentModulesList) {
+      return xdr->fail(JS::TranscodeResult_Throw);
+    }
+
+    module->initAsyncSlots(cx, isAsyncModule == 1, asyncParentModulesList);
   }
 
   modp.set(module);

js/src/builtin/ModuleObject.h

@@ -19,7 +19,9 @@
 #include "js/Modules.h"
 #include "js/UniquePtr.h"
 #include "vm/JSAtom.h"
+#include "vm/List.h"
 #include "vm/NativeObject.h"
+#include "vm/PromiseObject.h"  // js::PromiseObject
 #include "vm/ProxyObject.h"
 
 namespace js {
@@ -249,6 +251,10 @@ class ModuleObject : public NativeObject {
     DFSIndexSlot,
     DFSAncestorIndexSlot,
     AsyncSlot,
+    AsyncEvaluatingSlot,
+    TopLevelCapabilitySlot,
+    AsyncParentModulesSlot,
+    PendingAsyncDependenciesSlot,
     SlotCount
   };
 
@@ -262,6 +268,14 @@ class ModuleObject : public NativeObject {
                 "DFSIndexSlot must match self-hosting define");
   static_assert(DFSAncestorIndexSlot == MODULE_OBJECT_DFS_ANCESTOR_INDEX_SLOT,
                 "DFSAncestorIndexSlot must match self-hosting define");
+  static_assert(AsyncEvaluatingSlot == MODULE_OBJECT_ASYNC_EVALUATING_SLOT,
+                "AsyncEvaluatingSlot must match self-hosting define");
+  static_assert(TopLevelCapabilitySlot ==
+                    MODULE_OBJECT_TOP_LEVEL_CAPABILITY_SLOT,
+                "topLevelCapabilitySlot must match self-hosting define");
+  static_assert(PendingAsyncDependenciesSlot ==
+                    MODULE_OBJECT_PENDING_ASYNC_DEPENDENCIES_SLOT,
+                "PendingAsyncDependenciesSlot must match self-hosting define");
 
   static const JSClass class_;
 
@@ -294,7 +308,8 @@ class ModuleObject : public NativeObject {
   ModuleEnvironmentObject* environment() const;
   ModuleNamespaceObject* namespace_();
   ModuleStatus status() const;
-  bool isAsync() const;
+  uint32_t dfsIndex() const;
+  uint32_t dfsAncestorIndex() const;
   bool hadEvaluationError() const;
   Value evaluationError() const;
   JSObject* metaObject() const;
@@ -306,6 +321,28 @@ class ModuleObject : public NativeObject {
   ArrayObject& starExportEntries() const;
   IndirectBindingMap& importBindings();
 
+  static PromiseObject* createTopLevelCapability(JSContext* cx,
+                                                 HandleModuleObject module);
+  bool isAsync() const;
+  void setAsync(bool isAsync);
+  bool isAsyncEvaluating() const;
+  void setAsyncEvaluating(bool isEvaluating);
+  void setEvaluationError(HandleValue newValue);
+  void setPendingAsyncDependencies(uint32_t newValue);
+  void setInitialTopLevelCapability(HandleObject promiseObj);
+  bool hasTopLevelCapability() const;
+  JSObject* topLevelCapability() const;
+  ListObject* asyncParentModules() const;
+  uint32_t pendingAsyncDependencies() const;
+
+  static bool appendAsyncParentModule(JSContext* cx, HandleModuleObject self,
+                                      HandleModuleObject parent);
+
+  static bool topLevelCapabilityResolve(JSContext* cx,
+                                        HandleModuleObject module);
+  static bool topLevelCapabilityReject(JSContext* cx, HandleModuleObject module,
+                                       HandleValue error);
+
   static bool Instantiate(JSContext* cx, HandleModuleObject self);
   static bool Evaluate(JSContext* cx, HandleModuleObject self);
 
@@ -332,7 +369,8 @@ class ModuleObject : public NativeObject {
   frontend::FunctionDeclarationVector* functionDeclarations();
   void initFunctionDeclarations(frontend::FunctionDeclarationVector&& decls);
 
-  void initAsyncSlots(JSContext* cx, bool isAsync);
+  bool initAsyncSlots(JSContext* cx, bool isAsync,
+                      HandleObject asyncParentModulesList);
 
  private:
   static const JSClassOps classOps_;
@@ -348,6 +386,24 @@ JSObject* GetOrCreateModuleMetaObject(JSContext* cx, HandleObject module);
 JSObject* CallModuleResolveHook(JSContext* cx, HandleValue referencingPrivate,
                                 HandleString specifier);
 
+// https://tc39.es/proposal-top-level-await/#sec-getasynccycleroot
+ModuleObject* GetAsyncCycleRoot(ModuleObject* module);
+
+// https://tc39.es/proposal-top-level-await/#sec-asyncmodulexecutionfulfilled
+void AsyncModuleExecutionFulfilled(JSContext* cx, HandleModuleObject module);
+
+// https://tc39.es/proposal-top-level-await/#sec-asyncmodulexecutionrejected
+void AsyncModuleExecutionRejected(JSContext* cx, HandleModuleObject module,
+                                  HandleValue error);
+
+// https://tc39.es/proposal-top-level-await/#sec-asyncmodulexecutionfulfilled
+bool AsyncModuleExecutionFulfilledHandler(JSContext* cx, unsigned argc,
+                                          Value* vp);
+
+// https://tc39.es/proposal-top-level-await/#sec-asyncmodulexecutionrejected
+bool AsyncModuleExecutionRejectedHandler(JSContext* cx, unsigned argc,
+                                         Value* vp);
+
 JSObject* StartDynamicModuleImport(JSContext* cx, HandleScript script,
                                    HandleValue specifier);
 

js/src/builtin/Promise.cpp

@@ -5702,7 +5702,13 @@ static MOZ_MUST_USE bool IsTopMostAsyncFunctionCall(JSContext* cx) {
   if (iter.done()) {
     return false;
   }
-  MOZ_ASSERT(iter.isFunctionFrame());
+
+  if (!iter.isFunctionFrame() && iter.isModuleFrame()) {
+    // The iterator is not a function frame, it is a module frame.
+    // Ignore this optimization for now.
+    return true;
+  }
+
   MOZ_ASSERT(iter.calleeTemplate()->isAsync());
 
 #ifdef DEBUG

js/src/builtin/SelfHostingDefines.h

@@ -110,6 +110,9 @@
 #define MODULE_OBJECT_EVALUATION_ERROR_SLOT 4
 #define MODULE_OBJECT_DFS_INDEX_SLOT 14
 #define MODULE_OBJECT_DFS_ANCESTOR_INDEX_SLOT 15
+#define MODULE_OBJECT_ASYNC_EVALUATING_SLOT 17
+#define MODULE_OBJECT_TOP_LEVEL_CAPABILITY_SLOT 18
+#define MODULE_OBJECT_PENDING_ASYNC_DEPENDENCIES_SLOT 20
 
 // rev b012019fea18f29737a67c36911340a3e25bfc63
 // 15.2.1.16 Cyclic Module Records

js/src/frontend/BytecodeEmitter.cpp

@@ -2909,8 +2909,11 @@ bool BytecodeEmitter::emitIteratorNext(
     const Maybe<uint32_t>& callSourceCoordOffset,
     IteratorKind iterKind /* = IteratorKind::Sync */,
     bool allowSelfHosted /* = false */) {
-  MOZ_ASSERT(allowSelfHosted || emitterMode != BytecodeEmitter::SelfHosting,
-             ".next() iteration is prohibited in self-hosted code because it "
+  // TODO: migrate Module code to cpp, to avoid having the extra check here.
+  MOZ_ASSERT(allowSelfHosted || emitterMode != BytecodeEmitter::SelfHosting ||
+                 (sc->isModuleContext() && sc->asModuleContext()->isAsync()),
+             ".next() iteration is prohibited in non-module self-hosted code "
+             "because it"
              "can run user-modifiable iteration code");
 
   //                [stack] ... NEXT ITER

js/src/vm/AsyncFunction.cpp

@@ -225,6 +225,22 @@ AsyncFunctionGeneratorObject* AsyncFunctionGeneratorObject::create(
   return obj;
 }
 
+JSFunction* NewHandler(JSContext* cx, Native handler,
+                       JS::Handle<JSObject*> target) {
+  cx->check(target);
+
+  JS::Handle<PropertyName*> funName = cx->names().empty;
+  JS::Rooted<JSFunction*> handlerFun(
+      cx, NewNativeFunction(cx, handler, 0, funName,
+                            gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
+  if (!handlerFun) {
+    return nullptr;
+  }
+  handlerFun->setExtendedSlot(FunctionExtended::MODULE_SLOT,
+                              JS::ObjectValue(*target));
+  return handlerFun;
+}
+
 AsyncFunctionGeneratorObject* AsyncFunctionGeneratorObject::create(
     JSContext* cx, HandleModuleObject module) {
   // TODO: Module is currently hitching a ride with
@@ -247,6 +263,23 @@ AsyncFunctionGeneratorObject* AsyncFunctionGeneratorObject::create(
   }
   obj->initFixedSlot(PROMISE_SLOT, ObjectValue(*resultPromise));
 
+  RootedObject onFulfilled(
+      cx, NewHandler(cx, AsyncModuleExecutionFulfilledHandler, module));
+  if (!onFulfilled) {
+    return nullptr;
+  }
+
+  RootedObject onRejected(
+      cx, NewHandler(cx, AsyncModuleExecutionRejectedHandler, module));
+  if (!onRejected) {
+    return nullptr;
+  }
+
+  if (!JS::AddPromiseReactionsIgnoringUnhandledRejection(
+          cx, resultPromise, onFulfilled, onRejected)) {
+    return nullptr;
+  }
+
   // Starts in the running state.
   obj->setResumeIndex(AbstractGeneratorObject::RESUME_INDEX_RUNNING);
 

js/src/vm/JSFunction.h

@@ -826,6 +826,9 @@ class FunctionExtended : public JSFunction {
   // asm.js module functions store their WasmModuleObject in the first slot.
   static const unsigned ASMJS_MODULE_SLOT = 0;
 
+  // Async module callback handlers store their ModuleObject in the first slot.
+  static const unsigned MODULE_SLOT = 0;
+
   static inline size_t offsetOfExtendedSlot(unsigned which) {
     MOZ_ASSERT(which < NUM_EXTENDED_SLOTS);
     return offsetof(FunctionExtended, extendedSlots) +

js/src/vm/SelfHosting.cpp

@@ -1998,6 +1998,63 @@ static bool intrinsic_ExecuteModule(JSContext* cx, unsigned argc, Value* vp) {
   return ModuleObject::execute(cx, module, args.rval());
 }
 
+static bool intrinsic_GetAsyncCycleRoot(JSContext* cx, unsigned argc,
+                                        Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  RootedModuleObject module(cx, &args[0].toObject().as<ModuleObject>());
+  JSObject* result = js::GetAsyncCycleRoot(module);
+  if (!result) {
+    return false;
+  }
+  args.rval().setObject(*result);
+  return true;
+}
+
+static bool intrinsic_AppendAsyncParentModule(JSContext* cx, unsigned argc,
+                                              Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 2);
+  RootedModuleObject self(cx, &args[0].toObject().as<ModuleObject>());
+  RootedModuleObject parent(cx, &args[1].toObject().as<ModuleObject>());
+  return ModuleObject::appendAsyncParentModule(cx, self, parent);
+}
+
+static bool intrinsic_CreateTopLevelCapability(JSContext* cx, unsigned argc,
+                                               Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  RootedModuleObject self(cx, &args[0].toObject().as<ModuleObject>());
+  PromiseObject* result = ModuleObject::createTopLevelCapability(cx, self);
+  if (!result) {
+    return false;
+  }
+  args.rval().setObject(*result);
+  return true;
+}
+
+static bool intrinsic_ModuleTopLevelCapabilityResolve(JSContext* cx,
+                                                      unsigned argc,
+                                                      Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 1);
+  RootedModuleObject module(cx, &args[0].toObject().as<ModuleObject>());
+  ModuleObject::topLevelCapabilityResolve(cx, module);
+  args.rval().setUndefined();
+  return true;
+}
+
+static bool intrinsic_ModuleTopLevelCapabilityReject(JSContext* cx,
+                                                     unsigned argc, Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+  MOZ_ASSERT(args.length() == 2);
+  RootedModuleObject module(cx, &args[0].toObject().as<ModuleObject>());
+  HandleValue error = args[1];
+  ModuleObject::topLevelCapabilityReject(cx, module, error);
+  args.rval().setUndefined();
+  return true;
+}
+
 static bool intrinsic_NewModuleNamespace(JSContext* cx, unsigned argc,
                                          Value* vp) {
   CallArgs args = CallArgsFromVp(argc, vp);
@@ -2582,6 +2639,13 @@ static const JSFunctionSpec intrinsic_functions[] = {
     JS_FN("InstantiateModuleFunctionDeclarations",
           intrinsic_InstantiateModuleFunctionDeclarations, 1, 0),
     JS_FN("ExecuteModule", intrinsic_ExecuteModule, 1, 0),
+    JS_FN("GetAsyncCycleRoot", intrinsic_GetAsyncCycleRoot, 1, 0),
+    JS_FN("AppendAsyncParentModule", intrinsic_AppendAsyncParentModule, 2, 0),
+    JS_FN("CreateTopLevelCapability", intrinsic_CreateTopLevelCapability, 1, 0),
+    JS_FN("ModuleTopLevelCapabilityResolve",
+          intrinsic_ModuleTopLevelCapabilityResolve, 1, 0),
+    JS_FN("ModuleTopLevelCapabilityReject",
+          intrinsic_ModuleTopLevelCapabilityReject, 2, 0),
     JS_FN("NewModuleNamespace", intrinsic_NewModuleNamespace, 2, 0),
     JS_FN("AddModuleNamespaceBinding", intrinsic_AddModuleNamespaceBinding, 4,
           0),