1 files changed, 847 insertions, 0 deletions
diff --git a/js/src/builtin/FinalizationRegistryObject.cpp b/js/src/builtin/FinalizationRegistryObject.cpp
new file mode 100644
index 0000000000..194b858ea9
--- /dev/null
+++ b/js/src/builtin/FinalizationRegistryObject.cpp
@@ -0,0 +1,847 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Implementation of JS FinalizationRegistry objects.
+
+#include "builtin/FinalizationRegistryObject.h"
+
+#include "mozilla/ScopeExit.h"
+
+#include "jsapi.h"
+
+#include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
+#include "vm/GlobalObject.h"
+#include "vm/Interpreter.h"
+
+#include "gc/GCContext-inl.h"
+#include "vm/JSObject-inl.h"
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+
+///////////////////////////////////////////////////////////////////////////
+// FinalizationRecordObject
+
+const JSClass FinalizationRecordObject::class_ = {
+    "FinalizationRecord", JSCLASS_HAS_RESERVED_SLOTS(SlotCount)};
+
+/* static */
+FinalizationRecordObject* FinalizationRecordObject::create(
+    JSContext* cx, HandleFinalizationQueueObject queue, HandleValue heldValue) {
+  MOZ_ASSERT(queue);
+
+  auto record = NewObjectWithGivenProto<FinalizationRecordObject>(cx, nullptr);
+  if (!record) {
+    return nullptr;
+  }
+
+  MOZ_ASSERT(queue->compartment() == record->compartment());
+
+  record->initReservedSlot(QueueSlot, ObjectValue(*queue));
+  record->initReservedSlot(HeldValueSlot, heldValue);
+  record->initReservedSlot(InMapSlot, BooleanValue(false));
+
+  return record;
+}
+
+FinalizationQueueObject* FinalizationRecordObject::queue() const {
+  Value value = getReservedSlot(QueueSlot);
+  if (value.isUndefined()) {
+    return nullptr;
+  }
+  return &value.toObject().as<FinalizationQueueObject>();
+}
+
+Value FinalizationRecordObject::heldValue() const {
+  return getReservedSlot(HeldValueSlot);
+}
+
+bool FinalizationRecordObject::isRegistered() const {
+  MOZ_ASSERT_IF(!queue(), heldValue().isUndefined());
+  return queue();
+}
+
+bool FinalizationRecordObject::isInRecordMap() const {
+  return getReservedSlot(InMapSlot).toBoolean();
+}
+
+void FinalizationRecordObject::setInRecordMap(bool newValue) {
+  MOZ_ASSERT(newValue != isInRecordMap());
+  setReservedSlot(InMapSlot, BooleanValue(newValue));
+}
+
+void FinalizationRecordObject::clear() {
+  MOZ_ASSERT(queue());
+  setReservedSlot(QueueSlot, UndefinedValue());
+  setReservedSlot(HeldValueSlot, UndefinedValue());
+  MOZ_ASSERT(!isRegistered());
+}
+
+///////////////////////////////////////////////////////////////////////////
+// FinalizationRegistrationsObject
+
+const JSClass FinalizationRegistrationsObject::class_ = {
+    "FinalizationRegistrations",
+    JSCLASS_HAS_RESERVED_SLOTS(SlotCount) | JSCLASS_BACKGROUND_FINALIZE,
+    &classOps_, JS_NULL_CLASS_SPEC};
+
+const JSClassOps FinalizationRegistrationsObject::classOps_ = {
+    nullptr,                                    // addProperty
+    nullptr,                                    // delProperty
+    nullptr,                                    // enumerate
+    nullptr,                                    // newEnumerate
+    nullptr,                                    // resolve
+    nullptr,                                    // mayResolve
+    FinalizationRegistrationsObject::finalize,  // finalize
+    nullptr,                                    // call
+    nullptr,                                    // construct
+    FinalizationRegistrationsObject::trace,     // trace
+};
+
+/* static */
+FinalizationRegistrationsObject* FinalizationRegistrationsObject::create(
+    JSContext* cx) {
+  auto records = cx->make_unique<WeakFinalizationRecordVector>(cx->zone());
+  if (!records) {
+    return nullptr;
+  }
+
+  auto object =
+      NewObjectWithGivenProto<FinalizationRegistrationsObject>(cx, nullptr);
+  if (!object) {
+    return nullptr;
+  }
+
+  InitReservedSlot(object, RecordsSlot, records.release(),
+                   MemoryUse::FinalizationRecordVector);
+
+  return object;
+}
+
+/* static */
+void FinalizationRegistrationsObject::trace(JSTracer* trc, JSObject* obj) {
+  if (!trc->traceWeakEdges()) {
+    return;
+  }
+
+  auto* self = &obj->as<FinalizationRegistrationsObject>();
+  if (WeakFinalizationRecordVector* records = self->records()) {
+    TraceRange(trc, records->length(), records->begin(),
+               "FinalizationRegistrationsObject records");
+  }
+}
+
+/* static */
+void FinalizationRegistrationsObject::finalize(JS::GCContext* gcx,
+                                               JSObject* obj) {
+  auto* self = &obj->as<FinalizationRegistrationsObject>();
+  gcx->delete_(obj, self->records(), MemoryUse::FinalizationRecordVector);
+}
+
+inline WeakFinalizationRecordVector*
+FinalizationRegistrationsObject::records() {
+  return static_cast<WeakFinalizationRecordVector*>(privatePtr());
+}
+
+inline const WeakFinalizationRecordVector*
+FinalizationRegistrationsObject::records() const {
+  return static_cast<const WeakFinalizationRecordVector*>(privatePtr());
+}
+
+inline void* FinalizationRegistrationsObject::privatePtr() const {
+  Value value = getReservedSlot(RecordsSlot);
+  if (value.isUndefined()) {
+    return nullptr;
+  }
+  void* ptr = value.toPrivate();
+  MOZ_ASSERT(ptr);
+  return ptr;
+}
+
+inline bool FinalizationRegistrationsObject::isEmpty() const {
+  MOZ_ASSERT(records());
+  return records()->empty();
+}
+
+inline bool FinalizationRegistrationsObject::append(
+    HandleFinalizationRecordObject record) {
+  MOZ_ASSERT(records());
+  return records()->append(record);
+}
+
+inline void FinalizationRegistrationsObject::remove(
+    HandleFinalizationRecordObject record) {
+  MOZ_ASSERT(records());
+  records()->eraseIfEqual(record);
+}
+
+inline bool FinalizationRegistrationsObject::traceWeak(JSTracer* trc) {
+  MOZ_ASSERT(records());
+  return records()->traceWeak(trc);
+}
+
+///////////////////////////////////////////////////////////////////////////
+// FinalizationRegistryObject
+
+// Bug 1600300: FinalizationRegistryObject is foreground finalized so that
+// HeapPtr destructors never see referents with released arenas. When this is
+// fixed we may be able to make this background finalized again.
+const JSClass FinalizationRegistryObject::class_ = {
+    "FinalizationRegistry",
+    JSCLASS_HAS_CACHED_PROTO(JSProto_FinalizationRegistry) |
+        JSCLASS_HAS_RESERVED_SLOTS(SlotCount) | JSCLASS_FOREGROUND_FINALIZE,
+    &classOps_, &classSpec_};
+
+const JSClass FinalizationRegistryObject::protoClass_ = {
+    "FinalizationRegistry.prototype",
+    JSCLASS_HAS_CACHED_PROTO(JSProto_FinalizationRegistry), JS_NULL_CLASS_OPS,
+    &classSpec_};
+
+const JSClassOps FinalizationRegistryObject::classOps_ = {
+    nullptr,                               // addProperty
+    nullptr,                               // delProperty
+    nullptr,                               // enumerate
+    nullptr,                               // newEnumerate
+    nullptr,                               // resolve
+    nullptr,                               // mayResolve
+    FinalizationRegistryObject::finalize,  // finalize
+    nullptr,                               // call
+    nullptr,                               // construct
+    FinalizationRegistryObject::trace,     // trace
+};
+
+const ClassSpec FinalizationRegistryObject::classSpec_ = {
+    GenericCreateConstructor<construct, 1, gc::AllocKind::FUNCTION>,
+    GenericCreatePrototype<FinalizationRegistryObject>,
+    nullptr,
+    nullptr,
+    methods_,
+    properties_};
+
+const JSFunctionSpec FinalizationRegistryObject::methods_[] = {
+    JS_FN("register", register_, 2, 0), JS_FN("unregister", unregister, 1, 0),
+    JS_FN("cleanupSome", cleanupSome, 0, 0), JS_FS_END};
+
+const JSPropertySpec FinalizationRegistryObject::properties_[] = {
+    JS_STRING_SYM_PS(toStringTag, "FinalizationRegistry", JSPROP_READONLY),
+    JS_PS_END};
+
+/* static */
+bool FinalizationRegistryObject::construct(JSContext* cx, unsigned argc,
+                                           Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  if (!ThrowIfNotConstructing(cx, args, "FinalizationRegistry")) {
+    return false;
+  }
+
+  RootedObject cleanupCallback(
+      cx, ValueToCallable(cx, args.get(0), 1, NO_CONSTRUCT));
+  if (!cleanupCallback) {
+    return false;
+  }
+
+  RootedObject proto(cx);
+  if (!GetPrototypeFromBuiltinConstructor(
+          cx, args, JSProto_FinalizationRegistry, &proto)) {
+    return false;
+  }
+
+  Rooted<UniquePtr<ObjectWeakMap>> registrations(
+      cx, cx->make_unique<ObjectWeakMap>(cx));
+  if (!registrations) {
+    return false;
+  }
+
+  RootedFinalizationQueueObject queue(
+      cx, FinalizationQueueObject::create(cx, cleanupCallback));
+  if (!queue) {
+    return false;
+  }
+
+  RootedFinalizationRegistryObject registry(
+      cx, NewObjectWithClassProto<FinalizationRegistryObject>(cx, proto));
+  if (!registry) {
+    return false;
+  }
+
+  registry->initReservedSlot(QueueSlot, ObjectValue(*queue));
+  InitReservedSlot(registry, RegistrationsSlot, registrations.release(),
+                   MemoryUse::FinalizationRegistryRegistrations);
+
+  if (!cx->runtime()->gc.addFinalizationRegistry(cx, registry)) {
+    return false;
+  }
+
+  queue->setHasRegistry(true);
+
+  args.rval().setObject(*registry);
+  return true;
+}
+
+/* static */
+void FinalizationRegistryObject::trace(JSTracer* trc, JSObject* obj) {
+  // Trace the registrations weak map. At most this traces the
+  // FinalizationRegistrationsObject values of the map; the contents of those
+  // objects are weakly held and are not traced by this method.
+
+  auto* registry = &obj->as<FinalizationRegistryObject>();
+  if (ObjectWeakMap* registrations = registry->registrations()) {
+    registrations->trace(trc);
+  }
+}
+
+void FinalizationRegistryObject::traceWeak(JSTracer* trc) {
+  // Trace and update the contents of the registrations weak map's values, which
+  // are weakly held.
+
+  MOZ_ASSERT(registrations());
+  for (ObjectValueWeakMap::Enum e(registrations()->valueMap()); !e.empty();
+       e.popFront()) {
+    auto* registrations =
+        &e.front().value().toObject().as<FinalizationRegistrationsObject>();
+    if (!registrations->traceWeak(trc)) {
+      e.removeFront();
+    }
+  }
+}
+
+/* static */
+void FinalizationRegistryObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  auto registry = &obj->as<FinalizationRegistryObject>();
+
+  // The queue's flag should have been updated by
+  // GCRuntime::sweepFinalizationRegistries.
+  MOZ_ASSERT_IF(registry->queue(), !registry->queue()->hasRegistry());
+
+  gcx->delete_(obj, registry->registrations(),
+               MemoryUse::FinalizationRegistryRegistrations);
+}
+
+FinalizationQueueObject* FinalizationRegistryObject::queue() const {
+  Value value = getReservedSlot(QueueSlot);
+  if (value.isUndefined()) {
+    return nullptr;
+  }
+  return &value.toObject().as<FinalizationQueueObject>();
+}
+
+ObjectWeakMap* FinalizationRegistryObject::registrations() const {
+  Value value = getReservedSlot(RegistrationsSlot);
+  if (value.isUndefined()) {
+    return nullptr;
+  }
+  return static_cast<ObjectWeakMap*>(value.toPrivate());
+}
+
+// FinalizationRegistry.prototype.register(target, heldValue [, unregisterToken
+// ])
+// https://tc39.es/proposal-weakrefs/#sec-finalization-registry.prototype.register
+/* static */
+bool FinalizationRegistryObject::register_(JSContext* cx, unsigned argc,
+                                           Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  // 1. Let finalizationRegistry be the this value.
+  // 2. If Type(finalizationRegistry) is not Object, throw a TypeError
+  // exception.
+  // 3. If finalizationRegistry does not have a [[Cells]] internal slot, throw a
+  // TypeError exception.
+  if (!args.thisv().isObject() ||
+      !args.thisv().toObject().is<FinalizationRegistryObject>()) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_NOT_A_FINALIZATION_REGISTRY,
+                              "Receiver of FinalizationRegistry.register call");
+    return false;
+  }
+
+  RootedFinalizationRegistryObject registry(
+      cx, &args.thisv().toObject().as<FinalizationRegistryObject>());
+
+  // 4. If Type(target) is not Object, throw a TypeError exception.
+  if (!args.get(0).isObject()) {
+    JS_ReportErrorNumberASCII(
+        cx, GetErrorMessage, nullptr, JSMSG_OBJECT_REQUIRED,
+        "target argument to FinalizationRegistry.register");
+    return false;
+  }
+
+  RootedObject target(cx, &args[0].toObject());
+
+  // 5. If SameValue(target, heldValue), throw a TypeError exception.
+  if (args.get(1).isObject() && &args.get(1).toObject() == target) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_BAD_HELD_VALUE);
+    return false;
+  }
+
+  HandleValue heldValue = args.get(1);
+
+  // 6. If Type(unregisterToken) is not Object,
+  //    a. If unregisterToken is not undefined, throw a TypeError exception.
+  if (!args.get(2).isUndefined() && !args.get(2).isObject()) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_BAD_UNREGISTER_TOKEN,
+                              "FinalizationRegistry.register");
+    return false;
+  }
+
+  RootedObject unregisterToken(cx);
+  if (!args.get(2).isUndefined()) {
+    unregisterToken = &args[2].toObject();
+  }
+
+  // Create the finalization record representing this target and heldValue.
+  Rooted<FinalizationQueueObject*> queue(cx, registry->queue());
+  Rooted<FinalizationRecordObject*> record(
+      cx, FinalizationRecordObject::create(cx, queue, heldValue));
+  if (!record) {
+    return false;
+  }
+
+  // Add the record to the registrations if an unregister token was supplied.
+  if (unregisterToken &&
+      !addRegistration(cx, registry, unregisterToken, record)) {
+    return false;
+  }
+
+  auto registrationsGuard = mozilla::MakeScopeExit([&] {
+    if (unregisterToken) {
+      removeRegistrationOnError(registry, unregisterToken, record);
+    }
+  });
+
+  // Fully unwrap the target to pass it to the GC.
+  RootedObject unwrappedTarget(cx);
+  unwrappedTarget = CheckedUnwrapDynamic(target, cx);
+  if (!unwrappedTarget) {
+    ReportAccessDenied(cx);
+    return false;
+  }
+
+  // If the target is a DOM wrapper, preserve it.
+  if (!preserveDOMWrapper(cx, target)) {
+    return false;
+  }
+
+  // Wrap the record into the compartment of the target.
+  RootedObject wrappedRecord(cx, record);
+  AutoRealm ar(cx, unwrappedTarget);
+  if (!JS_WrapObject(cx, &wrappedRecord)) {
+    return false;
+  }
+
+  if (JS_IsDeadWrapper(wrappedRecord)) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_DEAD_OBJECT);
+    return false;
+  }
+
+  // Register the record with the target.
+  gc::GCRuntime* gc = &cx->runtime()->gc;
+  if (!gc->registerWithFinalizationRegistry(cx, unwrappedTarget,
+                                            wrappedRecord)) {
+    return false;
+  }
+
+  registrationsGuard.release();
+  args.rval().setUndefined();
+  return true;
+}
+
+/* static */
+bool FinalizationRegistryObject::preserveDOMWrapper(JSContext* cx,
+                                                    HandleObject obj) {
+  if (!MaybePreserveDOMWrapper(cx, obj)) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_BAD_FINALIZATION_REGISTRY_OBJECT);
+    return false;
+  }
+
+  return true;
+}
+
+/* static */
+bool FinalizationRegistryObject::addRegistration(
+    JSContext* cx, HandleFinalizationRegistryObject registry,
+    HandleObject unregisterToken, HandleFinalizationRecordObject record) {
+  // Add the record to the list of records associated with this unregister
+  // token.
+
+  MOZ_ASSERT(unregisterToken);
+  MOZ_ASSERT(registry->registrations());
+
+  auto& map = *registry->registrations();
+  Rooted<FinalizationRegistrationsObject*> recordsObject(cx);
+  JSObject* obj = map.lookup(unregisterToken);
+  if (obj) {
+    recordsObject = &obj->as<FinalizationRegistrationsObject>();
+  } else {
+    recordsObject = FinalizationRegistrationsObject::create(cx);
+    if (!recordsObject || !map.add(cx, unregisterToken, recordsObject)) {
+      return false;
+    }
+  }
+
+  if (!recordsObject->append(record)) {
+    ReportOutOfMemory(cx);
+    return false;
+  }
+
+  return true;
+}
+
+/* static */ void FinalizationRegistryObject::removeRegistrationOnError(
+    HandleFinalizationRegistryObject registry, HandleObject unregisterToken,
+    HandleFinalizationRecordObject record) {
+  // Remove a registration if something went wrong before we added it to the
+  // target zone's map. Note that this can't remove a registration after that
+  // point.
+
+  MOZ_ASSERT(unregisterToken);
+  MOZ_ASSERT(registry->registrations());
+  JS::AutoAssertNoGC nogc;
+
+  auto& map = *registry->registrations();
+  JSObject* obj = map.lookup(unregisterToken);
+  MOZ_ASSERT(obj);
+  auto records = &obj->as<FinalizationRegistrationsObject>();
+  records->remove(record);
+
+  if (records->empty()) {
+    map.remove(unregisterToken);
+  }
+}
+
+// FinalizationRegistry.prototype.unregister ( unregisterToken )
+// https://tc39.es/proposal-weakrefs/#sec-finalization-registry.prototype.unregister
+/* static */
+bool FinalizationRegistryObject::unregister(JSContext* cx, unsigned argc,
+                                            Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  // 1. Let finalizationRegistry be the this value.
+  // 2. If Type(finalizationRegistry) is not Object, throw a TypeError
+  //    exception.
+  // 3. If finalizationRegistry does not have a [[Cells]] internal slot, throw a
+  //    TypeError exception.
+  if (!args.thisv().isObject() ||
+      !args.thisv().toObject().is<FinalizationRegistryObject>()) {
+    JS_ReportErrorNumberASCII(
+        cx, GetErrorMessage, nullptr, JSMSG_NOT_A_FINALIZATION_REGISTRY,
+        "Receiver of FinalizationRegistry.unregister call");
+    return false;
+  }
+
+  RootedFinalizationRegistryObject registry(
+      cx, &args.thisv().toObject().as<FinalizationRegistryObject>());
+
+  // 4. If Type(unregisterToken) is not Object, throw a TypeError exception.
+  if (!args.get(0).isObject()) {
+    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
+                              JSMSG_BAD_UNREGISTER_TOKEN,
+                              "FinalizationRegistry.unregister");
+    return false;
+  }
+
+  RootedObject unregisterToken(cx, &args[0].toObject());
+
+  // 5. Let removed be false.
+  bool removed = false;
+
+  // 6. For each Record { [[Target]], [[HeldValue]], [[UnregisterToken]] } cell
+  //    that is an element of finalizationRegistry.[[Cells]], do
+  //    a. If SameValue(cell.[[UnregisterToken]], unregisterToken) is true, then
+  //       i. Remove cell from finalizationRegistry.[[Cells]].
+  //       ii. Set removed to true.
+
+  RootedObject obj(cx, registry->registrations()->lookup(unregisterToken));
+  if (obj) {
+    auto* records = obj->as<FinalizationRegistrationsObject>().records();
+    MOZ_ASSERT(records);
+    MOZ_ASSERT(!records->empty());
+    for (FinalizationRecordObject* record : *records) {
+      if (unregisterRecord(record)) {
+        removed = true;
+      }
+    }
+    registry->registrations()->remove(unregisterToken);
+  }
+
+  // 7. Return removed.
+  args.rval().setBoolean(removed);
+  return true;
+}
+
+/* static */
+bool FinalizationRegistryObject::unregisterRecord(
+    FinalizationRecordObject* record) {
+  if (!record->isRegistered()) {
+    return false;
+  }
+
+  // Clear the fields of this record; it will be removed from the target's
+  // list when it is next swept.
+  record->clear();
+  return true;
+}
+
+// FinalizationRegistry.prototype.cleanupSome ( [ callback ] )
+// https://tc39.es/proposal-weakrefs/#sec-finalization-registry.prototype.cleanupSome
+bool FinalizationRegistryObject::cleanupSome(JSContext* cx, unsigned argc,
+                                             Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  // 1. Let finalizationRegistry be the this value.
+  // 2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
+  if (!args.thisv().isObject() ||
+      !args.thisv().toObject().is<FinalizationRegistryObject>()) {
+    JS_ReportErrorNumberASCII(
+        cx, GetErrorMessage, nullptr, JSMSG_NOT_A_FINALIZATION_REGISTRY,
+        "Receiver of FinalizationRegistry.cleanupSome call");
+    return false;
+  }
+
+  RootedFinalizationRegistryObject registry(
+      cx, &args.thisv().toObject().as<FinalizationRegistryObject>());
+
+  // 3. If callback is not undefined and IsCallable(callback) is false, throw a
+  //    TypeError exception.
+  RootedObject cleanupCallback(cx);
+  if (!args.get(0).isUndefined()) {
+    cleanupCallback = ValueToCallable(cx, args.get(0), -1, NO_CONSTRUCT);
+    if (!cleanupCallback) {
+      return false;
+    }
+  }
+
+  RootedFinalizationQueueObject queue(cx, registry->queue());
+  if (!FinalizationQueueObject::cleanupQueuedRecords(cx, queue,
+                                                     cleanupCallback)) {
+    return false;
+  }
+
+  args.rval().setUndefined();
+  return true;
+}
+
+///////////////////////////////////////////////////////////////////////////
+// FinalizationQueueObject
+
+// Bug 1600300: FinalizationQueueObject is foreground finalized so that
+// HeapPtr destructors never see referents with released arenas. When this is
+// fixed we may be able to make this background finalized again.
+const JSClass FinalizationQueueObject::class_ = {
+    "FinalizationQueue",
+    JSCLASS_HAS_RESERVED_SLOTS(SlotCount) | JSCLASS_FOREGROUND_FINALIZE,
+    &classOps_};
+
+const JSClassOps FinalizationQueueObject::classOps_ = {
+    nullptr,                            // addProperty
+    nullptr,                            // delProperty
+    nullptr,                            // enumerate
+    nullptr,                            // newEnumerate
+    nullptr,                            // resolve
+    nullptr,                            // mayResolve
+    FinalizationQueueObject::finalize,  // finalize
+    nullptr,                            // call
+    nullptr,                            // construct
+    FinalizationQueueObject::trace,     // trace
+};
+
+/* static */
+FinalizationQueueObject* FinalizationQueueObject::create(
+    JSContext* cx, HandleObject cleanupCallback) {
+  MOZ_ASSERT(cleanupCallback);
+
+  Rooted<UniquePtr<FinalizationRecordVector>> recordsToBeCleanedUp(
+      cx, cx->make_unique<FinalizationRecordVector>(cx->zone()));
+  if (!recordsToBeCleanedUp) {
+    return nullptr;
+  }
+
+  Handle<PropertyName*> funName = cx->names().empty_;
+  RootedFunction doCleanupFunction(
+      cx, NewNativeFunction(cx, doCleanup, 0, funName,
+                            gc::AllocKind::FUNCTION_EXTENDED));
+  if (!doCleanupFunction) {
+    return nullptr;
+  }
+
+  // It's problematic storing a CCW to a global in another compartment because
+  // you don't know how far to unwrap it to get the original object
+  // back. Instead store a CCW to a plain object in the same compartment as the
+  // global (this uses Object.prototype).
+  RootedObject incumbentObject(cx);
+  if (!GetObjectFromIncumbentGlobal(cx, &incumbentObject) || !incumbentObject) {
+    return nullptr;
+  }
+
+  FinalizationQueueObject* queue =
+      NewObjectWithGivenProto<FinalizationQueueObject>(cx, nullptr);
+  if (!queue) {
+    return nullptr;
+  }
+
+  queue->initReservedSlot(CleanupCallbackSlot, ObjectValue(*cleanupCallback));
+  queue->initReservedSlot(IncumbentObjectSlot, ObjectValue(*incumbentObject));
+  InitReservedSlot(queue, RecordsToBeCleanedUpSlot,
+                   recordsToBeCleanedUp.release(),
+                   MemoryUse::FinalizationRegistryRecordVector);
+  queue->initReservedSlot(IsQueuedForCleanupSlot, BooleanValue(false));
+  queue->initReservedSlot(DoCleanupFunctionSlot,
+                          ObjectValue(*doCleanupFunction));
+  queue->initReservedSlot(HasRegistrySlot, BooleanValue(false));
+
+  doCleanupFunction->setExtendedSlot(DoCleanupFunction_QueueSlot,
+                                     ObjectValue(*queue));
+
+  return queue;
+}
+
+/* static */
+void FinalizationQueueObject::trace(JSTracer* trc, JSObject* obj) {
+  auto queue = &obj->as<FinalizationQueueObject>();
+
+  if (FinalizationRecordVector* records = queue->recordsToBeCleanedUp()) {
+    records->trace(trc);
+  }
+}
+
+/* static */
+void FinalizationQueueObject::finalize(JS::GCContext* gcx, JSObject* obj) {
+  auto queue = &obj->as<FinalizationQueueObject>();
+
+  gcx->delete_(obj, queue->recordsToBeCleanedUp(),
+               MemoryUse::FinalizationRegistryRecordVector);
+}
+
+void FinalizationQueueObject::setHasRegistry(bool newValue) {
+  MOZ_ASSERT(hasRegistry() != newValue);
+
+  // Suppress our assertions about touching grey things. It's OK for us to set a
+  // boolean slot even if this object is gray.
+  AutoTouchingGrayThings atgt;
+
+  setReservedSlot(HasRegistrySlot, BooleanValue(newValue));
+}
+
+bool FinalizationQueueObject::hasRegistry() const {
+  return getReservedSlot(HasRegistrySlot).toBoolean();
+}
+
+inline JSObject* FinalizationQueueObject::cleanupCallback() const {
+  Value value = getReservedSlot(CleanupCallbackSlot);
+  if (value.isUndefined()) {
+    return nullptr;
+  }
+  return &value.toObject();
+}
+
+JSObject* FinalizationQueueObject::incumbentObject() const {
+  Value value = getReservedSlot(IncumbentObjectSlot);
+  if (value.isUndefined()) {
+    return nullptr;
+  }
+  return &value.toObject();
+}
+
+FinalizationRecordVector* FinalizationQueueObject::recordsToBeCleanedUp()
+    const {
+  Value value = getReservedSlot(RecordsToBeCleanedUpSlot);
+  if (value.isUndefined()) {
+    return nullptr;
+  }
+  return static_cast<FinalizationRecordVector*>(value.toPrivate());
+}
+
+bool FinalizationQueueObject::isQueuedForCleanup() const {
+  return getReservedSlot(IsQueuedForCleanupSlot).toBoolean();
+}
+
+JSFunction* FinalizationQueueObject::doCleanupFunction() const {
+  Value value = getReservedSlot(DoCleanupFunctionSlot);
+  if (value.isUndefined()) {
+    return nullptr;
+  }
+  return &value.toObject().as<JSFunction>();
+}
+
+void FinalizationQueueObject::queueRecordToBeCleanedUp(
+    FinalizationRecordObject* record) {
+  AutoEnterOOMUnsafeRegion oomUnsafe;
+  if (!recordsToBeCleanedUp()->append(record)) {
+    oomUnsafe.crash("FinalizationQueueObject::queueRecordsToBeCleanedUp");
+  }
+}
+
+void FinalizationQueueObject::setQueuedForCleanup(bool value) {
+  MOZ_ASSERT(value != isQueuedForCleanup());
+  setReservedSlot(IsQueuedForCleanupSlot, BooleanValue(value));
+}
+
+/* static */
+bool FinalizationQueueObject::doCleanup(JSContext* cx, unsigned argc,
+                                        Value* vp) {
+  CallArgs args = CallArgsFromVp(argc, vp);
+
+  RootedFunction callee(cx, &args.callee().as<JSFunction>());
+
+  Value value = callee->getExtendedSlot(DoCleanupFunction_QueueSlot);
+  RootedFinalizationQueueObject queue(
+      cx, &value.toObject().as<FinalizationQueueObject>());
+
+  queue->setQueuedForCleanup(false);
+  return cleanupQueuedRecords(cx, queue);
+}
+
+// CleanupFinalizationRegistry ( finalizationRegistry [ , callback ] )
+// https://tc39.es/proposal-weakrefs/#sec-cleanup-finalization-registry
+/* static */
+bool FinalizationQueueObject::cleanupQueuedRecords(
+    JSContext* cx, HandleFinalizationQueueObject queue,
+    HandleObject callbackArg) {
+  MOZ_ASSERT(cx->compartment() == queue->compartment());
+
+  // 2. If callback is undefined, set callback to
+  //    finalizationRegistry.[[CleanupCallback]].
+  RootedValue callback(cx);
+  if (callbackArg) {
+    callback.setObject(*callbackArg);
+  } else {
+    JSObject* cleanupCallback = queue->cleanupCallback();
+    MOZ_ASSERT(cleanupCallback);
+    callback.setObject(*cleanupCallback);
+  }
+
+  // 3. While finalizationRegistry.[[Cells]] contains a Record cell such that
+  //    cell.[[WeakRefTarget]] is empty, then an implementation may perform the
+  //    following steps,
+  //    a. Choose any such cell.
+  //    b. Remove cell from finalizationRegistry.[[Cells]].
+  //    c. Perform ? Call(callback, undefined, « cell.[[HeldValue]] »).
+
+  RootedValue heldValue(cx);
+  RootedValue rval(cx);
+  FinalizationRecordVector* records = queue->recordsToBeCleanedUp();
+  while (!records->empty()) {
+    FinalizationRecordObject* record = records->popCopy();
+
+    // Skip over records that have been unregistered.
+    if (!record->isRegistered()) {
+      continue;
+    }
+
+    heldValue.set(record->heldValue());
+
+    record->clear();
+
+    if (!Call(cx, callback, UndefinedHandleValue, heldValue, &rval)) {
+      return false;
+    }
+  }
+
+  return true;
+}