path: root/js/src/builtin/Promise.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * 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/. */


#include "builtin/Promise.h"

#include "mozilla/Atomics.h"
#include "mozilla/Maybe.h"
#include "mozilla/TimeStamp.h"

#include "jscntxt.h"
#include "jsexn.h"
#include "jsiter.h"

#include "gc/Heap.h"
#include "js/Debug.h"
#include "vm/ArrayObject.h"
#include "vm/AsyncFunction.h"
#include "vm/AsyncIteration.h"
#include "vm/ErrorObject.h"

#include "jsobjinlines.h"

#include "vm/ErrorObject-inl.h"
#include "vm/NativeObject-inl.h"

using namespace js;

static double
MillisecondsSinceStartup()
{
    auto now = mozilla::TimeStamp::Now();
    bool ignored;
    return (now - mozilla::TimeStamp::ProcessCreation(ignored)).ToMilliseconds();
}

enum PromiseHandler {
    PromiseHandlerIdentity = 0,
    PromiseHandlerThrower,

    // ES 2018 draft 25.5.5.4-5.
    PromiseHandlerAsyncFunctionAwaitedFulfilled,
    PromiseHandlerAsyncFunctionAwaitedRejected,

    // Async Iteration proposal 4.1.
    PromiseHandlerAsyncGeneratorAwaitedFulfilled,
    PromiseHandlerAsyncGeneratorAwaitedRejected,

    // Async Iteration proposal 11.4.3.5.1-2.
    PromiseHandlerAsyncGeneratorResumeNextReturnFulfilled,
    PromiseHandlerAsyncGeneratorResumeNextReturnRejected,

    // Async Iteration proposal 11.4.3.7 steps 8.c-e.
    PromiseHandlerAsyncGeneratorYieldReturnAwaitedFulfilled,
    PromiseHandlerAsyncGeneratorYieldReturnAwaitedRejected,

    // Async Iteration proposal 11.1.3.2.5.
    // Async-from-Sync iterator handlers take the resolved value and create new
    // iterator objects.  To do so it needs to forward whether the iterator is
    // done. In spec, this is achieved via the [[Done]] internal slot. We
    // enumerate both true and false cases here.
    PromiseHandlerAsyncFromSyncIteratorValueUnwrapDone,
    PromiseHandlerAsyncFromSyncIteratorValueUnwrapNotDone,

    // One past the maximum allowed PromiseHandler value.
    PromiseHandlerLimit
};

enum ResolutionMode {
    ResolveMode,
    RejectMode
};

enum ResolveFunctionSlots {
    ResolveFunctionSlot_Promise = 0,
    ResolveFunctionSlot_RejectFunction,
};

enum RejectFunctionSlots {
    RejectFunctionSlot_Promise = 0,
    RejectFunctionSlot_ResolveFunction,
};

enum PromiseCombinatorElementFunctionSlots {
  PromiseCombinatorElementFunctionSlot_Data = 0,
  PromiseCombinatorElementFunctionSlot_ElementIndex,
};

enum ReactionJobSlots {
    ReactionJobSlot_ReactionRecord = 0,
};

enum ThenableJobSlots {
    // The handler to use as the Promise reaction. It is a callable object
    // that's guaranteed to be from the same compartment as the
    // PromiseReactionJob.
    ThenableJobSlot_Handler = 0,

    // JobData - a, potentially CCW-wrapped, dense list containing data
    // required for proper execution of the reaction.
    ThenableJobSlot_JobData,
};

enum ThenableJobDataIndices {
    // The Promise to resolve using the given thenable.
    ThenableJobDataIndex_Promise = 0,

    // The thenable to use as the receiver when calling the `then` function.
    ThenableJobDataIndex_Thenable,

    ThenableJobDataLength,
};

enum BuiltinThenableJobSlots {
    // The Promise to resolve using the given thenable.
    BuiltinThenableJobSlot_Promise = 0,

    // The thenable to use as the receiver when calling the built-in `then`
    // function.
    BuiltinThenableJobSlot_Thenable,
};

struct PromiseCapability {
    JSObject* promise = nullptr;
    JSObject* resolve = nullptr;
    JSObject* reject = nullptr;

    PromiseCapability() = default;

    static void trace(PromiseCapability* self, JSTracer* trc) { self->trace(trc); }
    void trace(JSTracer* trc);
};

void
PromiseCapability::trace(JSTracer* trc)
{
    if (promise)
        TraceRoot(trc, &promise, "PromiseCapability::promise");
    if (resolve)
        TraceRoot(trc, &resolve, "PromiseCapability::resolve");
    if (reject)
        TraceRoot(trc, &reject, "PromiseCapability::reject");
}

namespace js {

template <typename Wrapper>
class WrappedPtrOperations<PromiseCapability, Wrapper>
{
    const PromiseCapability& capability() const { return static_cast<const Wrapper*>(this)->get(); }

  public:
    HandleObject promise() const {
        return HandleObject::fromMarkedLocation(&capability().promise);
    }
    HandleObject resolve() const {
        return HandleObject::fromMarkedLocation(&capability().resolve);
    }
    HandleObject reject() const {
        return HandleObject::fromMarkedLocation(&capability().reject);
    }
};

template <typename Wrapper>
class MutableWrappedPtrOperations<PromiseCapability, Wrapper>
    : public WrappedPtrOperations<PromiseCapability, Wrapper>
{
    PromiseCapability& capability() { return static_cast<Wrapper*>(this)->get(); }

  public:
    MutableHandleObject promise() {
        return MutableHandleObject::fromMarkedLocation(&capability().promise);
    }
    MutableHandleObject resolve() {
        return MutableHandleObject::fromMarkedLocation(&capability().resolve);
    }
    MutableHandleObject reject() {
        return MutableHandleObject::fromMarkedLocation(&capability().reject);
    }
};

} // namespace js

struct PromiseCombinatorElements;

class PromiseCombinatorDataHolder : public NativeObject
{
  enum {
    Slot_Promise = 0,
    Slot_RemainingElements,
    Slot_ValuesArray,
    Slot_ResolveOrRejectFunction,
    SlotsCount,
  };

  public:
    static const Class class_;
    JSObject* promiseObj() { return &getFixedSlot(Slot_Promise).toObject(); }
    JSObject* resolveOrRejectObj() {
        return &getFixedSlot(Slot_ResolveOrRejectFunction).toObject();
    }
    Value valuesArray() { return getFixedSlot(Slot_ValuesArray); }
    int32_t remainingCount() {
        return getFixedSlot(Slot_RemainingElements).toInt32();
    }
    int32_t increaseRemainingCount() {
        int32_t remainingCount = getFixedSlot(Slot_RemainingElements).toInt32();
        remainingCount++;
        setFixedSlot(Slot_RemainingElements, Int32Value(remainingCount));
        return remainingCount;
    }
    int32_t decreaseRemainingCount() {
        int32_t remainingCount = getFixedSlot(Slot_RemainingElements).toInt32();
        remainingCount--;
        MOZ_ASSERT(remainingCount >= 0, "unpaired calls to decreaseRemainingCount");
        setFixedSlot(Slot_RemainingElements, Int32Value(remainingCount));
        return remainingCount;
    }

    static PromiseCombinatorDataHolder* New(JSContext* cx,
                                            HandleObject resultPromise,
                                            Handle<PromiseCombinatorElements> elements,
                                            HandleObject resolveOrReject);
};

const Class PromiseCombinatorDataHolder::class_ = {
    "PromiseCombinatorDataHolder",
    JSCLASS_HAS_RESERVED_SLOTS(SlotsCount)
};

// Smart pointer to the "F.[[Values]]" part of the state of a Promise.all or
// Promise.allSettled invocation, or the "F.[[Errors]]" part of the state of a
// Promise.any invocation. Copes with compartment issues when setting an
// element.
struct MOZ_STACK_CLASS PromiseCombinatorElements final
{
  // Object value holding the elements array. The object can be a wrapper.
  Value value;

  // Unwrapped elements array. May not belong to the current compartment!
  ArrayObject* unwrappedArray = nullptr;

  // Set to true if the |setElement| method needs to wrap its input value.
  bool setElementNeedsWrapping = false;

  PromiseCombinatorElements() = default;

  void trace(JSTracer* trc);
};

void PromiseCombinatorElements::trace(JSTracer* trc)
{
  TraceRoot(trc, &value, "PromiseCombinatorElements::value");
  if (unwrappedArray) {
    TraceRoot(trc, &unwrappedArray,
              "PromiseCombinatorElements::unwrappedArray");
  }
}

namespace js {

template <typename Wrapper>
class WrappedPtrOperations<PromiseCombinatorElements, Wrapper>
{
  const PromiseCombinatorElements& elements() const {
    return static_cast<const Wrapper*>(this)->get();
  }

 public:
  HandleValue value() const {
    return HandleValue::fromMarkedLocation(&elements().value);
  }

  HandleArrayObject unwrappedArray() const {
    return HandleArrayObject::fromMarkedLocation(&elements().unwrappedArray);
  }
};

template <typename Wrapper>
class MutableWrappedPtrOperations<PromiseCombinatorElements, Wrapper>
    : public WrappedPtrOperations<PromiseCombinatorElements, Wrapper>
{
  PromiseCombinatorElements& elements() {
    return static_cast<Wrapper*>(this)->get();
  }

 public:
  MutableHandleValue value() {
    return MutableHandleValue::fromMarkedLocation(&elements().value);
  }

  MutableHandle<ArrayObject*> unwrappedArray() {
    return MutableHandle<ArrayObject*>::fromMarkedLocation(
        &elements().unwrappedArray);
  }

  void initialize(ArrayObject* arrayObj) {
    unwrappedArray().set(arrayObj);
    value().setObject(*arrayObj);

    // |needsWrapping| isn't tracked here, because all modifications on the
    // initial elements don't require any wrapping.
  }

  void initialize(PromiseCombinatorDataHolder* data, ArrayObject* arrayObj,
                  bool needsWrapping) {
    unwrappedArray().set(arrayObj);
    value().set(data->valuesArray());
    elements().setElementNeedsWrapping = needsWrapping;
  }

  MOZ_MUST_USE bool pushUndefined(JSContext* cx) {
    // Helper for the AutoCompartment we need to work with |array|. We mostly do this
    // for performance; we could go ahead and do the define via a cross-
    // compartment proxy instead...
    AutoCompartment ac(cx, unwrappedArray());

    HandleArrayObject arrayObj = unwrappedArray();
    return js::NewbornArrayPush(cx, arrayObj, UndefinedValue());
  }

  // `Promise.all` Resolve Element Functions
  // Step 9. Set values[index] to x.
  //
  // `Promise.allSettled` Resolve Element Functions
  // `Promise.allSettled` Reject Element Functions
  // Step 12. Set values[index] to obj.
  //
  // `Promise.any` Reject Element Functions
  // Step 9. Set errors[index] to x.
  //
  // These handler functions are always created in the compartment of the
  // Promise.all/allSettled/any function, which isn't necessarily the same
  // compartment as unwrappedArray as explained in NewPromiseCombinatorElements.
  // So before storing |val| we may need to enter unwrappedArray's compartment.
  MOZ_MUST_USE bool setElement(JSContext* cx, uint32_t index, HandleValue val) {
    // The index is guaranteed to be initialized to `undefined`.
    MOZ_ASSERT(unwrappedArray()->getDenseElement(index).isUndefined());

    if (elements().setElementNeedsWrapping) {
      AutoCompartment ac(cx, unwrappedArray());

      RootedValue rootedVal(cx, val);
      if (!cx->compartment()->wrap(cx, &rootedVal)) {
        return false;
      }
      unwrappedArray()->setDenseElement(index, rootedVal);
    } else {
      unwrappedArray()->setDenseElement(index, val);
    }
    return true;
  }
};

}  // namespace js

PromiseCombinatorDataHolder*
PromiseCombinatorDataHolder::New(
    JSContext* cx, HandleObject resultPromise, Handle<PromiseCombinatorElements> elements,
    HandleObject resolveOrReject)
{
    auto* dataHolder = NewBuiltinClassInstance<PromiseCombinatorDataHolder>(cx);
    if (!dataHolder)
        return nullptr;

    assertSameCompartment(cx, resultPromise);
    assertSameCompartment(cx, elements.value());
    assertSameCompartment(cx, resolveOrReject);

    dataHolder->setFixedSlot(Slot_Promise, ObjectValue(*resultPromise));
    dataHolder->setFixedSlot(Slot_RemainingElements, Int32Value(1));
    dataHolder->setFixedSlot(Slot_ValuesArray, elements.value());
    dataHolder->setFixedSlot(Slot_ResolveOrRejectFunction, ObjectValue(*resolveOrReject));
    return dataHolder;
}

namespace {
// Generator used by PromiseObject::getID.
mozilla::Atomic<uint64_t> gIDGenerator(0);
} // namespace

static MOZ_ALWAYS_INLINE bool
ShouldCaptureDebugInfo(JSContext* cx)
{
    return cx->options().asyncStack() || cx->compartment()->isDebuggee();
}

class PromiseDebugInfo : public NativeObject
{
  private:
    enum Slots {
        Slot_AllocationSite,
        Slot_ResolutionSite,
        Slot_AllocationTime,
        Slot_ResolutionTime,
        Slot_Id,
        SlotCount
    };

  public:
    static const Class class_;
    static PromiseDebugInfo* create(JSContext* cx, Handle<PromiseObject*> promise) {
        Rooted<PromiseDebugInfo*> debugInfo(cx, NewObjectWithClassProto<PromiseDebugInfo>(cx));
        if (!debugInfo)
            return nullptr;

        RootedObject stack(cx);
        if (!JS::CaptureCurrentStack(cx, &stack, JS::StackCapture(JS::AllFrames())))
            return nullptr;
        debugInfo->setFixedSlot(Slot_AllocationSite, ObjectOrNullValue(stack));
        debugInfo->setFixedSlot(Slot_ResolutionSite, NullValue());
        debugInfo->setFixedSlot(Slot_AllocationTime, DoubleValue(MillisecondsSinceStartup()));
        debugInfo->setFixedSlot(Slot_ResolutionTime, NumberValue(0));
        promise->setFixedSlot(PromiseSlot_DebugInfo, ObjectValue(*debugInfo));

        return debugInfo;
    }

    static PromiseDebugInfo* FromPromise(PromiseObject* promise) {
        Value val = promise->getFixedSlot(PromiseSlot_DebugInfo);
        if (val.isObject())
            return &val.toObject().as<PromiseDebugInfo>();
        return nullptr;
    }

    /**
     * Returns the given PromiseObject's process-unique ID.
     * The ID is lazily assigned when first queried, and then either stored
     * in the DebugInfo slot if no debug info was recorded for this Promise,
     * or in the Id slot of the DebugInfo object.
     */
    static uint64_t id(PromiseObject* promise) {
        Value idVal(promise->getFixedSlot(PromiseSlot_DebugInfo));
        if (idVal.isUndefined()) {
            idVal.setDouble(++gIDGenerator);
            promise->setFixedSlot(PromiseSlot_DebugInfo, idVal);
        } else if (idVal.isObject()) {
            PromiseDebugInfo* debugInfo = FromPromise(promise);
            idVal = debugInfo->getFixedSlot(Slot_Id);
            if (idVal.isUndefined()) {
                idVal.setDouble(++gIDGenerator);
                debugInfo->setFixedSlot(Slot_Id, idVal);
            }
        }
        return uint64_t(idVal.toNumber());
    }

    double allocationTime() { return getFixedSlot(Slot_AllocationTime).toNumber(); }
    double resolutionTime() { return getFixedSlot(Slot_ResolutionTime).toNumber(); }
    JSObject* allocationSite() { return getFixedSlot(Slot_AllocationSite).toObjectOrNull(); }
    JSObject* resolutionSite() { return getFixedSlot(Slot_ResolutionSite).toObjectOrNull(); }

    static void setResolutionInfo(JSContext* cx, Handle<PromiseObject*> promise) {
        if (!ShouldCaptureDebugInfo(cx))
            return;

        // If async stacks weren't enabled and the Promise's global wasn't a
        // debuggee when the Promise was created, we won't have a debugInfo
        // object. We still want to capture the resolution stack, so we
        // create the object now and change it's slots' values around a bit.
        Rooted<PromiseDebugInfo*> debugInfo(cx, FromPromise(promise));
        if (!debugInfo) {
            RootedValue idVal(cx, promise->getFixedSlot(PromiseSlot_DebugInfo));
            debugInfo = create(cx, promise);
            if (!debugInfo) {
                cx->clearPendingException();
                return;
            }

            // The current stack was stored in the AllocationSite slot, move
            // it to ResolutionSite as that's what it really is.
            debugInfo->setFixedSlot(Slot_ResolutionSite,
                                    debugInfo->getFixedSlot(Slot_AllocationSite));
            debugInfo->setFixedSlot(Slot_AllocationSite, NullValue());

            // There's no good default for a missing AllocationTime, so
            // instead of resetting that, ensure that it's the same as
            // ResolutionTime, so that the diff shows as 0, which isn't great,
            // but bearable.
            debugInfo->setFixedSlot(Slot_ResolutionTime,
                                    debugInfo->getFixedSlot(Slot_AllocationTime));

            // The Promise's ID might've been queried earlier, in which case
            // it's stored in the DebugInfo slot. We saved that earlier, so
            // now we can store it in the right place (or leave it as
            // undefined if it wasn't ever initialized.)
            debugInfo->setFixedSlot(Slot_Id, idVal);
            return;
        }

        RootedObject stack(cx);
        if (!JS::CaptureCurrentStack(cx, &stack, JS::StackCapture(JS::AllFrames()))) {
            cx->clearPendingException();
            return;
        }

        debugInfo->setFixedSlot(Slot_ResolutionSite, ObjectOrNullValue(stack));
        debugInfo->setFixedSlot(Slot_ResolutionTime, DoubleValue(MillisecondsSinceStartup()));
    }
};

const Class PromiseDebugInfo::class_ = {
    "PromiseDebugInfo",
    JSCLASS_HAS_RESERVED_SLOTS(SlotCount)
};

double
PromiseObject::allocationTime()
{
    auto debugInfo = PromiseDebugInfo::FromPromise(this);
    if (debugInfo)
        return debugInfo->allocationTime();
    return 0;
}

double
PromiseObject::resolutionTime()
{
    auto debugInfo = PromiseDebugInfo::FromPromise(this);
    if (debugInfo)
        return debugInfo->resolutionTime();
    return 0;
}

JSObject*
PromiseObject::allocationSite()
{
    auto debugInfo = PromiseDebugInfo::FromPromise(this);
    if (debugInfo)
        return debugInfo->allocationSite();
    return nullptr;
}

JSObject*
PromiseObject::resolutionSite()
{
    auto debugInfo = PromiseDebugInfo::FromPromise(this);
    if (debugInfo) {
        JSObject* site = debugInfo->resolutionSite();
        if (site && !JS_IsDeadWrapper(site)) {
            return site;
        }
    }
    return nullptr;
}

/**
 * Wrapper for GetAndClearException that handles cases where no exception is
 * pending, but an error occurred. This can be the case if an OOM was
 * encountered while throwing the error.
 */
static bool
MaybeGetAndClearException(JSContext* cx, MutableHandleValue rval)
{
    if (!cx->isExceptionPending())
        return false;

    return GetAndClearException(cx, rval);
}

static MOZ_MUST_USE bool RunResolutionFunction(JSContext *cx, HandleObject resolutionFun,
                                               HandleValue result, ResolutionMode mode,
                                               HandleObject promiseObj);

// ES2016, 25.4.1.1.1, Steps 1.a-b.
// Extracting all of this internal spec algorithm into a helper function would
// be tedious, so the check in step 1 and the entirety of step 2 aren't
// included.
static bool
AbruptRejectPromise(JSContext* cx, CallArgs& args, HandleObject promiseObj, HandleObject reject)
{
    // Step 1.a.
    RootedValue reason(cx);
    if (!MaybeGetAndClearException(cx, &reason))
        return false;

    if (!RunResolutionFunction(cx, reject, reason, RejectMode, promiseObj))
        return false;

    // Step 1.b.
    args.rval().setObject(*promiseObj);
    return true;
}

static bool
AbruptRejectPromise(JSContext* cx, CallArgs& args, Handle<PromiseCapability> capability)
{
    return AbruptRejectPromise(cx, args, capability.promise(), capability.reject());
}

enum ReactionRecordSlots {
    // The promise for which this record provides a reaction handler.
    // Matches the [[Capability]].[[Promise]] field from the spec.
    //
    // The slot value is either an object, but not necessarily a built-in
    // Promise object, or null. The latter case is only possible for async
    // generator functions, in which case the REACTION_FLAG_ASYNC_GENERATOR
    // flag must be set.
    ReactionRecordSlot_Promise = 0,

    // The [[Handler]] field(s) of a PromiseReaction record. We create a
    // single reaction record for fulfillment and rejection, therefore our
    // PromiseReaction implementation needs two [[Handler]] fields.
    //
    // The slot value is either a callable object, an integer constant from
    // the |PromiseHandler| enum, or null. If the value is null, either the
    // REACTION_FLAG_DEBUGGER_DUMMY or the
    // REACTION_FLAG_DEFAULT_RESOLVING_HANDLER flag must be set.
    //
    // After setting the target state for a PromiseReaction, the slot of the
    // no longer used handler gets reused to store the argument of the active
    // handler.
    ReactionRecordSlot_OnFulfilled,
    ReactionRecordSlot_OnRejectedArg = ReactionRecordSlot_OnFulfilled,
    ReactionRecordSlot_OnRejected,
    ReactionRecordSlot_OnFulfilledArg = ReactionRecordSlot_OnRejected,

    // The functions to resolve or reject the promise. Matches the
    // [[Capability]].[[Resolve]] and [[Capability]].[[Reject]] fields from
    // the spec.
    //
    // The slot values are either callable objects or null, but the latter
    // case is only allowed if the promise is either a built-in Promise object
    // or null.
    ReactionRecordSlot_Resolve,
    ReactionRecordSlot_Reject,

    // The incumbent global for this reaction record. Can be null.
    ReactionRecordSlot_IncumbentGlobalObject,

    // Bitmask of the REACTION_FLAG values.
    ReactionRecordSlot_Flags,

    // Additional slot to store extra data for specific reaction record types.
    //
    // - When the REACTION_FLAG_ASYNC_GENERATOR flag is set, this slot store
    //   the async generator function for this promise reaction.
    // - When the REACTION_FLAG_DEFAULT_RESOLVING_HANDLER flag is set, this
    //   slot stores the promise to resolve when conceptually "calling" the
    //   OnFulfilled or OnRejected handlers.
    ReactionRecordSlot_GeneratorOrPromiseToResolve,

    ReactionRecordSlots,
};

// ES2016, 25.4.1.2.
class PromiseReactionRecord : public NativeObject
{
    static constexpr uint32_t REACTION_FLAG_RESOLVED = 0x1;
    static constexpr uint32_t REACTION_FLAG_FULFILLED = 0x2;
    static constexpr uint32_t REACTION_FLAG_DEFAULT_RESOLVING_HANDLER = 0x4;
    static constexpr uint32_t REACTION_FLAG_ASYNC_FUNCTION = 0x8;
    static constexpr uint32_t REACTION_FLAG_ASYNC_GENERATOR = 0x10;
    static constexpr uint32_t REACTION_FLAG_DEBUGGER_DUMMY = 0x20;

    void setFlagOnInitialState(uint32_t flag) {
        int32_t flags = this->flags();
        MOZ_ASSERT(flags == 0, "Can't modify with non-default flags");
        flags |= flag;
        setFixedSlot(ReactionRecordSlot_Flags, Int32Value(flags));
    }

    uint32_t handlerSlot() {
        MOZ_ASSERT(targetState() != JS::PromiseState::Pending);
        return targetState() == JS::PromiseState::Fulfilled
               ? ReactionRecordSlot_OnFulfilled
               : ReactionRecordSlot_OnRejected;
    }

    uint32_t handlerArgSlot() {
        MOZ_ASSERT(targetState() != JS::PromiseState::Pending);
        return targetState() == JS::PromiseState::Fulfilled
               ? ReactionRecordSlot_OnFulfilledArg
               : ReactionRecordSlot_OnRejectedArg;
    }

  public:
    static const Class class_;

    JSObject* promise() { return getFixedSlot(ReactionRecordSlot_Promise).toObjectOrNull(); }
    int32_t flags() { return getFixedSlot(ReactionRecordSlot_Flags).toInt32(); }
    JS::PromiseState targetState() {
        int32_t flags = this->flags();
        if (!(flags & REACTION_FLAG_RESOLVED))
            return JS::PromiseState::Pending;
        return flags & REACTION_FLAG_FULFILLED
               ? JS::PromiseState::Fulfilled
               : JS::PromiseState::Rejected;
    }
    void setTargetStateAndHandlerArg(JS::PromiseState state, const Value& arg) {
        MOZ_ASSERT(targetState() == JS::PromiseState::Pending);
        MOZ_ASSERT(state != JS::PromiseState::Pending, "Can't revert a reaction to pending.");

        int32_t flags = this->flags();
        flags |= REACTION_FLAG_RESOLVED;
        if (state == JS::PromiseState::Fulfilled)
            flags |= REACTION_FLAG_FULFILLED;

        setFixedSlot(ReactionRecordSlot_Flags, Int32Value(flags));
        setFixedSlot(handlerArgSlot(), arg);
    }
    void setIsDefaultResolvingHandler(PromiseObject* promiseToResolve) {
        setFlagOnInitialState(REACTION_FLAG_DEFAULT_RESOLVING_HANDLER);
        setFixedSlot(ReactionRecordSlot_GeneratorOrPromiseToResolve, ObjectValue(*promiseToResolve));
    }
    bool isDefaultResolvingHandler() {
        int32_t flags = this->flags();
        return flags & REACTION_FLAG_DEFAULT_RESOLVING_HANDLER;
    }
    PromiseObject* defaultResolvingPromise() {
        MOZ_ASSERT(isDefaultResolvingHandler());
        const Value& promiseToResolve = getFixedSlot(ReactionRecordSlot_GeneratorOrPromiseToResolve);
        return &promiseToResolve.toObject().as<PromiseObject>();
    }
    void setIsAsyncFunction() {
        setFlagOnInitialState(REACTION_FLAG_ASYNC_FUNCTION);
    }
    bool isAsyncFunction() {
        int32_t flags = this->flags();
        return flags & REACTION_FLAG_ASYNC_FUNCTION;
    }
    void setIsAsyncGenerator(AsyncGeneratorObject* asyncGenObj) {
        setFlagOnInitialState(REACTION_FLAG_ASYNC_GENERATOR);
        setFixedSlot(ReactionRecordSlot_GeneratorOrPromiseToResolve, ObjectValue(*asyncGenObj));
    }
    bool isAsyncGenerator() {
        int32_t flags = this->flags();
        return flags & REACTION_FLAG_ASYNC_GENERATOR;
    }
    AsyncGeneratorObject* asyncGenerator() {
        MOZ_ASSERT(isAsyncGenerator());
        const Value& generator = getFixedSlot(ReactionRecordSlot_GeneratorOrPromiseToResolve);
        return &generator.toObject().as<AsyncGeneratorObject>();
    }
    void setIsDebuggerDummy() {
        setFlagOnInitialState(REACTION_FLAG_DEBUGGER_DUMMY);
    }
    bool isDebuggerDummy() {
        int32_t flags = this->flags();
        return flags & REACTION_FLAG_DEBUGGER_DUMMY;
    }
    Value handler() {
        MOZ_ASSERT(targetState() != JS::PromiseState::Pending);
        return getFixedSlot(handlerSlot());
    }
    Value handlerArg() {
        MOZ_ASSERT(targetState() != JS::PromiseState::Pending);
        return getFixedSlot(handlerArgSlot());
    }
    JSObject* getAndClearIncumbentGlobalObject() {
        JSObject* obj = getFixedSlot(ReactionRecordSlot_IncumbentGlobalObject).toObjectOrNull();
        setFixedSlot(ReactionRecordSlot_IncumbentGlobalObject, UndefinedValue());
        return obj;
    }
};

const Class PromiseReactionRecord::class_ = {
    "PromiseReactionRecord",
    JSCLASS_HAS_RESERVED_SLOTS(ReactionRecordSlots)
};

static void
AddPromiseFlags(PromiseObject& promise, int32_t flag)
{
    int32_t flags = promise.flags();
    promise.setFixedSlot(PromiseSlot_Flags, Int32Value(flags | flag));
}

static bool
PromiseHasAnyFlag(PromiseObject& promise, int32_t flag)
{
    return promise.flags() & flag;
}

static bool ResolvePromiseFunction(JSContext* cx, unsigned argc, Value* vp);
static bool RejectPromiseFunction(JSContext* cx, unsigned argc, Value* vp);

// ES2016, 25.4.1.3.
static MOZ_MUST_USE bool
CreateResolvingFunctions(JSContext* cx, HandleObject promise,
                         MutableHandleObject resolveFn,
                         MutableHandleObject rejectFn)
{
    RootedAtom funName(cx, cx->names().empty);
    RootedFunction resolve(cx, NewNativeFunction(cx, ResolvePromiseFunction, 1, funName,
                                                 gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
    if (!resolve)
        return false;

    RootedFunction reject(cx, NewNativeFunction(cx, RejectPromiseFunction, 1, funName,
                                                 gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
    if (!reject)
        return false;

    resolve->setExtendedSlot(ResolveFunctionSlot_Promise, ObjectValue(*promise));
    resolve->setExtendedSlot(ResolveFunctionSlot_RejectFunction, ObjectValue(*reject));

    reject->setExtendedSlot(RejectFunctionSlot_Promise, ObjectValue(*promise));
    reject->setExtendedSlot(RejectFunctionSlot_ResolveFunction, ObjectValue(*resolve));

    resolveFn.set(resolve);
    rejectFn.set(reject);

    return true;
}

static void ClearResolutionFunctionSlots(JSFunction* resolutionFun);

static bool
IsSettledMaybeWrappedPromise(JSObject* promise)
{
    if (IsProxy(promise)) {
        promise = UncheckedUnwrap(promise);

        // Caller needs to handle dead wrappers.
        if (JS_IsDeadWrapper(promise))
            return false;
    }

    return promise->as<PromiseObject>().state() != JS::PromiseState::Pending;
}

// ES2016, 25.4.1.7.
static MOZ_MUST_USE bool
RejectMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj, HandleValue reason);

// ES2016, 25.4.1.7.
static MOZ_MUST_USE bool
RejectPromiseInternal(JSContext* cx, Handle<PromiseObject*> promise, HandleValue reason);

// ES2016, 25.4.1.3.1.
static bool
RejectPromiseFunction(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);

    JSFunction* reject = &args.callee().as<JSFunction>();
    HandleValue reasonVal = args.get(0);

    // Steps 1-2.
    const Value& promiseVal = reject->getExtendedSlot(RejectFunctionSlot_Promise);

    // Steps 3-4.
    // If the Promise isn't available anymore, it has been resolved and the
    // reference to it removed to make it eligible for collection.
    if (promiseVal.isUndefined()) {
        args.rval().setUndefined();
        return true;
    }

    // Store the promise value in |promise| before ClearResolutionFunctionSlots
    // removes the reference.
    RootedObject promise(cx, &promiseVal.toObject());

    // Step 5.
    // Here, we only remove the Promise reference from the resolution
    // functions. Actually marking it as fulfilled/rejected happens later.
    ClearResolutionFunctionSlots(reject);

    // In some cases the Promise reference on the resolution function won't
    // have been removed during resolution, so we need to check that here,
    // too.
    if (IsSettledMaybeWrappedPromise(promise)) {
        args.rval().setUndefined();
        return true;
    }

    // Step 6.
    if (!RejectMaybeWrappedPromise(cx, promise, reasonVal))
        return false;
    args.rval().setUndefined();
    return true;
}

static MOZ_MUST_USE bool FulfillMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj,
                                                    HandleValue value_);

static MOZ_MUST_USE bool EnqueuePromiseResolveThenableJob(JSContext* cx,
                                                          HandleValue promiseToResolve,
                                                          HandleValue thenable,
                                                          HandleValue thenVal);

static MOZ_MUST_USE bool EnqueuePromiseResolveThenableBuiltinJob(JSContext* cx,
                                                                 HandleObject promiseToResolve,
                                                                 HandleObject thenable);

static bool Promise_then(JSContext* cx, unsigned argc, Value* vp);
static bool Promise_then_impl(JSContext* cx, HandleValue promiseVal, HandleValue onFulfilled,
                              HandleValue onRejected, MutableHandleValue rval, bool rvalUsed);

// ES2016, 25.4.1.3.2, steps 6-13.
static MOZ_MUST_USE bool
ResolvePromiseInternal(JSContext* cx, HandleObject promise, HandleValue resolutionVal)
{
    assertSameCompartment(cx, promise, resolutionVal);
    MOZ_ASSERT(!IsSettledMaybeWrappedPromise(promise));

    // Step 7 (reordered).
    if (!resolutionVal.isObject())
        return FulfillMaybeWrappedPromise(cx, promise, resolutionVal);

    RootedObject resolution(cx, &resolutionVal.toObject());

    // Step 6.
    if (resolution == promise) {
        // Step 6.a.
        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
                                  JSMSG_CANNOT_RESOLVE_PROMISE_WITH_ITSELF);
        RootedValue selfResolutionError(cx);
        MOZ_ALWAYS_TRUE(GetAndClearException(cx, &selfResolutionError));

        // Step 6.b.
        return RejectMaybeWrappedPromise(cx, promise, selfResolutionError);
    }

    // Step 8.
    RootedValue thenVal(cx);
    bool status = GetProperty(cx, resolution, resolution, cx->names().then, &thenVal);

    RootedValue error(cx);
    if (!status) {
        if (!MaybeGetAndClearException(cx, &error))
            return false;
    }

    // Testing functions allow to directly settle a promise without going
    // through the resolving functions. In that case the normal bookkeeping to
    // ensure only pending promises can be resolved doesn't apply and we need
    // to manually check for already settled promises. The exception is simply
    // dropped when this case happens.
    if (IsSettledMaybeWrappedPromise(promise))
        return true;

    // Step 9.
    if (!status)
        return RejectMaybeWrappedPromise(cx, promise, error);

    // Step 10 (implicit).

    // Step 11.
    if (!IsCallable(thenVal))
        return FulfillMaybeWrappedPromise(cx, promise, resolutionVal);

    // If the resolution object is a built-in Promise object and the
    // `then` property is the original Promise.prototype.then function
    // from the current realm, we skip storing/calling it.
    // Additionally we require that |promise| itself is also a built-in
    // Promise object, so the fast path doesn't need to cope with wrappers.
    bool isBuiltinThen = false;
    if (resolution->is<PromiseObject>() &&
        promise->is<PromiseObject>() &&
        IsNativeFunction(thenVal, Promise_then) &&
        thenVal.toObject().as<JSFunction>().compartment() == cx->compartment())
    {
        thenVal = UndefinedValue();
        isBuiltinThen = true;
    }

    // Step 12.
    if (!isBuiltinThen) {
        RootedValue promiseVal(cx, ObjectValue(*promise));
        if (!EnqueuePromiseResolveThenableJob(cx, promiseVal, resolutionVal, thenVal))
            return false;
    } else {
        if (!EnqueuePromiseResolveThenableBuiltinJob(cx, promise, resolution))
            return false;
    }

    // Step 13.
    return true;
}

// ES2016, 25.4.1.3.2.
static bool
ResolvePromiseFunction(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);

    JSFunction* resolve = &args.callee().as<JSFunction>();
    HandleValue resolutionVal = args.get(0);

    // Steps 3-4 (reordered).
    // We use the reference to the reject function as a signal for whether
    // the resolve or reject function was already called, at which point
    // the references on each of the functions are cleared.
    if (!resolve->getExtendedSlot(ResolveFunctionSlot_RejectFunction).isObject()) {
        args.rval().setUndefined();
        return true;
    }

    // Steps 1-2 (reordered).
    RootedObject promise(cx, &resolve->getExtendedSlot(ResolveFunctionSlot_Promise).toObject());

    // Step 5.
    // Here, we only remove the Promise reference from the resolution
    // functions. Actually marking it as fulfilled/rejected happens later.
    ClearResolutionFunctionSlots(resolve);

    // In some cases the Promise reference on the resolution function won't
    // have been removed during resolution, so we need to check that here,
    // too.
    if (IsSettledMaybeWrappedPromise(promise)) {
        args.rval().setUndefined();
        return true;
    }

    // Steps 6-13.
    if (!ResolvePromiseInternal(cx, promise, resolutionVal))
        return false;
    args.rval().setUndefined();
    return true;
}

static bool PromiseReactionJob(JSContext* cx, unsigned argc, Value* vp);

/**
 * Tells the embedding to enqueue a Promise reaction job, based on
 * three parameters:
 * reactionObj - The reaction record.
 * handlerArg_ - The first and only argument to pass to the handler invoked by
 *              the job. This will be stored on the reaction record.
 * targetState - The PromiseState this reaction job targets. This decides
 *               whether the onFulfilled or onRejected handler is called.
 */
MOZ_MUST_USE static bool
EnqueuePromiseReactionJob(JSContext* cx, HandleObject reactionObj,
                          HandleValue handlerArg_, JS::PromiseState targetState)
{
    MOZ_ASSERT(targetState == JS::PromiseState::Fulfilled ||
               targetState == JS::PromiseState::Rejected);

    // The reaction might have been stored on a Promise from another
    // compartment, which means it would've been wrapped in a CCW.
    // To properly handle that case here, unwrap it and enter its
    // compartment, where the job creation should take place anyway.
    Rooted<PromiseReactionRecord*> reaction(cx);
    RootedValue handlerArg(cx, handlerArg_);
    mozilla::Maybe<AutoCompartment> ac;
    if (!IsProxy(reactionObj)) {
        MOZ_RELEASE_ASSERT(reactionObj->is<PromiseReactionRecord>());
        reaction = &reactionObj->as<PromiseReactionRecord>();
    } else {
        JSObject* unwrappedReactionObj = UncheckedUnwrap(reactionObj);
        if (JS_IsDeadWrapper(unwrappedReactionObj)) {
            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_DEAD_OBJECT);
            return false;
        }
        reaction = &unwrappedReactionObj->as<PromiseReactionRecord>();
        MOZ_RELEASE_ASSERT(reaction->is<PromiseReactionRecord>());
        ac.emplace(cx, reaction);
        if (!cx->compartment()->wrap(cx, &handlerArg))
            return false;
    }

    // Must not enqueue a reaction job more than once.
    MOZ_ASSERT(reaction->targetState() == JS::PromiseState::Pending);

    assertSameCompartment(cx, handlerArg);
    reaction->setTargetStateAndHandlerArg(targetState, handlerArg);

    RootedValue reactionVal(cx, ObjectValue(*reaction));
    RootedValue handler(cx, reaction->handler());

    // If we have a handler callback, we enter that handler's compartment so
    // that the promise reaction job function is created in that compartment.
    // That guarantees that the embedding ends up with the right entry global.
    // This is relevant for some html APIs like fetch that derive information
    // from said global.
    mozilla::Maybe<AutoCompartment> ac2;
    if (handler.isObject()) {
        // The unwrapping has to be unchecked because we specifically want to
        // be able to use handlers with wrappers that would only allow calls.
        // E.g., it's ok to have a handler from a chrome compartment in a
        // reaction to a content compartment's Promise instance.
        JSObject* handlerObj = UncheckedUnwrap(&handler.toObject());
        MOZ_ASSERT(handlerObj);
        ac2.emplace(cx, handlerObj);

        // We need to wrap the reaction to store it on the job function.
        if (!cx->compartment()->wrap(cx, &reactionVal))
            return false;
    }

    // Create the JS function to call when the job is triggered.
    HandlePropertyName funName = cx->names().empty;
    RootedFunction job(cx, NewNativeFunction(cx, PromiseReactionJob, 0, funName,
                                             gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
    if (!job)
        return false;

    // Store the reaction on the reaction job.
    job->setExtendedSlot(ReactionJobSlot_ReactionRecord, reactionVal);

    // When using JS::AddPromiseReactions, no actual promise is created, so we
    // might not have one here.
    // Additionally, we might have an object here that isn't an instance of
    // Promise. This can happen if content overrides the value of
    // Promise[@@species] (or invokes Promise#then on a Promise subclass
    // instance with a non-default @@species value on the constructor) with a
    // function that returns objects that're not Promise (subclass) instances.
    // In that case, we just pretend we didn't have an object in the first
    // place.
    // If after all this we do have an object, wrap it in case we entered the
    // handler's compartment above, because we should pass objects from a
    // single compartment to the enqueuePromiseJob callback.
    RootedObject promise(cx, reaction->promise());
    if (promise && promise->is<PromiseObject>()) {
        if (!cx->compartment()->wrap(cx, &promise))
            return false;
    }

    // Using objectFromIncumbentGlobal, we can derive the incumbent global by
    // unwrapping and then getting the global. This is very convoluted, but
    // much better than having to store the original global as a private value
    // because we couldn't wrap it to store it as a normal JS value.
    RootedObject global(cx);
    if (JSObject* objectFromIncumbentGlobal = reaction->getAndClearIncumbentGlobalObject()) {
        objectFromIncumbentGlobal = CheckedUnwrap(objectFromIncumbentGlobal);
        MOZ_ASSERT(objectFromIncumbentGlobal);
        global = &objectFromIncumbentGlobal->global();
    }

    // Note: the global we pass here might be from a different compartment
    // than job and promise. While it's somewhat unusual to pass objects
    // from multiple compartments, in this case we specifically need the
    // global to be unwrapped because wrapping and unwrapping aren't
    // necessarily symmetric for globals.
    return cx->runtime()->enqueuePromiseJob(cx, job, promise, global);
}

static MOZ_MUST_USE bool TriggerPromiseReactions(JSContext* cx, HandleValue reactionsVal,
                                                 JS::PromiseState state, HandleValue valueOrReason);

// ES2016, Commoned-out implementation of 25.4.1.4. and 25.4.1.7.
static MOZ_MUST_USE bool
ResolvePromise(JSContext* cx, Handle<PromiseObject*> promise, HandleValue valueOrReason,
               JS::PromiseState state)
{
    // Step 1.
    MOZ_ASSERT(promise->state() == JS::PromiseState::Pending);
    MOZ_ASSERT(state == JS::PromiseState::Fulfilled || state == JS::PromiseState::Rejected);

    // Step 2.
    // We only have one list of reactions for both resolution types. So
    // instead of getting the right list of reactions, we determine the
    // resolution type to retrieve the right information from the
    // reaction records.
    RootedValue reactionsVal(cx, promise->reactions());

    // Steps 3-5.
    // The same slot is used for the reactions list and the result, so setting
    // the result also removes the reactions list.
    promise->setFixedSlot(PromiseSlot_ReactionsOrResult, valueOrReason);

    // Step 6.
    int32_t flags = promise->flags();
    flags |= PROMISE_FLAG_RESOLVED;
    if (state == JS::PromiseState::Fulfilled)
        flags |= PROMISE_FLAG_FULFILLED;
    promise->setFixedSlot(PromiseSlot_Flags, Int32Value(flags));

    // Also null out the resolve/reject functions so they can be GC'd.
    promise->setFixedSlot(PromiseSlot_RejectFunction, UndefinedValue());

    // Now that everything else is done, do the things the debugger needs.
    // Step 7 of RejectPromise implemented in onSettled.
    PromiseObject::onSettled(cx, promise);

    // Step 7 of FulfillPromise.
    // Step 8 of RejectPromise.
    if (reactionsVal.isObject())
        return TriggerPromiseReactions(cx, reactionsVal, state, valueOrReason);

    return true;
}

// ES2016, 25.4.1.7.
static MOZ_MUST_USE bool
RejectPromiseInternal(JSContext* cx, Handle<PromiseObject*> promise, HandleValue reason)
{
    return ResolvePromise(cx, promise, reason, JS::PromiseState::Rejected);
}

// ES2016, 25.4.1.4.
static MOZ_MUST_USE bool
FulfillMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj, HandleValue value_)
{
    Rooted<PromiseObject*> promise(cx);
    RootedValue value(cx, value_);

    mozilla::Maybe<AutoCompartment> ac;
    if (!IsProxy(promiseObj)) {
        promise = &promiseObj->as<PromiseObject>();
    } else {
        JSObject* unwrappedPromiseObj = UncheckedUnwrap(promiseObj);
        if (JS_IsDeadWrapper(unwrappedPromiseObj)) {
            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_DEAD_OBJECT);
            return false;
        }
        promise = &unwrappedPromiseObj->as<PromiseObject>();
        ac.emplace(cx, promise);
        if (!cx->compartment()->wrap(cx, &value))
            return false;
    }

    return ResolvePromise(cx, promise, value, JS::PromiseState::Fulfilled);
}

static bool GetCapabilitiesExecutor(JSContext* cx, unsigned argc, Value* vp);
static bool PromiseConstructor(JSContext* cx, unsigned argc, Value* vp);
static MOZ_MUST_USE PromiseObject* CreatePromiseObjectInternal(JSContext* cx,
                                                               HandleObject proto = nullptr,
                                                               bool protoIsWrapped = false,
                                                               bool informDebugger = true);

enum GetCapabilitiesExecutorSlots {
    GetCapabilitiesExecutorSlots_Resolve,
    GetCapabilitiesExecutorSlots_Reject
};

static MOZ_MUST_USE PromiseObject*
CreatePromiseObjectWithoutResolutionFunctions(JSContext* cx)
{
    Rooted<PromiseObject*> promise(cx, CreatePromiseObjectInternal(cx));
    if (!promise)
        return nullptr;

    AddPromiseFlags(*promise, PROMISE_FLAG_DEFAULT_RESOLVING_FUNCTIONS);
    return promise;
}

static MOZ_MUST_USE PromiseObject*
CreatePromiseWithDefaultResolutionFunctions(JSContext* cx, MutableHandleObject resolve,
                                            MutableHandleObject reject)
{
    // ES2016, 25.4.3.1., as if called with GetCapabilitiesExecutor as the
    // executor argument.

    // Steps 1-2 (Not applicable).

    // Steps 3-7.
    Rooted<PromiseObject*> promise(cx, CreatePromiseObjectInternal(cx));
    if (!promise)
        return nullptr;

    // Step 8.
    if (!CreateResolvingFunctions(cx, promise, resolve, reject))
        return nullptr;

    promise->setFixedSlot(PromiseSlot_RejectFunction, ObjectValue(*reject));

    // Steps 9-10 (Not applicable).

    // Step 11.
    return promise;
}

// ES2016, 25.4.1.5.
static MOZ_MUST_USE bool
NewPromiseCapability(JSContext* cx, HandleObject C, MutableHandle<PromiseCapability> capability,
                     bool canOmitResolutionFunctions)
{
    RootedValue cVal(cx, ObjectValue(*C));

    // Steps 1-2.
    if (!IsConstructor(C)) {
        ReportValueError(cx, JSMSG_NOT_CONSTRUCTOR, -1, cVal, nullptr);
        return false;
    }

    // If we'd call the original Promise constructor and know that the
    // resolve/reject functions won't ever escape to content, we can skip
    // creating and calling the executor function and instead return a Promise
    // marked as having default resolve/reject functions.
    //
    // This can't be used in Promise.all and Promise.race because we have to
    // pass the reject (and resolve, in the race case) function to thenables
    // in the list passed to all/race, which (potentially) means exposing them
    // to content.
    //
    // For Promise.all and Promise.race we can only optimize away the creation
    // of the GetCapabilitiesExecutor function, and directly allocate the
    // result promise instead of invoking the Promise constructor.
    if (IsNativeFunction(cVal, PromiseConstructor)) {
        PromiseObject* promise;
        if (canOmitResolutionFunctions) {
            promise = CreatePromiseObjectWithoutResolutionFunctions(cx);
        } else {
            promise = CreatePromiseWithDefaultResolutionFunctions(cx, capability.resolve(),
                                                                  capability.reject());
        }
        if (!promise)
            return false;

        capability.promise().set(promise);
        return true;
    }

    // Step 3 (omitted).

    // Step 4.
    HandlePropertyName funName = cx->names().empty;
    RootedFunction executor(cx, NewNativeFunction(cx, GetCapabilitiesExecutor, 2, funName,
                                                  gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
    if (!executor)
        return false;

    // Step 5 (omitted).

    // Step 6.
    FixedConstructArgs<1> cargs(cx);
    cargs[0].setObject(*executor);
    if (!Construct(cx, cVal, cargs, cVal, capability.promise()))
        return false;

    // Step 7.
    const Value& resolveVal = executor->getExtendedSlot(GetCapabilitiesExecutorSlots_Resolve);
    if (!IsCallable(resolveVal)) {
        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
                                  JSMSG_PROMISE_RESOLVE_FUNCTION_NOT_CALLABLE);
        return false;
    }

    // Step 8.
    const Value& rejectVal = executor->getExtendedSlot(GetCapabilitiesExecutorSlots_Reject);
    if (!IsCallable(rejectVal)) {
        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
                                  JSMSG_PROMISE_REJECT_FUNCTION_NOT_CALLABLE);
        return false;
    }

    // Step 9 (well, the equivalent for all of promiseCapabilities' fields.)
    capability.resolve().set(&resolveVal.toObject());
    capability.reject().set(&rejectVal.toObject());

    // Step 10.
    return true;
}

// ES2016, 25.4.1.5.1.
static bool
GetCapabilitiesExecutor(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);
    JSFunction* F = &args.callee().as<JSFunction>();

    // Steps 1-2 (implicit).

    // Steps 3-4.
    if (!F->getExtendedSlot(GetCapabilitiesExecutorSlots_Resolve).isUndefined() ||
        !F->getExtendedSlot(GetCapabilitiesExecutorSlots_Reject).isUndefined())
    {
        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
                                  JSMSG_PROMISE_CAPABILITY_HAS_SOMETHING_ALREADY);
        return false;
    }

    // Step 5.
    F->setExtendedSlot(GetCapabilitiesExecutorSlots_Resolve, args.get(0));

    // Step 6.
    F->setExtendedSlot(GetCapabilitiesExecutorSlots_Reject, args.get(1));

    // Step 7.
    args.rval().setUndefined();
    return true;
}

// ES2016, 25.4.1.7.
static MOZ_MUST_USE bool
RejectMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj, HandleValue reason_)
{
    Rooted<PromiseObject*> promise(cx);
    RootedValue reason(cx, reason_);

    mozilla::Maybe<AutoCompartment> ac;
    if (!IsProxy(promiseObj)) {
        promise = &promiseObj->as<PromiseObject>();
    } else {
        JSObject* unwrappedPromiseObj = UncheckedUnwrap(promiseObj);
        if (JS_IsDeadWrapper(unwrappedPromiseObj)) {
            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_DEAD_OBJECT);
            return false;
        }
        promise = &unwrappedPromiseObj->as<PromiseObject>();
        ac.emplace(cx, promise);

        // The rejection reason might've been created in a compartment with higher
        // privileges than the Promise's. In that case, object-type rejection
        // values might be wrapped into a wrapper that throws whenever the
        // Promise's reaction handler wants to do anything useful with it. To
        // avoid that situation, we synthesize a generic error that doesn't
        // expose any privileged information but can safely be used in the
        // rejection handler.
        if (!cx->compartment()->wrap(cx, &reason))
            return false;
        if (reason.isObject() && !CheckedUnwrap(&reason.toObject())) {
            // Async stacks are only properly adopted if there's at least one
            // interpreter frame active right now. If a thenable job with a
            // throwing `then` function got us here, that'll not be the case,
            // so we add one by throwing the error from self-hosted code.
            if (!GetInternalError(cx, JSMSG_PROMISE_ERROR_IN_WRAPPED_REJECTION_REASON, &reason))
                return false;
        }
    }

    return ResolvePromise(cx, promise, reason, JS::PromiseState::Rejected);
}

// ES2016, 25.4.1.8.
static MOZ_MUST_USE bool
TriggerPromiseReactions(JSContext* cx, HandleValue reactionsVal, JS::PromiseState state,
                        HandleValue valueOrReason)
{
    MOZ_ASSERT(state == JS::PromiseState::Fulfilled || state == JS::PromiseState::Rejected);

    RootedObject reactions(cx, &reactionsVal.toObject());

    if (reactions->is<PromiseReactionRecord>() || IsWrapper(reactions))
        return EnqueuePromiseReactionJob(cx, reactions, valueOrReason, state);

    HandleNativeObject reactionsList = reactions.as<NativeObject>();
    uint32_t reactionsCount = reactionsList->getDenseInitializedLength();
    MOZ_ASSERT(reactionsCount > 1, "Reactions list should be created lazily");

    RootedObject reaction(cx);
    for (uint32_t i = 0; i < reactionsCount; i++) {
        const Value& reactionVal = reactionsList->getDenseElement(i);
        MOZ_RELEASE_ASSERT(reactionVal.isObject());
        reaction = &reactionVal.toObject();
        if (!EnqueuePromiseReactionJob(cx, reaction, valueOrReason, state))
            return false;
    }

    return true;
}

// Implements PromiseReactionJob optimized for the case when the reaction
// handler is one of the default resolving functions as created by the
// CreateResolvingFunctions abstract operation.
static MOZ_MUST_USE bool
DefaultResolvingPromiseReactionJob(JSContext* cx, Handle<PromiseReactionRecord*> reaction,
                                   MutableHandleValue rval)
{
    MOZ_ASSERT(reaction->targetState() != JS::PromiseState::Pending);

    Rooted<PromiseObject*> promiseToResolve(cx, reaction->defaultResolvingPromise());

    // Testing functions allow to directly settle a promise without going
    // through the resolving functions. In that case the normal bookkeeping to
    // ensure only pending promises can be resolved doesn't apply and we need
    // to manually check for already settled promises. We still call
    // RunResolutionFunction for consistency with PromiseReactionJob.
    ResolutionMode resolutionMode = ResolveMode;
    RootedValue handlerResult(cx, UndefinedValue());
    if (promiseToResolve->state() == JS::PromiseState::Pending) {
        RootedValue argument(cx, reaction->handlerArg());

        // Step 6.
        bool ok;
        if (reaction->targetState() == JS::PromiseState::Fulfilled)
            ok = ResolvePromiseInternal(cx, promiseToResolve, argument);
        else
            ok = RejectPromiseInternal(cx, promiseToResolve, argument);

        if (!ok) {
            resolutionMode = RejectMode;
            if (!MaybeGetAndClearException(cx, &handlerResult))
                return false;
        }
    }

    // Steps 7-9.
    uint32_t hookSlot = resolutionMode == RejectMode
                        ? ReactionRecordSlot_Reject
                        : ReactionRecordSlot_Resolve;
    RootedObject callee(cx, reaction->getFixedSlot(hookSlot).toObjectOrNull());
    RootedObject promiseObj(cx, reaction->promise());
    if (!RunResolutionFunction(cx, callee, handlerResult, resolutionMode, promiseObj))
        return false;

    rval.setUndefined();
    return true;
}

static MOZ_MUST_USE bool
AsyncFunctionPromiseReactionJob(JSContext* cx, Handle<PromiseReactionRecord*> reaction,
                                MutableHandleValue rval)
{
    MOZ_ASSERT(reaction->isAsyncFunction());

    RootedValue handlerVal(cx, reaction->handler());
    RootedValue argument(cx, reaction->handlerArg());
    Rooted<PromiseObject*> resultPromise(cx, &reaction->promise()->as<PromiseObject>());
    RootedValue generatorVal(cx, resultPromise->getFixedSlot(PromiseSlot_AwaitGenerator));

    int32_t handlerNum = handlerVal.toInt32();

    // Await's handlers don't return a value, nor throw exception.
    // They fail only on OOM.
    if (handlerNum == PromiseHandlerAsyncFunctionAwaitedFulfilled) {
        if (!AsyncFunctionAwaitedFulfilled(cx, resultPromise, generatorVal, argument))
            return false;
    } else {
        MOZ_ASSERT(handlerNum == PromiseHandlerAsyncFunctionAwaitedRejected);
        if (!AsyncFunctionAwaitedRejected(cx, resultPromise, generatorVal, argument))
            return false;
    }

    rval.setUndefined();
    return true;
}

static MOZ_MUST_USE bool
AsyncGeneratorPromiseReactionJob(JSContext* cx, Handle<PromiseReactionRecord*> reaction,
                                 MutableHandleValue rval)
{
    MOZ_ASSERT(reaction->isAsyncGenerator());

    RootedValue handlerVal(cx, reaction->handler());
    RootedValue argument(cx, reaction->handlerArg());
    Rooted<AsyncGeneratorObject*> asyncGenObj(cx, reaction->asyncGenerator());

    int32_t handlerNum = handlerVal.toInt32();

    // Await's handlers don't return a value, nor throw exception.
    // They fail only on OOM.
    if (handlerNum == PromiseHandlerAsyncGeneratorAwaitedFulfilled) {
        // 4.1.1.
        if (!AsyncGeneratorAwaitedFulfilled(cx, asyncGenObj, argument))
            return false;
    } else if (handlerNum == PromiseHandlerAsyncGeneratorAwaitedRejected) {
        // 4.1.2.
        if (!AsyncGeneratorAwaitedRejected(cx, asyncGenObj, argument))
            return false;
    } else if (handlerNum == PromiseHandlerAsyncGeneratorResumeNextReturnFulfilled) {
        asyncGenObj->setCompleted();
        // 11.4.3.5.1 step 1.
        if (!AsyncGeneratorResolve(cx, asyncGenObj, argument, true))
            return false;
    } else if (handlerNum == PromiseHandlerAsyncGeneratorResumeNextReturnRejected) {
        asyncGenObj->setCompleted();
        // 11.4.3.5.2 step 1.
        if (!AsyncGeneratorReject(cx, asyncGenObj, argument))
            return false;
    } else if (handlerNum == PromiseHandlerAsyncGeneratorYieldReturnAwaitedFulfilled) {
        asyncGenObj->setExecuting();
        // 11.4.3.7 steps 8.d-e.
        if (!AsyncGeneratorYieldReturnAwaitedFulfilled(cx, asyncGenObj, argument))
            return false;
    } else {
        MOZ_ASSERT(handlerNum == PromiseHandlerAsyncGeneratorYieldReturnAwaitedRejected);
        asyncGenObj->setExecuting();
        // 11.4.3.7 step 8.c.
        if (!AsyncGeneratorYieldReturnAwaitedRejected(cx, asyncGenObj, argument))
            return false;
    }

    rval.setUndefined();
    return true;
}

// ES2016, 25.4.2.1.
/**
 * Callback triggering the fulfill/reject reaction for a resolved Promise,
 * to be invoked by the embedding during its processing of the Promise job
 * queue.
 *
 * See http://www.ecma-international.org/ecma-262/7.0/index.html#sec-jobs-and-job-queues
 *
 * A PromiseReactionJob is set as the native function of an extended
 * JSFunction object, with all information required for the job's
 * execution stored in in a reaction record in its first extended slot.
 */
static bool
PromiseReactionJob(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);

    RootedFunction job(cx, &args.callee().as<JSFunction>());

    RootedObject reactionObj(cx, &job->getExtendedSlot(ReactionJobSlot_ReactionRecord).toObject());

    // To ensure that the embedding ends up with the right entry global, we're
    // guaranteeing that the reaction job function gets created in the same
    // compartment as the handler function. That's not necessarily the global
    // that the job was triggered from, though.
    // We can find the triggering global via the job's reaction record. To go
    // back, we check if the reaction is a wrapper and if so, unwrap it and
    // enter its compartment.
    mozilla::Maybe<AutoCompartment> ac;
    if (!IsProxy(reactionObj)) {
        MOZ_RELEASE_ASSERT(reactionObj->is<PromiseReactionRecord>());
    } else {
        reactionObj = UncheckedUnwrap(reactionObj);
        if (JS_IsDeadWrapper(reactionObj)) {
            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_DEAD_OBJECT);
            return false;
        }
        MOZ_RELEASE_ASSERT(reactionObj->is<PromiseReactionRecord>());
        ac.emplace(cx, reactionObj);
    }

    // Steps 1-2.
    Handle<PromiseReactionRecord*> reaction = reactionObj.as<PromiseReactionRecord>();
    if (reaction->isDefaultResolvingHandler())
        return DefaultResolvingPromiseReactionJob(cx, reaction, args.rval());
    if (reaction->isAsyncFunction())
        return AsyncFunctionPromiseReactionJob(cx, reaction, args.rval());
    if (reaction->isAsyncGenerator())
        return AsyncGeneratorPromiseReactionJob(cx, reaction, args.rval());
    if (reaction->isDebuggerDummy())
        return true;

    // Step 3.
    RootedValue handlerVal(cx, reaction->handler());

    RootedValue argument(cx, reaction->handlerArg());

    RootedValue handlerResult(cx);
    ResolutionMode resolutionMode = ResolveMode;

    // Steps 4-6.
    if (handlerVal.isInt32()) {
        int32_t handlerNum = handlerVal.toInt32();

        // Step 4.
        if (handlerNum == PromiseHandlerIdentity) {
            handlerResult = argument;
        } else if (handlerNum == PromiseHandlerThrower) {
            // Step 5.
            resolutionMode = RejectMode;
            handlerResult = argument;
        } else {
            MOZ_ASSERT(handlerNum == PromiseHandlerAsyncFromSyncIteratorValueUnwrapDone ||
                       handlerNum == PromiseHandlerAsyncFromSyncIteratorValueUnwrapNotDone);

            bool done = handlerNum == PromiseHandlerAsyncFromSyncIteratorValueUnwrapDone;
            // Async Iteration proposal 11.1.3.2.5 step 1.
            JSObject* resultObj = CreateIterResultObject(cx, argument, done);
            if (!resultObj)
                return false;

            handlerResult = ObjectValue(*resultObj);
        }
    } else {
        MOZ_ASSERT(handlerVal.isObject());
        MOZ_ASSERT(IsCallable(handlerVal));

        // Step 6.
        if (!Call(cx, handlerVal, UndefinedHandleValue, argument, &handlerResult)) {
            resolutionMode = RejectMode;
            if (!MaybeGetAndClearException(cx, &handlerResult))
                return false;
        }
    }

    // Steps 7-9.
    uint32_t hookSlot = resolutionMode == RejectMode
                        ? ReactionRecordSlot_Reject
                        : ReactionRecordSlot_Resolve;
    RootedObject callee(cx, reaction->getFixedSlot(hookSlot).toObjectOrNull());
    RootedObject promiseObj(cx, reaction->promise());
    if (!RunResolutionFunction(cx, callee, handlerResult, resolutionMode, promiseObj))
        return false;

    args.rval().setUndefined();
    return true;
}

// ES2016, 25.4.2.2.
/**
 * Callback for resolving a thenable, to be invoked by the embedding during
 * its processing of the Promise job queue.
 *
 * See https://tc39.github.io/ecma262/#sec-jobs-and-job-queues
 *
 * A PromiseResolveThenableJob is set as the native function of an extended
 * JSFunction object, with all information required for the job's
 * execution stored in the function's extended slots.
 *
 * Usage of the function's extended slots is described in the ThenableJobSlots
 * enum.
 */
static bool
PromiseResolveThenableJob(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);

    RootedFunction job(cx, &args.callee().as<JSFunction>());
    RootedValue then(cx, job->getExtendedSlot(ThenableJobSlot_Handler));
    MOZ_ASSERT(then.isObject());
    MOZ_ASSERT(!IsWrapper(&then.toObject()));
    RootedNativeObject jobArgs(cx, &job->getExtendedSlot(ThenableJobSlot_JobData)
                                    .toObject().as<NativeObject>());

    RootedObject promise(cx, &jobArgs->getDenseElement(ThenableJobDataIndex_Promise).toObject());
    RootedValue thenable(cx, jobArgs->getDenseElement(ThenableJobDataIndex_Thenable));

    // Step 1.
    RootedObject resolveFn(cx);
    RootedObject rejectFn(cx);
    if (!CreateResolvingFunctions(cx, promise, &resolveFn, &rejectFn))
        return false;

    // Step 2.
    FixedInvokeArgs<2> args2(cx);
    args2[0].setObject(*resolveFn);
    args2[1].setObject(*rejectFn);

    // In difference to the usual pattern, we return immediately on success.
    RootedValue rval(cx);
    if (Call(cx, then, thenable, args2, &rval))
        return true;

    // Steps 3-4.
    if (!MaybeGetAndClearException(cx, &rval))
        return false;

    RootedValue rejectVal(cx, ObjectValue(*rejectFn));
    return Call(cx, rejectVal, UndefinedHandleValue, rval, &rval);
}

static MOZ_MUST_USE bool
OriginalPromiseThenWithoutSettleHandlers(JSContext* cx, Handle<PromiseObject*> promise,
                                         Handle<PromiseObject*> promiseToResolve);

/**
 * Specialization of PromiseResolveThenableJob when the `thenable` is a
 * built-in Promise object and the `then` property is the built-in
 * `Promise.prototype.then` function.
 *
 * A PromiseResolveBuiltinThenableJob is set as the native function of an
 * extended JSFunction object, with all information required for the job's
 * execution stored in the function's extended slots.
 *
 * Usage of the function's extended slots is described in the
 * BuiltinThenableJobSlots enum.
 */
static bool
PromiseResolveBuiltinThenableJob(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);

    RootedFunction job(cx, &args.callee().as<JSFunction>());
    RootedObject promise(cx, &job->getExtendedSlot(BuiltinThenableJobSlot_Promise).toObject());
    RootedObject thenable(cx, &job->getExtendedSlot(BuiltinThenableJobSlot_Thenable).toObject());

    assertSameCompartment(cx, promise, thenable);
    MOZ_ASSERT(promise->is<PromiseObject>());
    MOZ_ASSERT(thenable->is<PromiseObject>());

    // Step 1 (Skipped).

    // Step 2.
    // In difference to the usual pattern, we return immediately on success.
    if (OriginalPromiseThenWithoutSettleHandlers(cx, thenable.as<PromiseObject>(),
                                                 promise.as<PromiseObject>()))
    {
        return true;
    }

    // Steps 3-4.
    RootedValue exception(cx);
    if (!MaybeGetAndClearException(cx, &exception))
        return false;

    // Testing functions allow to directly settle a promise without going
    // through the resolving functions. In that case the normal bookkeeping to
    // ensure only pending promises can be resolved doesn't apply and we need
    // to manually check for already settled promises. The exception is simply
    // dropped when this case happens.
    if (promise->as<PromiseObject>().state() != JS::PromiseState::Pending)
        return true;

    return RejectPromiseInternal(cx, promise.as<PromiseObject>(), exception);
}

/**
 * Tells the embedding to enqueue a Promise resolve thenable job, based on
 * three parameters:
 * promiseToResolve_ - The promise to resolve, obviously.
 * thenable_ - The thenable to resolve the Promise with.
 * thenVal - The `then` function to invoke with the `thenable` as the receiver.
 */
static MOZ_MUST_USE bool
EnqueuePromiseResolveThenableJob(JSContext* cx, HandleValue promiseToResolve_,
                                 HandleValue thenable_, HandleValue thenVal)
{
    // Need to re-root these to enable wrapping them below.
    RootedValue promiseToResolve(cx, promiseToResolve_);
    RootedValue thenable(cx, thenable_);

    // We enter the `then` callable's compartment so that the job function is
    // created in that compartment.
    // That guarantees that the embedding ends up with the right entry global.
    // This is relevant for some html APIs like fetch that derive information
    // from said global.
    RootedObject then(cx, CheckedUnwrap(&thenVal.toObject()));
    AutoCompartment ac(cx, then);

    // Wrap the `promiseToResolve` and `thenable` arguments.
    if (!cx->compartment()->wrap(cx, &promiseToResolve))
        return false;

    MOZ_ASSERT(thenable.isObject());
    if (!cx->compartment()->wrap(cx, &thenable))
        return false;

    HandlePropertyName funName = cx->names().empty;
    RootedFunction job(cx, NewNativeFunction(cx, PromiseResolveThenableJob, 0, funName,
                                             gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
    if (!job)
        return false;

    // Store the `then` function on the callback.
    job->setExtendedSlot(ThenableJobSlot_Handler, ObjectValue(*then));

    // Create a dense array to hold the data needed for the reaction job to
    // work.
    // The layout is described in the ThenableJobDataIndices enum.
    RootedArrayObject data(cx, NewDenseFullyAllocatedArray(cx, ThenableJobDataLength));
    if (!data)
        return false;

    // Set the `promiseToResolve` and `thenable` arguments.
    data->setDenseInitializedLength(ThenableJobDataLength);
    data->initDenseElement(ThenableJobDataIndex_Promise, promiseToResolve);
    data->initDenseElement(ThenableJobDataIndex_Thenable, thenable);

    // Store the data array on the reaction job.
    job->setExtendedSlot(ThenableJobSlot_JobData, ObjectValue(*data));

    // At this point the promise is guaranteed to be wrapped into the job's
    // compartment.
    RootedObject promise(cx, &promiseToResolve.toObject());

    RootedObject incumbentGlobal(cx, cx->runtime()->getIncumbentGlobal(cx));
    return cx->runtime()->enqueuePromiseJob(cx, job, promise, incumbentGlobal);
}

/**
 * Tells the embedding to enqueue a Promise resolve thenable built-in job,
 * based on two parameters:
 * promiseToResolve - The promise to resolve, obviously.
 * thenable - The thenable to resolve the Promise with.
 */
static MOZ_MUST_USE bool
EnqueuePromiseResolveThenableBuiltinJob(JSContext* cx, HandleObject promiseToResolve,
                                        HandleObject thenable)
{
    assertSameCompartment(cx, promiseToResolve, thenable);
    MOZ_ASSERT(promiseToResolve->is<PromiseObject>());
    MOZ_ASSERT(thenable->is<PromiseObject>());

    HandlePropertyName funName = cx->names().empty;
    RootedFunction job(cx, NewNativeFunction(cx, PromiseResolveBuiltinThenableJob, 0, funName,
                                             gc::AllocKind::FUNCTION_EXTENDED, GenericObject));
    if (!job)
        return false;

    // Store the promise and the thenable on the reaction job.
    job->setExtendedSlot(BuiltinThenableJobSlot_Promise, ObjectValue(*promiseToResolve));
    job->setExtendedSlot(BuiltinThenableJobSlot_Thenable, ObjectValue(*thenable));

    RootedObject incumbentGlobal(cx, cx->runtime()->getIncumbentGlobal(cx));
    return cx->runtime()->enqueuePromiseJob(cx, job, promiseToResolve, incumbentGlobal);
}

static MOZ_MUST_USE bool
AddDummyPromiseReactionForDebugger(JSContext* cx, Handle<PromiseObject*> promise,
                                   HandleObject dependentPromise);

static MOZ_MUST_USE bool
AddPromiseReaction(JSContext* cx, Handle<PromiseObject*> promise,
                   Handle<PromiseReactionRecord*> reaction);

static JSFunction*
GetResolveFunctionFromReject(JSFunction* reject)
{
    MOZ_ASSERT(reject->maybeNative() == RejectPromiseFunction);
    Value resolveFunVal = reject->getExtendedSlot(RejectFunctionSlot_ResolveFunction);
    MOZ_ASSERT(IsNativeFunction(resolveFunVal, ResolvePromiseFunction));
    return &resolveFunVal.toObject().as<JSFunction>();
}

static JSFunction*
GetRejectFunctionFromResolve(JSFunction* resolve)
{
    MOZ_ASSERT(resolve->maybeNative() == ResolvePromiseFunction);
    Value rejectFunVal = resolve->getExtendedSlot(ResolveFunctionSlot_RejectFunction);
    MOZ_ASSERT(IsNativeFunction(rejectFunVal, RejectPromiseFunction));
    return &rejectFunVal.toObject().as<JSFunction>();
}

static JSFunction*
GetResolveFunctionFromPromise(PromiseObject* promise)
{
    Value rejectFunVal = promise->getFixedSlot(PromiseSlot_RejectFunction);
    if (rejectFunVal.isUndefined())
        return nullptr;
    JSObject* rejectFunObj = &rejectFunVal.toObject();

    // We can safely unwrap it because all we want is to get the resolve
    // function.
    if (IsWrapper(rejectFunObj))
        rejectFunObj = UncheckedUnwrap(rejectFunObj);

    if (!rejectFunObj->is<JSFunction>())
        return nullptr;

    JSFunction* rejectFun = &rejectFunObj->as<JSFunction>();

    // Only the original RejectPromiseFunction has a reference to the resolve
    // function.
    if (rejectFun->maybeNative() != &RejectPromiseFunction)
        return nullptr;

    // The reject function was already called and cleared its resolve-function
    // extended slot.
    if (rejectFun->getExtendedSlot(RejectFunctionSlot_ResolveFunction).isUndefined())
        return nullptr;

    return GetResolveFunctionFromReject(rejectFun);
}

static void
ClearResolutionFunctionSlots(JSFunction* resolutionFun)
{
    JSFunction* resolve;
    JSFunction* reject;
    if (resolutionFun->maybeNative() == ResolvePromiseFunction) {
        resolve = resolutionFun;
        reject = GetRejectFunctionFromResolve(resolutionFun);
    } else {
        resolve = GetResolveFunctionFromReject(resolutionFun);
        reject = resolutionFun;
    }

    resolve->setExtendedSlot(ResolveFunctionSlot_Promise, UndefinedValue());
    resolve->setExtendedSlot(ResolveFunctionSlot_RejectFunction, UndefinedValue());

    reject->setExtendedSlot(RejectFunctionSlot_Promise, UndefinedValue());
    reject->setExtendedSlot(RejectFunctionSlot_ResolveFunction, UndefinedValue());
}

// ES2016, 25.4.3.1. steps 3-7.
static MOZ_MUST_USE PromiseObject*
CreatePromiseObjectInternal(JSContext* cx, HandleObject proto /* = nullptr */,
                            bool protoIsWrapped /* = false */, bool informDebugger /* = true */)
{
    // Step 3.
    Rooted<PromiseObject*> promise(cx);
    // Enter the unwrapped proto's compartment, if that's different from
    // the current one.
    // All state stored in a Promise's fixed slots must be created in the
    // same compartment, so we get all of that out of the way here.
    // (Except for the resolution functions, which are created below.)
    mozilla::Maybe<AutoCompartment> ac;
    if (protoIsWrapped)
        ac.emplace(cx, proto);

    promise = NewObjectWithClassProto<PromiseObject>(cx, proto);
    if (!promise)
        return nullptr;

    // Step 4.
    promise->setFixedSlot(PromiseSlot_Flags, Int32Value(0));

    // Steps 5-6.
    // Omitted, we allocate our single list of reaction records lazily.

    // Step 7.
    // Implicit, the handled flag is unset by default.

    // Store an allocation stack so we can later figure out what the
    // control flow was for some unexpected results. Frightfully expensive,
    // but oh well.
    if (ShouldCaptureDebugInfo(cx)) {
        PromiseDebugInfo* debugInfo = PromiseDebugInfo::create(cx, promise);
        if (!debugInfo)
            return nullptr;
    }

    // Let the Debugger know about this Promise.
    if (informDebugger)
        JS::dbg::onNewPromise(cx, promise);

    return promise;
}

// ES2016, 25.4.3.1.
static bool
PromiseConstructor(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);

    // Step 1.
    if (!ThrowIfNotConstructing(cx, args, "Promise"))
        return false;

    // Step 2.
    HandleValue executorVal = args.get(0);
    if (!IsCallable(executorVal))
        return ReportIsNotFunction(cx, executorVal);
    RootedObject executor(cx, &executorVal.toObject());

    // Steps 3-10.
    RootedObject newTarget(cx, &args.newTarget().toObject());
    RootedObject originalNewTarget(cx, newTarget);
    bool needsWrapping = false;

    // If the constructor is called via an Xray wrapper, then the newTarget
    // hasn't been unwrapped. We want that because, while the actual instance
    // should be created in the target compartment, the constructor's code
    // should run in the wrapper's compartment.
    //
    // This is so that the resolve and reject callbacks get created in the
    // wrapper's compartment, which is required for code in that compartment
    // to freely interact with it, and, e.g., pass objects as arguments, which
    // it wouldn't be able to if the callbacks were themselves wrapped in Xray
    // wrappers.
    //
    // At the same time, just creating the Promise itself in the wrapper's
    // compartment wouldn't be helpful: if the wrapper forbids interactions
    // with objects except for specific actions, such as calling them, then
    // the code we want to expose it to can't actually treat it as a Promise:
    // calling .then on it would throw, for example.
    //
    // Another scenario where it's important to create the Promise in a
    // different compartment from the resolution functions is when we want to
    // give non-privileged code a Promise resolved with the result of a
    // Promise from privileged code; as a return value of a JS-implemented
    // API, say. If the resolution functions were unprivileged, then resolving
    // with a privileged Promise would cause `resolve` to attempt accessing
    // .then on the passed Promise, which would throw an exception, so we'd
    // just end up with a rejected Promise. Really, we want to chain the two
    // Promises, with the unprivileged one resolved with the resolution of the
    // privileged one.
    if (IsWrapper(newTarget)) {
        JSObject* unwrappedNewTarget = CheckedUnwrap(newTarget);
        MOZ_ASSERT(unwrappedNewTarget);
        MOZ_ASSERT(unwrappedNewTarget != newTarget);

        newTarget = unwrappedNewTarget;
        {
            AutoCompartment ac(cx, newTarget);
            Handle<GlobalObject*> global = cx->global();
            JSFunction* promiseCtor = GlobalObject::getOrCreatePromiseConstructor(cx, global);
            if (!promiseCtor)
                return false;

            // Promise subclasses don't get the special Xray treatment, so
            // we only need to do the complex wrapping and unwrapping scheme
            // described above for instances of Promise itself.
            if (newTarget == promiseCtor)
                needsWrapping = true;
        }
    }

    RootedObject proto(cx);
    if (!GetPrototypeFromConstructor(cx, needsWrapping ? newTarget : originalNewTarget, &proto))
        return false;
    if (needsWrapping && !cx->compartment()->wrap(cx, &proto))
        return false;
    Rooted<PromiseObject*> promise(cx, PromiseObject::create(cx, executor, proto, needsWrapping));
    if (!promise)
        return false;

    // Step 11.
    args.rval().setObject(*promise);
    if (needsWrapping)
        return cx->compartment()->wrap(cx, args.rval());
    return true;
}

// ES2016, 25.4.3.1. steps 3-11.
/* static */ PromiseObject*
PromiseObject::create(JSContext* cx, HandleObject executor, HandleObject proto /* = nullptr */,
                      bool needsWrapping /* = false */)
{
    MOZ_ASSERT(executor->isCallable());

    RootedObject usedProto(cx, proto);
    // If the proto is wrapped, that means the current function is running
    // with a different compartment active from the one the Promise instance
    // is to be created in.
    // See the comment in PromiseConstructor for details.
    if (needsWrapping) {
        MOZ_ASSERT(proto);
        usedProto = CheckedUnwrap(proto);
        if (!usedProto) {
            /* ReportAccessDenied(cx); */
            return nullptr;
        }
    }


    // Steps 3-7.
    Rooted<PromiseObject*> promise(cx, CreatePromiseObjectInternal(cx, usedProto, needsWrapping,
                                                                   false));
    if (!promise)
        return nullptr;

    RootedObject promiseObj(cx, promise);
    if (needsWrapping && !cx->compartment()->wrap(cx, &promiseObj))
        return nullptr;

    // Step 8.
    // The resolving functions are created in the compartment active when the
    // (maybe wrapped) Promise constructor was called. They contain checks and
    // can unwrap the Promise if required.
    RootedObject resolveFn(cx);
    RootedObject rejectFn(cx);
    if (!CreateResolvingFunctions(cx, promiseObj, &resolveFn, &rejectFn))
        return nullptr;

    // Need to wrap the resolution functions before storing them on the Promise.
    if (needsWrapping) {
        AutoCompartment ac(cx, promise);
        RootedObject wrappedRejectFn(cx, rejectFn);
        if (!cx->compartment()->wrap(cx, &wrappedRejectFn))
            return nullptr;
        promise->setFixedSlot(PromiseSlot_RejectFunction, ObjectValue(*wrappedRejectFn));
    } else {
        promise->setFixedSlot(PromiseSlot_RejectFunction, ObjectValue(*rejectFn));
    }

    // Step 9.
    bool success;
    {
        FixedInvokeArgs<2> args(cx);
        args[0].setObject(*resolveFn);
        args[1].setObject(*rejectFn);

        RootedValue calleeOrRval(cx, ObjectValue(*executor));
        success = Call(cx, calleeOrRval, UndefinedHandleValue, args, &calleeOrRval);
    }

    // Step 10.
    if (!success) {
        RootedValue exceptionVal(cx);
        if (!MaybeGetAndClearException(cx, &exceptionVal))
            return nullptr;

        RootedValue calleeOrRval(cx, ObjectValue(*rejectFn));
        if (!Call(cx, calleeOrRval, UndefinedHandleValue, exceptionVal, &calleeOrRval))
            return nullptr;
    }

    // Let the Debugger know about this Promise.
    JS::dbg::onNewPromise(cx, promise);

    // Step 11.
    return promise;
}

// ES2016, 25.4.3.1. skipping creation of resolution functions and executor
// function invocation.
/* static */ PromiseObject*
PromiseObject::createSkippingExecutor(JSContext* cx)
{
    return CreatePromiseObjectWithoutResolutionFunctions(cx);
}

class MOZ_STACK_CLASS PromiseForOfIterator : public JS::ForOfIterator {
  public:
    using JS::ForOfIterator::ForOfIterator;

    bool isOptimizedDenseArrayIteration() {
        MOZ_ASSERT(valueIsIterable());
        return index != NOT_ARRAY && IsPackedArray(iterator);
    }
};

static MOZ_MUST_USE bool
PerformPromiseAll(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                  Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done);

static MOZ_MUST_USE bool
PerformPromiseAllSettled(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                         Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done);

static MOZ_MUST_USE bool
PerformPromiseAny(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                  Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done);

static MOZ_MUST_USE bool
PerformPromiseRace(JSContext* cx, PromiseForOfIterator& iterator, HandleObject C,
                   Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done);

enum class CombinatorKind { All, AllSettled, Any, Race };

// ES2020 draft rev e97c95d064750fb949b6778584702dd658cf5624
//
// Unified implementation of
// 25.6.4.1 Promise.all ( iterable )
// 25.6.4.2 Promise.allSettled ( iterable )
// 25.6.4.4 Promise.race ( iterable )
//
// Promise.any (Stage 3 proposal)
// https://tc39.es/proposal-promise-any/
//
// Promise.any ( iterable )
static MOZ_MUST_USE bool
CommonPromiseCombinator(JSContext* cx, CallArgs& args, CombinatorKind mode)
{
    HandleValue iterable = args.get(0);

    // Step 2 (reordered).
    HandleValue CVal = args.thisv();
    if (!CVal.isObject()) {
        const char* message;
        switch (mode) {
          case CombinatorKind::All:
            message = "Receiver of Promise.all call";
            break;
          case CombinatorKind::AllSettled:
            message = "Receiver of Promise.allSettled call";
            break;
          case CombinatorKind::Any:
            message = "Receiver of Promise.any call";
            break;
          case CombinatorKind::Race:
            message = "Receiver of Promise.race call";
            break;
        }
        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_NOT_NONNULL_OBJECT,
                                  message);
        return false;
    }

    // Step 1.
    RootedObject C(cx, &CVal.toObject());

    // Step 3.
    Rooted<PromiseCapability> promiseCapability(cx);
    if (!NewPromiseCapability(cx, C, &promiseCapability, false))
        return false;

    // Regardless of whether we got a fast-path promise from NewPromiseCapability,
    // the spec requires us to throw TypeError if |Promise.reject| is not callable,
    // ie. overwritten in this context. Check the prototype here.
    RootedValue promiseResolve(cx, UndefinedValue());
    {
        JSObject* promiseCtor = GlobalObject::getOrCreatePromiseConstructor(cx, cx->global());
        if (!promiseCtor) {
            return false;
        }

        PromiseLookup& promiseLookup = cx->compartment()->promiseLookup;
        if (C != promiseCtor || !promiseLookup.isDefaultPromiseState(cx)) {
            // Step 3. Let promiseResolve be GetPromiseResolve(C).

            // GetPromiseResolve
            // Step 1. Let promiseResolve be ? Get(promiseConstructor, "resolve").
            if (!GetProperty(cx, C, C, cx->names().resolve, &promiseResolve)) {
                // Step 4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
                return AbruptRejectPromise(cx, args, promiseCapability);
            }

            // GetPromiseResolve
            // Step 2. If IsCallable(promiseResolve) is false,
            //         throw a TypeError exception.
            if (!IsCallable(promiseResolve)) {
                ReportIsNotFunction(cx, promiseResolve);

                // Step 4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
                return AbruptRejectPromise(cx, args, promiseCapability);
            }
        }
    }

    // Steps 5.
    PromiseForOfIterator iter(cx);
    if (!iter.init(iterable, JS::ForOfIterator::AllowNonIterable))
        return AbruptRejectPromise(cx, args, promiseCapability);

    if (!iter.valueIsIterable()) {
        const char* message;
        switch (mode) {
          case CombinatorKind::All:
            message = "Argument of Promise.all";
            break;
          case CombinatorKind::AllSettled:
            message = "Argument of Promise.allSettled";
            break;
          case CombinatorKind::Any:
            message = "Argument of Promise.any";
            break;
          case CombinatorKind::Race:
            message = "Argument of Promise.race";
            break;
        }
        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_NOT_ITERABLE,
                                  message);
        return AbruptRejectPromise(cx, args, promiseCapability);
    }

    // Step 6 (implicit).

    // Step 7.
    bool done, result;
    switch (mode) {
      case CombinatorKind::All:
        result = PerformPromiseAll(cx, iter, C, promiseCapability, promiseResolve, &done);
        break;
      case CombinatorKind::AllSettled:
        result = PerformPromiseAllSettled(cx, iter, C, promiseCapability, promiseResolve, &done);
        break;
      case CombinatorKind::Any:
        result = PerformPromiseAny(cx, iter, C, promiseCapability, promiseResolve, &done);
        break;
      case CombinatorKind::Race:
        result = PerformPromiseRace(cx, iter, C, promiseCapability, promiseResolve, &done);
        break;
    }

    // Step 8.
    if (!result) {
        // Step 8.a.
        if (!done)
            iter.closeThrow();

        // Step 8.b.
        return AbruptRejectPromise(cx, args, promiseCapability);
    }

    // Step 9.
    args.rval().setObject(*promiseCapability.promise());
    return true;
}

// ES2020 draft rev a09fc232c137800dbf51b6204f37fdede4ba1646
// 25.6.4.1 Promise.all ( iterable )
static bool Promise_static_all(JSContext* cx, unsigned argc, Value* vp) {
  CallArgs args = CallArgsFromVp(argc, vp);
  return CommonPromiseCombinator(cx, args, CombinatorKind::All);
}

static MOZ_MUST_USE bool
PerformPromiseThen(JSContext* cx, Handle<PromiseObject*> promise, HandleValue onFulfilled_,
                   HandleValue onRejected_, Handle<PromiseCapability> resultCapability);

static MOZ_MUST_USE bool
PerformPromiseThenWithoutSettleHandlers(JSContext* cx, Handle<PromiseObject*> promise,
                                        Handle<PromiseObject*> promiseToResolve,
                                        Handle<PromiseCapability> resultCapability);

static JSFunction* NewPromiseCombinatorElementFunction(
    JSContext* cx, Native native,
    Handle<PromiseCombinatorDataHolder*> dataHolder, uint32_t index);

static bool PromiseAllResolveElementFunction(JSContext* cx, unsigned argc, Value* vp);

// Unforgeable version of ES2016, 25.4.4.1.
MOZ_MUST_USE JSObject*
js::GetWaitForAllPromise(JSContext* cx, const JS::AutoObjectVector& promises)
{
#ifdef DEBUG
    for (size_t i = 0, len = promises.length(); i < len; i++) {
        JSObject* obj = promises[i];
        assertSameCompartment(cx, obj);
        MOZ_ASSERT(UncheckedUnwrap(obj)->is<PromiseObject>());
    }
#endif

    // Step 1.
    RootedObject C(cx, GlobalObject::getOrCreatePromiseConstructor(cx, cx->global()));
    if (!C)
        return nullptr;

    // Step 2 (omitted).

    // Step 3.
    Rooted<PromiseCapability> resultCapability(cx);
    if (!NewPromiseCapability(cx, C, &resultCapability, false))
        return nullptr;

    // Steps 4-6 (omitted).

    // Step 7.
    // Implemented as an inlined, simplied version of ES2016 25.4.4.1.1, PerformPromiseAll.
    {
        uint32_t promiseCount = promises.length();
        // Sub-steps 1-2 (omitted).

        // Sub-step 3.
        Rooted<PromiseCombinatorElements> values(cx);
        {
          auto* valuesArray = NewDenseFullyAllocatedArray(cx, promiseCount);
          if (!valuesArray) {
            return nullptr;
          }
          valuesArray->ensureDenseInitializedLength(cx, 0, promiseCount);

          values.initialize(valuesArray);
        }

        // Sub-step 4.
        // Create our data holder that holds all the things shared across
        // every step of the iterator.  In particular, this holds the
        // remainingElementsCount (as an integer reserved slot), the array of
        // values, and the resolve function from our PromiseCapability.
        Rooted<PromiseCombinatorDataHolder*> dataHolder(cx);
        dataHolder = PromiseCombinatorDataHolder::New(cx, resultCapability.promise(),
                                                      values, resultCapability.resolve());
        if (!dataHolder)
            return nullptr;

        // Call PerformPromiseThen with resolve and reject set to nullptr.
        Rooted<PromiseCapability> resultCapabilityWithoutResolving(cx);
        resultCapabilityWithoutResolving.promise().set(resultCapability.promise());

        // Sub-step 5 (inline in loop-header below).

        // Sub-step 6.
        for (uint32_t index = 0; index < promiseCount; index++) {
            // Steps a-c (omitted).
            // Step d (implemented after the loop).
            // Steps e-g (omitted).

            // Step h.
            values.unwrappedArray()->setDenseElement(index, UndefinedHandleValue);

            // Step i, vastly simplified.
            RootedObject nextPromiseObj(cx, promises[index]);

            // Steps j-o.
            JSFunction* resolveFunc = NewPromiseCombinatorElementFunction(
                                          cx, PromiseAllResolveElementFunction, dataHolder, index);
            if (!resolveFunc)
                return nullptr;

            // Step p.
            dataHolder->increaseRemainingCount();

            // Step q, very roughly.
            RootedValue resolveFunVal(cx, ObjectValue(*resolveFunc));
            RootedValue rejectFunVal(cx, ObjectValue(*resultCapability.reject()));
            Rooted<PromiseObject*> nextPromise(cx);

            // GetWaitForAllPromise is used internally only and must not
            // trigger content-observable effects when registering a reaction.
            // It's also meant to work on wrapped Promises, potentially from
            // compartments with principals inaccessible from the current
            // compartment. To make that work, it unwraps promises with
            // UncheckedUnwrap,
            nextPromise = &UncheckedUnwrap(nextPromiseObj)->as<PromiseObject>();

            if (!PerformPromiseThen(cx, nextPromise, resolveFunVal, rejectFunVal,
                                    resultCapabilityWithoutResolving))
            {
                return nullptr;
            }

            // Step r (inline in loop-header).
        }

        // Sub-step d.i (implicit).
        // Sub-step d.ii.
        int32_t remainingCount = dataHolder->decreaseRemainingCount();

        // Sub-step d.iii-iv.
        if (remainingCount == 0) {
            if (!ResolvePromiseInternal(cx, resultCapability.promise(), values.value()))
                return nullptr;
        }
    }

    // Step 8 (omitted).

    // Step 9.
    return resultCapability.promise();
}

static MOZ_MUST_USE bool
RunResolutionFunction(JSContext *cx, HandleObject resolutionFun, HandleValue result,
                      ResolutionMode mode, HandleObject promiseObj)
{
    // The absence of a resolve/reject function can mean that, as an
    // optimization, those weren't created. In that case, a flag is set on
    // the Promise object. (It's also possible to not have a resolution
    // function without that flag being set. This can occur if a Promise
    // subclass constructor passes null/undefined to `super()`.)
    // There are also reactions where the Promise itself is missing. For
    // those, there's nothing left to do here.
    assertSameCompartment(cx, resolutionFun);
    assertSameCompartment(cx, result);
    assertSameCompartment(cx, promiseObj);
    if (resolutionFun) {
        RootedValue calleeOrRval(cx, ObjectValue(*resolutionFun));
        return Call(cx, calleeOrRval, UndefinedHandleValue, result, &calleeOrRval);
    }

    if (!promiseObj) {
        if (mode == RejectMode) {
            // The rejection will never be handled, given the returned promise
            // is known to be unused, and already optimized away.
            //
            // Create temporary Promise object and reject it, in order to
            // report the unhandled rejection.
            //
            // Allocation time points wrong time, but won't matter much.
            Rooted<PromiseObject*> temporaryPromise(cx);
            temporaryPromise = CreatePromiseObjectWithoutResolutionFunctions(cx);
            if (!temporaryPromise) {
                cx->clearPendingException();
                return true;
            }
            return RejectPromiseInternal(cx, temporaryPromise, result);
        }
        return true;
    }

    Handle<PromiseObject*> promise = promiseObj.as<PromiseObject>();
    if (promise->state() != JS::PromiseState::Pending)
        return true;

    if (!PromiseHasAnyFlag(*promise, PROMISE_FLAG_DEFAULT_RESOLVING_FUNCTIONS))
        return true;

    if (mode == ResolveMode)
        return ResolvePromiseInternal(cx, promise, result);

    return RejectPromiseInternal(cx, promise, result);
}

static MOZ_MUST_USE JSObject*
CommonStaticResolveRejectImpl(JSContext* cx, HandleValue thisVal, HandleValue argVal,
                              ResolutionMode mode);

static bool
IsPromiseSpecies(JSContext* cx, JSFunction* species);

// ES2020 draft rev e97c95d064750fb949b6778584702dd658cf5624
// 25.6.4.1.1 Runtime Semantics: PerformPromiseAll, steps 5-6 and step 8.
// 25.6.4.2.1 Runtime Semantics: PerformPromiseAllSettled, steps 5-6 and step 8.
// 25.6.4.4.1 Runtime Semantics: PerformPromiseRace, steps 3-5.
//
// Promise.any (Stage 3 proposal)
// https://tc39.es/proposal-promise-any/
// Runtime Semantics: PerformPromiseAny, steps 6-8.
template <typename T>
static MOZ_MUST_USE bool
CommonPerformPromiseCombinator(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                               HandleObject resultPromise, HandleValue promiseResolve, bool* done,
                               bool resolveReturnsUndefined, T getResolveAndReject)
{
    RootedObject promiseCtor(cx, GlobalObject::getOrCreatePromiseConstructor(cx, cx->global()));
    if (!promiseCtor)
        return false;

    // Optimized dense array iteration ensures no side-effects take place
    // during the iteration.
    bool iterationMayHaveSideEffects = !iterator.isOptimizedDenseArrayIteration();

    PromiseLookup& promiseLookup = cx->compartment()->promiseLookup;

    // Try to optimize when the Promise object is in its default state, seeded
    // with |C == promiseCtor| because we can only perform this optimization
    // for the builtin Promise constructor.
    bool isDefaultPromiseState = C == promiseCtor && promiseLookup.isDefaultPromiseState(cx);
    bool validatePromiseState = iterationMayHaveSideEffects;

    RootedValue CVal(cx, ObjectValue(*C));
    RootedValue resolveFunVal(cx);
    RootedValue rejectFunVal(cx);

    // We're reusing rooted variables in the loop below, so we don't need to
    // declare a gazillion different rooted variables here. Rooted variables
    // which are reused include "Or" in their name.
    RootedValue nextValueOrNextPromise(cx);
    RootedObject nextPromiseObj(cx);
    RootedValue resolveOrThen(cx);
    RootedObject thenSpeciesOrBlockedPromise(cx);
    Rooted<PromiseCapability> thenCapability(cx);

    while (true) {
        // Steps a-c, e-g.
        RootedValue& nextValue = nextValueOrNextPromise;
        if (!iterator.next(&nextValue, done)) {
            // Steps b, f.
            *done = true;

            // Steps c, g.
            return false;
        }

        // Step d.
        if (*done)
            return true;

        // Set to false when we can skip the [[Get]] for "then" and instead
        // use the built-in Promise.prototype.then function.
        bool getThen = true;

        if (isDefaultPromiseState && validatePromiseState)
            isDefaultPromiseState = promiseLookup.isDefaultPromiseState(cx);

        RootedValue& nextPromise = nextValueOrNextPromise;
        if (isDefaultPromiseState) {
            PromiseObject* nextValuePromise = nullptr;
            if (nextValue.isObject() && nextValue.toObject().is<PromiseObject>())
                nextValuePromise = &nextValue.toObject().as<PromiseObject>();

            if (nextValuePromise &&
                promiseLookup.isDefaultInstanceWhenPromiseStateIsSane(cx, nextValuePromise))
            {
                // The below steps don't produce any side-effects, so we can
                // skip the Promise state revalidation in the next iteration
                // when the iterator itself also doesn't produce any
                // side-effects.
                validatePromiseState = iterationMayHaveSideEffects;

                // 25.6.4.1.1, step 6.i.
                // 25.6.4.3.1, step 3.h.
                // Promise.resolve is a no-op for the default case.
                MOZ_ASSERT(&nextPromise.toObject() == nextValuePromise);

                // `nextPromise` uses the built-in `then` function.
                getThen = false;
            } else {
                // Need to revalidate the Promise state in the next iteration,
                // because CommonStaticResolveRejectImpl may have modified it.
                validatePromiseState = true;

                // 25.6.4.1.1, step 6.i.
                // 25.6.4.3.1, step 3.h.
                // Inline the call to Promise.resolve.
                JSObject* res = CommonStaticResolveRejectImpl(cx, CVal, nextValue, ResolveMode);
                if (!res)
                    return false;

                nextPromise.setObject(*res);
            }
        } else {
            // |promiseResolve| gets passed in from |CommonPromiseCombinator| via |PerformPromise*|.
            // It is undefined when the Promise constructor was initially in its default state.
            // Otherwise it is the value of |Promise.resolve|.
            if (promiseResolve.isUndefined()) {
                // 25.6.4.1.1, step 6.i.
                // 25.6.4.3.1, step 3.h.
                // Inline the call to Promise.resolve.
                JSObject* res = CommonStaticResolveRejectImpl(cx, CVal, nextValue, ResolveMode);
                if (!res)
                    return false;

                nextPromise.setObject(*res);
            } else {
                // 25.6.4.1.1, step 6.i.
                // 25.6.4.3.1, step 3.h.
                // Sadly, because someone could have overridden
                // "resolve" on the canonical Promise constructor.
                
                // Step {i, h}. Let nextPromise be
                //              ? Call(promiseResolve, constructor, « nextValue »).
                if (!Call(cx, promiseResolve, CVal, nextValue, &nextPromise))
                    return false;
            }
        }

        // Get the resolving functions for this iteration.
        // 25.6.4.1.1, steps 6.j-q.
        if (!getResolveAndReject(&resolveFunVal, &rejectFunVal))
            return false;

        // Call |nextPromise.then| with the provided hooks and add
        // |resultPromise| to the list of dependent promises.
        //
        // If |nextPromise.then| is the original |Promise.prototype.then|
        // function and the call to |nextPromise.then| would use the original
        // |Promise| constructor to create the resulting promise, we skip the
        // call to |nextPromise.then| and thus creating a new promise that
        // would not be observable by content.

        // 25.6.4.1.1, step 6.r.
        // 25.6.4.3.1, step 3.i.
        nextPromiseObj = ToObject(cx, nextPromise);
        if (!nextPromiseObj)
            return false;

        RootedValue& thenVal = resolveOrThen;
        bool isBuiltinThen;
        if (getThen) {
            // We don't use the Promise lookup cache here, because this code
            // is only called when we had a lookup cache miss, so it's likely
            // we'd get another cache miss when trying to use the cache here.
            if (!GetProperty(cx, nextPromiseObj, nextPromise, cx->names().then, &thenVal))
                return false;

            // |nextPromise| is an unwrapped Promise, and |then| is the
            // original |Promise.prototype.then|, inline it here.
            isBuiltinThen = nextPromiseObj->is<PromiseObject>() &&
                            IsNativeFunction(thenVal, Promise_then);
        } else {
            isBuiltinThen = true;
        }

        // By default, the blocked promise is added as an extra entry to the
        // rejected promises list.
        bool addToDependent = true;

        if (isBuiltinThen) {
            MOZ_ASSERT(nextPromise.isObject());
            MOZ_ASSERT(&nextPromise.toObject() == nextPromiseObj);

            // 25.6.5.4, step 3.
            RootedObject& thenSpecies = thenSpeciesOrBlockedPromise;
            if (getThen) {
                thenSpecies = SpeciesConstructor(cx, nextPromiseObj, JSProto_Promise,
                                                 IsPromiseSpecies);
                if (!thenSpecies)
                    return false;
            } else {
                thenSpecies = promiseCtor;
            }

            // The fast path here and the one in NewPromiseCapability may not
            // set the resolve and reject handlers, so we need to clear the
            // fields in case they were set in the previous iteration.
            thenCapability.resolve().set(nullptr);
            thenCapability.reject().set(nullptr);

            // Skip the creation of a built-in Promise object if:
            // 1. `thenSpecies` is the built-in Promise constructor.
            // 2. `resolveFun` doesn't return an object, which ensures no
            //    side-effects take place in ResolvePromiseInternal.
            // 3. The result promise is a built-in Promise object.
            // 4. The result promise doesn't use the default resolving
            //    functions, which in turn means RunResolutionFunction when
            //    called from PromiseRectionJob won't try to resolve the
            //    promise.
            if (thenSpecies == promiseCtor &&
                resolveReturnsUndefined &&
                resultPromise->is<PromiseObject>() &&
                !PromiseHasAnyFlag(resultPromise->as<PromiseObject>(),
                                   PROMISE_FLAG_DEFAULT_RESOLVING_FUNCTIONS))
            {
                thenCapability.promise().set(resultPromise);
                addToDependent = false;
            } else {
                // 25.6.5.4, step 4.
                if (!NewPromiseCapability(cx, thenSpecies, &thenCapability, true))
                    return false;
            }

            // 25.6.5.4, step 5.
            Handle<PromiseObject*> promise = nextPromiseObj.as<PromiseObject>();
            if (!PerformPromiseThen(cx, promise, resolveFunVal, rejectFunVal, thenCapability))
                return false;
        } else {
            // Optimization failed, do the normal call.
            RootedValue& ignored = thenVal;
            if (!Call(cx, thenVal, nextPromise, resolveFunVal, rejectFunVal, &ignored))
                return false;

            // In case the value to depend on isn't an object at all, there's
            // nothing more to do here: we can only add reactions to Promise
            // objects (potentially after unwrapping them), and non-object
            // values can't be Promise objects. This can happen if Promise.all
            // is called on an object with a `resolve` method that returns
            // primitives.
            if (!nextPromise.isObject())
                addToDependent = false;
        }

        // Adds |resultPromise| to the list of dependent promises.
        if (addToDependent) {
            // The object created by the |promise.then| call or the inlined
            // version of it above is visible to content (either because
            // |promise.then| was overridden by content and could leak it,
            // or because a constructor other than the original value of
            // |Promise| was used to create it). To have both that object and
            // |resultPromise| show up as dependent promises in the debugger,
            // add a dummy reaction to the list of reject reactions that
            // contains |resultPromise|, but otherwise does nothing.
            RootedObject& blockedPromise = thenSpeciesOrBlockedPromise;
            blockedPromise = resultPromise;

            mozilla::Maybe<AutoCompartment> ac;
            if (IsProxy(nextPromiseObj)) {
                nextPromiseObj = CheckedUnwrap(nextPromiseObj);
                if (!nextPromiseObj) {
                    return false;
                }
                if (JS_IsDeadWrapper(nextPromiseObj)) {
                    JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_DEAD_OBJECT);
                    return false;
                }
                ac.emplace(cx, nextPromiseObj);
                if (!cx->compartment()->wrap(cx, &blockedPromise))
                    return false;
            }

            // If either the object to depend on or the object that gets
            // blocked isn't a, maybe-wrapped, Promise instance, we ignore it.
            // All this does is lose some small amount of debug information in
            // scenarios that are highly unlikely to occur in useful code.
            if (nextPromiseObj->is<PromiseObject>() && resultPromise->is<PromiseObject>()) {
                Handle<PromiseObject*> promise = nextPromiseObj.as<PromiseObject>();
                if (!AddDummyPromiseReactionForDebugger(cx, promise, blockedPromise))
                    return false;
            }
        }
    }
}

// Create the elements for the Promise combinators Promise.all and
// Promise.allSettled.
static MOZ_MUST_USE bool
NewPromiseCombinatorElements(JSContext* cx, Handle<PromiseCapability> resultCapability,
                             MutableHandle<PromiseCombinatorElements> elements)
{
  // We have to be very careful about which compartments we create things for
  // the Promise combinators. In particular, we have to maintain the invariant
  // that anything stored in a reserved slot is same-compartment with the object
  // whose reserved slot it's in. But we want to create the values array in the
  // compartment of the result capability's Promise, because that array can get
  // exposed as the Promise's resolution value to code that has access to the
  // Promise (in particular code from that compartment), and that should work,
  // even if the Promise compartment is less-privileged than our caller
  // compartment.
  //
  // So the plan is as follows: Create the values array in the promise
  // compartment. Create the promise resolving functions and the data holder in
  // our current compartment, i.e. the compartment of the Promise combinator
  // function. Store a cross-compartment wrapper to the values array in the
  // holder. This should be OK because the only things we hand the promise
  // resolving functions to are the "then" calls we do and in the case when the
  // Promise's compartment is not the current compartment those are happening
  // over Xrays anyway, which means they get the canonical "then" function and
  // content can't see our promise resolving functions.

  if (IsWrapper(resultCapability.promise())) {
    JSObject* unwrappedPromiseObj = CheckedUnwrap(resultCapability.promise());
    MOZ_ASSERT(unwrappedPromiseObj);

    {
      AutoCompartment ac(cx, unwrappedPromiseObj);
      auto* array = NewDenseEmptyArray(cx);
      if (!array) {
        return false;
      }
      elements.initialize(array);
    }

    if (!cx->compartment()->wrap(cx, elements.value())) {
      return false;
    }
  } else {
    auto* array = NewDenseEmptyArray(cx);
    if (!array) {
      return false;
    }

    elements.initialize(array);
  }
  return true;
}

// Retrieve the combinator elements from the data holder.
static MOZ_MUST_USE bool
GetPromiseCombinatorElements(JSContext* cx, Handle<PromiseCombinatorDataHolder*> data,
                             MutableHandle<PromiseCombinatorElements> elements)
{
  bool needsWrapping = false;
  JSObject* valuesObj = &data->valuesArray().toObject();
  if (IsProxy(valuesObj)) {
    // See comment for NewPromiseCombinatorElements for why we unwrap here.
    valuesObj = UncheckedUnwrap(valuesObj);

    if (JS_IsDeadWrapper(valuesObj)) {
      JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr,
                                JSMSG_DEAD_OBJECT);
      return false;
    }

    needsWrapping = true;
  }

  elements.initialize(data, &valuesObj->as<ArrayObject>(), needsWrapping);
  return true;
}

static JSFunction*
NewPromiseCombinatorElementFunction(JSContext* cx, Native native,
                                    Handle<PromiseCombinatorDataHolder*> dataHolder,
                                    uint32_t index)
{
    JSFunction* fn = NewNativeFunction(cx, native, 1, nullptr,
                                       gc::AllocKind::FUNCTION_EXTENDED, GenericObject);
    if (!fn) {
        return nullptr;
    }

    fn->setExtendedSlot(PromiseCombinatorElementFunctionSlot_Data,
                        ObjectValue(*dataHolder));
    fn->setExtendedSlot(PromiseCombinatorElementFunctionSlot_ElementIndex,
                        Int32Value(index));
    return fn;
}

// ES2020 draft rev e97c95d064750fb949b6778584702dd658cf5624
// 25.6.4.1.2 Promise.all Resolve Element Functions
// 25.6.4.2.2 Promise.allSettled Resolve Element Functions
// 25.6.4.2.3 Promise.allSettled Reject Element Functions
//
// Common implementation for Promise combinator element functions to check if
// they've already been called.
static bool
PromiseCombinatorElementFunctionAlreadyCalled(const CallArgs& args,
    MutableHandle<PromiseCombinatorDataHolder*> data, uint32_t* index)
{
  // Step 1.
  JSFunction* fn = &args.callee().as<JSFunction>();

  // Step 2.
  const Value& dataVal =
      fn->getExtendedSlot(PromiseCombinatorElementFunctionSlot_Data);

  // Step 3.
  // We use the existence of the data holder as a signal for whether the Promise
  // combinator element function was already called. Upon resolution, it's reset
  // to `undefined`.
  if (dataVal.isUndefined()) {
    return true;
  }

  data.set(&dataVal.toObject().as<PromiseCombinatorDataHolder>());

  // Step 4.
  fn->setExtendedSlot(PromiseCombinatorElementFunctionSlot_Data,
                      UndefinedValue());

  // Step 5.
  int32_t idx =
      fn->getExtendedSlot(PromiseCombinatorElementFunctionSlot_ElementIndex)
          .toInt32();
  MOZ_ASSERT(idx >= 0);
  *index = uint32_t(idx);

  return false;
}

// ES2020 draft rev a09fc232c137800dbf51b6204f37fdede4ba1646
// 25.6.4.1.1 PerformPromiseAll (iteratorRecord, constructor, resultCapability)
static MOZ_MUST_USE bool
PerformPromiseAll(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                  Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done)
{
    *done = false;

    // Step 1.
    MOZ_ASSERT(C->isConstructor());

    // Step 2 (omitted).

    // Step 3.
    Rooted<PromiseCombinatorElements> values(cx);
    if (!NewPromiseCombinatorElements(cx, resultCapability, &values)) {
        return false;
    }

    // Step 4.
    // Create our data holder that holds all the things shared across
    // every step of the iterator.  In particular, this holds the
    // remainingElementsCount (as an integer reserved slot), the array of
    // values, and the resolve function from our PromiseCapability.
    Rooted<PromiseCombinatorDataHolder*> dataHolder(cx);
    dataHolder = PromiseCombinatorDataHolder::New(cx, resultCapability.promise(),
                                                  values, resultCapability.resolve());
    if (!dataHolder)
        return false;

    // Step 5.
    uint32_t index = 0;

    auto getResolveAndReject = [cx, &resultCapability, &values, &dataHolder,
                                &index](MutableHandleValue resolveFunVal,
                                        MutableHandleValue rejectFunVal) {
        // Step 8.h.
        if (!values.pushUndefined(cx)) {
            return false;
        }

        // Steps 8.j-p.
        JSFunction* resolveFunc = NewPromiseCombinatorElementFunction(cx,
                                      PromiseAllResolveElementFunction, dataHolder, index);
        if (!resolveFunc)
            return false;

        // Step 8.q.
        dataHolder->increaseRemainingCount();

        // Step 8.s.
        index++;
        MOZ_ASSERT(index > 0);

        resolveFunVal.setObject(*resolveFunc);
        rejectFunVal.setObject(*resultCapability.reject());
        return true;
    };

    // Step 8.
    if (!CommonPerformPromiseCombinator(cx, iterator, C, resultCapability.promise(),
                                        promiseResolve, done, true, getResolveAndReject))
        return false;

    // Step 8.d.ii.
    int32_t remainingCount = dataHolder->decreaseRemainingCount();

    // Steps 8.d.iii-iv.
    if (remainingCount == 0) {
        return RunResolutionFunction(cx, resultCapability.resolve(), values.value(), ResolveMode,
                                     resultCapability.promise());
    }

    return true;
}

// ES2020 draft rev e97c95d064750fb949b6778584702dd658cf5624
// 25.6.4.1.2 Promise.all Resolve Element Functions
static bool
PromiseAllResolveElementFunction(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);

    HandleValue xVal = args.get(0);

    // Steps 1-5.
    Rooted<PromiseCombinatorDataHolder*> data(cx);
    uint32_t index;
    if (PromiseCombinatorElementFunctionAlreadyCalled(args, &data, &index)) {
        args.rval().setUndefined();
        return true;
    }

    // Step 6.
    Rooted<PromiseCombinatorElements> values(cx);
    if (!GetPromiseCombinatorElements(cx, data, &values)) {
        return false;
    }

    // Step 7 (moved under step 11).
    // Step 8 (moved to step 10).

    // Step 9.
    if (!values.setElement(cx, index, xVal)) {
        return false;
    }

    // Steps 8,10.
    uint32_t remainingCount = data->decreaseRemainingCount();

    // Step 11.
    if (remainingCount == 0) {
        // Step 11.a. (Omitted, happened in PerformPromiseAll.)
        // Step 11.b.

        // Step 7 (Adapted to work with PromiseCombinatorDataHolder's layout).
        RootedObject resolveAllFun(cx, data->resolveOrRejectObj());
        RootedObject promiseObj(cx, data->promiseObj());
        if (!RunResolutionFunction(cx, resolveAllFun, values.value(), ResolveMode, promiseObj))
            return false;
    }

    // Step 12.
    args.rval().setUndefined();
    return true;
}

// ES2020 draft rev a09fc232c137800dbf51b6204f37fdede4ba1646
// 25.6.4.3 Promise.race ( iterable )
static bool
Promise_static_race(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);
    return CommonPromiseCombinator(cx, args, CombinatorKind::Race);
}

// ES2020 draft rev a09fc232c137800dbf51b6204f37fdede4ba1646
// 25.6.4.3.1 PerformPromiseRace (iteratorRecord, constructor, resultCapability)
static MOZ_MUST_USE bool
PerformPromiseRace(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                   Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done)
{
    *done = false;

    // Step 1.
    MOZ_ASSERT(C->isConstructor());

    // Step 2 (omitted).

    // BlockOnPromise fast path requires the passed onFulfilled function
    // doesn't return an object value, because otherwise the skipped promise
    // creation is detectable due to missing property lookups.
    bool isDefaultResolveFn = IsNativeFunction(resultCapability.resolve(),
                                               ResolvePromiseFunction);

    auto getResolveAndReject = [&resultCapability](
                                   MutableHandleValue resolveFunVal,
                                   MutableHandleValue rejectFunVal) {
        resolveFunVal.setObject(*resultCapability.resolve());
        rejectFunVal.setObject(*resultCapability.reject());
        return true;
    };

    // Step 3-5.
    return CommonPerformPromiseCombinator(cx, iterator, C, resultCapability.promise(),
                                          promiseResolve, done,
                                          isDefaultResolveFn, getResolveAndReject);
}

enum class PromiseAllSettledElementFunctionKind { Resolve, Reject };

// ES2020 draft rev e97c95d064750fb949b6778584702dd658cf5624
// 25.6.4.2 Promise.allSettled ( iterable )
//
// Promise.allSettled Resolve Element Functions
// Promise.allSettled Reject Element Functions
template <PromiseAllSettledElementFunctionKind Kind>
static bool PromiseAllSettledElementFunction(JSContext* cx, unsigned argc,
                                             Value* vp);

// ES2020 draft rev e97c95d064750fb949b6778584702dd658cf5624
// 25.6.4.2 Promise.allSettled ( iterable )
//
// Promise.allSettled ( iterable )
static bool Promise_static_allSettled(JSContext* cx, unsigned argc, Value* vp) {
  CallArgs args = CallArgsFromVp(argc, vp);
  return CommonPromiseCombinator(cx, args, CombinatorKind::AllSettled);
}

// ES2020 draft rev e97c95d064750fb949b6778584702dd658cf5624
// 25.6.4.2 Promise.allSettled ( iterable )
//
// PerformPromiseAllSettled ( iteratorRecord, constructor, resultCapability )
static MOZ_MUST_USE bool PerformPromiseAllSettled(
    JSContext* cx, PromiseForOfIterator& iterator, HandleObject C,
    Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done) {
  *done = false;

  // Step 1.
  MOZ_ASSERT(C->isConstructor());

  // Step 2 (omitted).

  // Step 3.
  Rooted<PromiseCombinatorElements> values(cx);
  if (!NewPromiseCombinatorElements(cx, resultCapability, &values)) {
    return false;
  }

  // Step 4.
  // Create our data holder that holds all the things shared across every step
  // of the iterator. In particular, this holds the remainingElementsCount
  // (as an integer reserved slot), the array of values, and the resolve
  // function from our PromiseCapability.
  Rooted<PromiseCombinatorDataHolder*> dataHolder(cx);
  dataHolder = PromiseCombinatorDataHolder::New(cx, resultCapability.promise(),
                                                values,
                                                resultCapability.resolve());
  if (!dataHolder) {
    return false;
  }

  // Step 5.
  uint32_t index = 0;

  auto getResolveAndReject = [cx, &values, &dataHolder, &index](
                                 MutableHandleValue resolveFunVal,
                                 MutableHandleValue rejectFunVal) {
    // Step 8.h.
    if (!values.pushUndefined(cx)) {
      return false;
    }

    auto PromiseAllSettledResolveElementFunction =
        PromiseAllSettledElementFunction<
            PromiseAllSettledElementFunctionKind::Resolve>;
    auto PromiseAllSettledRejectElementFunction =
        PromiseAllSettledElementFunction<
            PromiseAllSettledElementFunctionKind::Reject>;

    // Steps 8.j-q.
    JSFunction* resolveFunc = NewPromiseCombinatorElementFunction(
        cx, PromiseAllSettledResolveElementFunction, dataHolder, index);
    if (!resolveFunc) {
      return false;
    }
    resolveFunVal.setObject(*resolveFunc);

    // Steps 8.r-x.
    JSFunction* rejectFunc = NewPromiseCombinatorElementFunction(
        cx, PromiseAllSettledRejectElementFunction, dataHolder, index);
    if (!rejectFunc) {
      return false;
    }
    rejectFunVal.setObject(*rejectFunc);

    // Step 6.y.
    dataHolder->increaseRemainingCount();

    // Step 6.aa.
    index++;
    MOZ_ASSERT(index > 0);

    return true;
  };

  // Step 5-6 and 8.
  if (!CommonPerformPromiseCombinator(cx, iterator, C, resultCapability.promise(),
                                      promiseResolve, done, true, getResolveAndReject)) {
    return false;
  }

  // Step 8.d.ii.
  int32_t remainingCount = dataHolder->decreaseRemainingCount();

  // Steps 8.d.iii-iv.
  if (remainingCount == 0) {
    return RunResolutionFunction(cx, resultCapability.resolve(), values.value(),
                                 ResolveMode, resultCapability.promise());
  }

  return true;
}

// ES2020 draft rev e97c95d064750fb949b6778584702dd658cf5624
// 25.6.4.2.2 Promise.allSettled Resolve Element Functions
// 25.6.4.2.3 Promise.allSettled Reject Element Functions
template <PromiseAllSettledElementFunctionKind Kind>
static bool PromiseAllSettledElementFunction(JSContext* cx, unsigned argc,
                                             Value* vp) {
  CallArgs args = CallArgsFromVp(argc, vp);
  HandleValue valueOrReason = args.get(0);

  // Steps 1-5.
  Rooted<PromiseCombinatorDataHolder*> data(cx);
  uint32_t index;
  if (PromiseCombinatorElementFunctionAlreadyCalled(args, &data, &index)) {
    args.rval().setUndefined();
    return true;
  }

  // Step 6.
  Rooted<PromiseCombinatorElements> values(cx);
  if (!GetPromiseCombinatorElements(cx, data, &values)) {
    return false;
  }

  // Steps 2-3.
  // The already-called check above only handles the case when |this| function
  // is called repeatedly, so we still need to check if the other pair of this
  // resolving function was already called:
  // We use the element value as a signal for whether the Promise was already
  // fulfilled. Upon resolution, it's set to the result object created below.
  if (!values.unwrappedArray()->getDenseElement(index).isUndefined()) {
    args.rval().setUndefined();
    return true;
  }

  // Steps 7-8 (moved below).

  // Step 9.
  RootedPlainObject obj(cx, NewBuiltinClassInstance<PlainObject>(cx));
  if (!obj) {
    return false;
  }

  // Step 10.
  RootedId id(cx, NameToId(cx->names().status));
  RootedValue statusValue(cx);
  if (Kind == PromiseAllSettledElementFunctionKind::Resolve) {
    statusValue.setString(cx->names().fulfilled);
  } else {
    statusValue.setString(cx->names().rejected);
  }
  if (!::JS_DefinePropertyById(cx, obj, id, statusValue, JSPROP_ENUMERATE)) {
    return false;
  }

  // Step 11.
  if (Kind == PromiseAllSettledElementFunctionKind::Resolve) {
    id = NameToId(cx->names().value);
  } else {
    id = NameToId(cx->names().reason);
  }
  if (!::JS_DefinePropertyById(cx, obj, id, valueOrReason, JSPROP_ENUMERATE)) {
    return false;
  }

  // Steps 4, 12.
  RootedValue objVal(cx, ObjectValue(*obj));
  if (!values.setElement(cx, index, objVal)) {
    return false;
  }

  // Steps 8, 13.
  uint32_t remainingCount = data->decreaseRemainingCount();

  // Step 14.
  if (remainingCount == 0) {
    // Step 14.a. (Omitted, happened in PerformPromiseAllSettled.)
    // Step 14.b.

    // Step 7 (Adapted to work with PromiseCombinatorDataHolder's layout).
    RootedObject resolveAllFun(cx, data->resolveOrRejectObj());
    RootedObject promiseObj(cx, data->promiseObj());
    if (!RunResolutionFunction(cx, resolveAllFun, values.value(), ResolveMode,
                               promiseObj)) {
      return false;
    }
  }

  // Step 15.
  args.rval().setUndefined();
  return true;
}

// Promise.any (Stage 3 proposal)
// https://tc39.es/proposal-promise-any/
//
// Promise.any ( iterable )
static bool Promise_static_any(JSContext* cx, unsigned argc, Value* vp) {
  CallArgs args = CallArgsFromVp(argc, vp);
  return CommonPromiseCombinator(cx, args, CombinatorKind::Any);
}

// Promise.any (Stage 3 proposal)
// https://tc39.es/proposal-promise-any/
//
// Promise.any Reject Element Functions
static bool PromiseAnyRejectElementFunction(JSContext* cx, unsigned argc,
                                            Value* vp);

// Promise.any (Stage 3 proposal)
// https://tc39.es/proposal-promise-any/
//
// ThrowAggregateError ( errors )
static void ThrowAggregateError(JSContext* cx,
                                Handle<PromiseCombinatorElements> errors,
                                HandleObject promise);

// Promise.any (Stage 3 proposal)
// https://tc39.es/proposal-promise-any/
//
// PerformPromiseAny ( iteratorRecord, constructor, resultCapability )
static MOZ_MUST_USE bool
PerformPromiseAny(JSContext* cx, PromiseForOfIterator& iterator, HandleObject C,
                  Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done)
{
  *done = false;

  // Step 1.
  MOZ_ASSERT(C->isConstructor());

  // Step 2 (omitted).

  // Step 3.
  Rooted<PromiseCombinatorElements> errors(cx);
  if (!NewPromiseCombinatorElements(cx, resultCapability, &errors)) {
    return false;
  }

  // Step 4.
  // Create our data holder that holds all the things shared across every step
  // of the iterator. In particular, this holds the remainingElementsCount (as
  // an integer reserved slot), the array of errors, and the reject function
  // from our PromiseCapability.
  Rooted<PromiseCombinatorDataHolder*> dataHolder(cx);
  dataHolder = PromiseCombinatorDataHolder::New(
      cx, resultCapability.promise(), errors, resultCapability.reject());
  if (!dataHolder) {
    return false;
  }

  // Step 5.
  uint32_t index = 0;

  auto getResolveAndReject = [cx, &resultCapability, &errors, &dataHolder,
                              &index](MutableHandleValue resolveFunVal,
                                      MutableHandleValue rejectFunVal) {
    // Step 8.h.
    if (!errors.pushUndefined(cx)) {
      return false;
    }

    // Steps 8.j-p.
    JSFunction* rejectFunc = NewPromiseCombinatorElementFunction(
        cx, PromiseAnyRejectElementFunction, dataHolder, index);
    if (!rejectFunc) {
      return false;
    }

    // Step 8.q.
    dataHolder->increaseRemainingCount();

    // Step 8.s.
    index++;
    MOZ_ASSERT(index > 0);

    resolveFunVal.setObject(*resultCapability.resolve());
    rejectFunVal.setObject(*rejectFunc);
    return true;
  };

  // BlockOnPromise fast path requires the passed onFulfilled function doesn't
  // return an object value, because otherwise the skipped promise creation is
  // detectable due to missing property lookups.
  bool isDefaultResolveFn =
      IsNativeFunction(resultCapability.resolve(), ResolvePromiseFunction);

  // Steps 6-8.
  if (!CommonPerformPromiseCombinator(
          cx, iterator, C, resultCapability.promise(), promiseResolve, done, isDefaultResolveFn,
          getResolveAndReject)) {
    return false;
  }

  // Step 8.d.ii.
  int32_t remainingCount = dataHolder->decreaseRemainingCount();

  // Step 8.d.iii.
  if (remainingCount == 0) {
    ThrowAggregateError(cx, errors, resultCapability.promise());
    return false;
  }

  // Step 8.d.iv.
  return true;
}

// Promise.any (Stage 3 proposal)
// https://tc39.es/proposal-promise-any/
//
// Promise.any Reject Element Functions
static bool
PromiseAnyRejectElementFunction(JSContext* cx, unsigned argc, Value* vp)
{
  CallArgs args = CallArgsFromVp(argc, vp);
  HandleValue xVal = args.get(0);

  // Steps 1-5.
  Rooted<PromiseCombinatorDataHolder*> data(cx);
  uint32_t index;
  if (PromiseCombinatorElementFunctionAlreadyCalled(args, &data, &index)) {
    args.rval().setUndefined();
    return true;
  }

  // Step 6.
  Rooted<PromiseCombinatorElements> errors(cx);
  if (!GetPromiseCombinatorElements(cx, data, &errors)) {
    return false;
  }

  // Step 9.
  if (!errors.setElement(cx, index, xVal)) {
    return false;
  }

  // Steps 8, 10.
  uint32_t remainingCount = data->decreaseRemainingCount();

  // Step 11.
  if (remainingCount == 0) {
    // Step 7 (Adapted to work with PromiseCombinatorDataHolder's layout).
    RootedObject rejectFun(cx, data->resolveOrRejectObj());
    RootedObject promiseObj(cx, data->promiseObj());

    ThrowAggregateError(cx, errors, promiseObj);

    RootedValue reason(cx);
    if (!MaybeGetAndClearException(cx, &reason)) {
      return false;
    }

    if (!RunResolutionFunction(cx, rejectFun, reason, RejectMode, promiseObj)) {
      return false;
    }
  }

  // Step 12.
  args.rval().setUndefined();
  return true;
}

// Promise.any (Stage 3 proposal)
// https://tc39.es/proposal-promise-any/
//
// ThrowAggregateError ( errors )
static void
ThrowAggregateError(JSContext* cx, Handle<PromiseCombinatorElements> errors, HandleObject promise)
{
  MOZ_ASSERT(!cx->isExceptionPending());

  // Create the AggregateError in the same compartment as the array object.
  AutoCompartment ac(cx, errors.unwrappedArray());

  RootedObject allocationSite(cx);
  mozilla::Maybe<JS::AutoSetAsyncStackForNewCalls> asyncStack;

  // Provide a more useful error stack if possible: This function is typically
  // called from Promise job queue, which doesn't have any JS frames on the
  // stack. So when we create the AggregateError below, its stack property will
  // be set to the empty string, which makes it harder to debug the error cause.
  // To avoid this situation set-up an async stack based on the Promise
  // allocation site, which should point to calling site of |Promise.any|.
  if (promise->is<PromiseObject>()) {
    allocationSite = promise->as<PromiseObject>().allocationSite();
    if (allocationSite) {
      asyncStack.emplace(
          cx, allocationSite, "Promise.any",
          JS::AutoSetAsyncStackForNewCalls::AsyncCallKind::IMPLICIT);
    }
  }

  // AutoSetAsyncStackForNewCalls requires a new activation before it takes
  // effect, so call into the self-hosting helper to set-up new call frames.
  RootedValue error(cx);
  if (!GetAggregateError(cx, JSMSG_PROMISE_ANY_REJECTION, &error)) {
    return;
  }

  // |error| isn't guaranteed to be an ErrorObject in case of OOM.
  RootedSavedFrame stack(cx);
  if (error.isObject() && error.toObject().is<ErrorObject>()) {
    Rooted<ErrorObject*> errorObj(cx, &error.toObject().as<ErrorObject>());
    MOZ_ASSERT(errorObj->type() == JSEXN_AGGREGATEERR);

    RootedValue errorsVal(cx, JS::ObjectValue(*errors.unwrappedArray()));
    if (!NativeDefineDataProperty(cx, errorObj, cx->names().errors, errorsVal,
                                  0)) {
      return;
    }

    // Adopt the existing saved frames when present.
    if (JSObject* errorStack = errorObj->stack()) {
      stack = &errorStack->as<SavedFrame>();
    }
  }

  cx->setPendingException(error, stack);
}

// https://tc39.github.io/ecma262/#sec-promise.reject
//
// Unified implementation of
// 25.6.4.4 Promise.reject ( r )
// 25.6.4.5 Promise.resolve ( x )
// 25.6.4.5.1 PromiseResolve ( C, x )
static MOZ_MUST_USE JSObject*
CommonStaticResolveRejectImpl(JSContext* cx, HandleValue thisVal, HandleValue argVal,
                              ResolutionMode mode)
{
    // Steps 1-2.
    if (!thisVal.isObject()) {
        const char* msg = mode == ResolveMode
                          ? "Receiver of Promise.resolve call"
                          : "Receiver of Promise.reject call";
        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_NOT_NONNULL_OBJECT, msg);
        return nullptr;
    }
    RootedObject C(cx, &thisVal.toObject());

    // Step 3 of Resolve.
    if (mode == ResolveMode && argVal.isObject()) {
        RootedObject xObj(cx, &argVal.toObject());
        bool isPromise = false;
        if (xObj->is<PromiseObject>()) {
            isPromise = true;
        } else if (IsWrapper(xObj)) {
            // Treat instances of Promise from other compartments as Promises
            // here, too.
            // It's important to do the GetProperty for the `constructor`
            // below through the wrapper, because wrappers can change the
            // outcome, so instead of unwrapping and then performing the
            // GetProperty, just check here and then operate on the original
            // object again.
            JSObject* unwrappedObject = CheckedUnwrap(xObj);
            if (unwrappedObject && unwrappedObject->is<PromiseObject>())
                isPromise = true;
        }
        if (isPromise) {
            RootedValue ctorVal(cx);
            if (!GetProperty(cx, xObj, xObj, cx->names().constructor, &ctorVal))
                return nullptr;
            if (ctorVal == thisVal)
                return xObj;
        }
    }

    // Step 4 of Resolve, 3 of Reject.
    Rooted<PromiseCapability> capability(cx);
    if (!NewPromiseCapability(cx, C, &capability, true))
        return nullptr;

    // Step 5 of Resolve, 4 of Reject.
    if (!RunResolutionFunction(cx, mode == ResolveMode ? capability.resolve() : capability.reject(),
                               argVal, mode, capability.promise()))
    {
        return nullptr;
    }

    // Step 6 of Resolve, 4 of Reject.
    return capability.promise();
}

MOZ_MUST_USE JSObject*
js::PromiseResolve(JSContext* cx, HandleObject constructor, HandleValue value)
{
    RootedValue C(cx, ObjectValue(*constructor));
    return CommonStaticResolveRejectImpl(cx, C, value, ResolveMode);
}

/**
 * ES2016, 25.4.4.4, Promise.reject.
 */
static bool
Promise_reject(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);
    HandleValue thisVal = args.thisv();
    HandleValue argVal = args.get(0);
    JSObject* result = CommonStaticResolveRejectImpl(cx, thisVal, argVal, RejectMode);
    if (!result)
        return false;
    args.rval().setObject(*result);
    return true;
}

/**
 * Unforgeable version of ES2016, 25.4.4.4, Promise.reject.
 */
/* static */ JSObject*
PromiseObject::unforgeableReject(JSContext* cx, HandleValue value)
{
    JSObject* promiseCtor = JS::GetPromiseConstructor(cx);
    if (!promiseCtor)
        return nullptr;
    RootedValue cVal(cx, ObjectValue(*promiseCtor));
    return CommonStaticResolveRejectImpl(cx, cVal, value, RejectMode);
}

/**
 * ES2016, 25.4.4.5, Promise.resolve.
 */
static bool
Promise_static_resolve(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);
    HandleValue thisVal = args.thisv();
    HandleValue argVal = args.get(0);
    JSObject* result = CommonStaticResolveRejectImpl(cx, thisVal, argVal, ResolveMode);
    if (!result)
        return false;
    args.rval().setObject(*result);
    return true;
}

/**
 * Unforgeable version of ES2016, 25.4.4.5, Promise.resolve.
 */
/* static */ JSObject*
PromiseObject::unforgeableResolve(JSContext* cx, HandleValue value)
{
    JSObject* promiseCtor = JS::GetPromiseConstructor(cx);
    if (!promiseCtor)
        return nullptr;
    RootedValue cVal(cx, ObjectValue(*promiseCtor));
    return CommonStaticResolveRejectImpl(cx, cVal, value, ResolveMode);
}

/**
 * ES2016, 25.4.4.6 get Promise [ @@species ]
 */
static bool
Promise_static_species(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);

    // Step 1: Return the this value.
    args.rval().set(args.thisv());
    return true;
}

// ES2016, 25.4.5.1, implemented in Promise.js.

enum class IncumbentGlobalObject {
    Yes, No
};

static PromiseReactionRecord*
NewReactionRecord(JSContext* cx, Handle<PromiseCapability> resultCapability,
                  HandleValue onFulfilled, HandleValue onRejected,
                  IncumbentGlobalObject incumbentGlobalObjectOption)
{
    // Either of the following conditions must be met:
    //   * resultCapability.promise is a PromiseObject
    //   * resultCapability.resolve and resultCapability.resolve are callable
    // except for Async Generator, there resultPromise can be nullptr.
#ifdef DEBUG
    if (resultCapability.promise() && !resultCapability.promise()->is<PromiseObject>()) {
        MOZ_ASSERT(resultCapability.resolve());
        MOZ_ASSERT(IsCallable(resultCapability.resolve()));
        MOZ_ASSERT(resultCapability.reject());
        MOZ_ASSERT(IsCallable(resultCapability.reject()));
    }
#endif

    // Ensure the onFulfilled handler has the expected type.
    MOZ_ASSERT(onFulfilled.isInt32() || onFulfilled.isObjectOrNull());
    MOZ_ASSERT_IF(onFulfilled.isObject(), IsCallable(onFulfilled));
    MOZ_ASSERT_IF(onFulfilled.isInt32(),
                  0 <= onFulfilled.toInt32() && onFulfilled.toInt32() < PromiseHandlerLimit);

    // Ensure the onRejected handler has the expected type.
    MOZ_ASSERT(onRejected.isInt32() || onRejected.isObjectOrNull());
    MOZ_ASSERT_IF(onRejected.isObject(), IsCallable(onRejected));
    MOZ_ASSERT_IF(onRejected.isInt32(),
                  0 <= onRejected.toInt32() && onRejected.toInt32() < PromiseHandlerLimit);

    // Handlers must either both be present or both be absent.
    MOZ_ASSERT(onFulfilled.isNull() == onRejected.isNull());

    RootedObject incumbentGlobalObject(cx);
    if (incumbentGlobalObjectOption == IncumbentGlobalObject::Yes) {
        if (!GetObjectFromIncumbentGlobal(cx, &incumbentGlobalObject))
            return nullptr;
    }

    PromiseReactionRecord* reaction = NewObjectWithClassProto<PromiseReactionRecord>(cx);
    if (!reaction)
        return nullptr;

    assertSameCompartment(cx, resultCapability.promise());
    assertSameCompartment(cx, onFulfilled);
    assertSameCompartment(cx, onRejected);
    assertSameCompartment(cx, resultCapability.resolve());
    assertSameCompartment(cx, resultCapability.reject());
    assertSameCompartment(cx, incumbentGlobalObject);

    reaction->setFixedSlot(ReactionRecordSlot_Promise,
                           ObjectOrNullValue(resultCapability.promise()));
    reaction->setFixedSlot(ReactionRecordSlot_Flags, Int32Value(0));
    reaction->setFixedSlot(ReactionRecordSlot_OnFulfilled, onFulfilled);
    reaction->setFixedSlot(ReactionRecordSlot_OnRejected, onRejected);
    reaction->setFixedSlot(ReactionRecordSlot_Resolve,
                           ObjectOrNullValue(resultCapability.resolve()));
    reaction->setFixedSlot(ReactionRecordSlot_Reject,
                           ObjectOrNullValue(resultCapability.reject()));
    reaction->setFixedSlot(ReactionRecordSlot_IncumbentGlobalObject,
                           ObjectOrNullValue(incumbentGlobalObject));

    return reaction;
}

static bool
IsPromiseSpecies(JSContext* cx, JSFunction* species)
{
    return species->maybeNative() == Promise_static_species;
}

static bool
PromiseThenNewPromiseCapability(JSContext* cx, HandleObject promiseObj,
                                CreateDependentPromise createDependent,
                                MutableHandle<PromiseCapability> resultCapability)
{
    if (createDependent != CreateDependentPromise::Never) {
        // Step 3.
        RootedObject C(cx, SpeciesConstructor(cx, promiseObj, JSProto_Promise, IsPromiseSpecies));
        if (!C)
            return false;

        if (createDependent == CreateDependentPromise::Always ||
            !IsNativeFunction(C, PromiseConstructor))
        {
            // Step 4.
            if (!NewPromiseCapability(cx, C, resultCapability, true))
                return false;
        }
    }

    return true;
}

// ES2016, 25.4.5.3., steps 3-5.
MOZ_MUST_USE bool
js::OriginalPromiseThen(JSContext* cx, Handle<PromiseObject*> promise,
                        HandleValue onFulfilled, HandleValue onRejected,
                        MutableHandleObject dependent, CreateDependentPromise createDependent)
{
    RootedObject promiseObj(cx, promise);
    if (promise->compartment() != cx->compartment()) {
        if (!cx->compartment()->wrap(cx, &promiseObj))
            return false;
    }

    // Steps 3-4.
    Rooted<PromiseCapability> resultCapability(cx);
    if (!PromiseThenNewPromiseCapability(cx, promiseObj, createDependent, &resultCapability))
        return false;

    // Step 5.
    if (!PerformPromiseThen(cx, promise, onFulfilled, onRejected, resultCapability))
        return false;

    dependent.set(resultCapability.promise());
    return true;
}

static MOZ_MUST_USE bool
OriginalPromiseThenWithoutSettleHandlers(JSContext* cx, Handle<PromiseObject*> promise,
                                         Handle<PromiseObject*> promiseToResolve)
{
    assertSameCompartment(cx, promise);

    // Steps 3-4.
    Rooted<PromiseCapability> resultCapability(cx);
    if (!PromiseThenNewPromiseCapability(cx, promise, CreateDependentPromise::SkipIfCtorUnobservable,
                                         &resultCapability))
    {
        return false;
    }

    // Step 5.
    return PerformPromiseThenWithoutSettleHandlers(cx, promise, promiseToResolve, resultCapability);
}

static bool
CanCallOriginalPromiseThenBuiltin(JSContext* cx, HandleValue promise)
{
    return promise.isObject() &&
           promise.toObject().is<PromiseObject>() &&
           cx->compartment()->promiseLookup.isDefaultInstance(cx, &promise.toObject().as<PromiseObject>());
}

// ES2016, 25.4.5.3., steps 3-5.
static bool
OriginalPromiseThenBuiltin(JSContext* cx, HandleValue promiseVal, HandleValue onFulfilled,
                           HandleValue onRejected, MutableHandleValue rval, bool rvalUsed)
{
    assertSameCompartment(cx, promiseVal, onFulfilled, onRejected);
    MOZ_ASSERT(CanCallOriginalPromiseThenBuiltin(cx, promiseVal));

    Rooted<PromiseObject*> promise(cx, &promiseVal.toObject().as<PromiseObject>());

    // Steps 3-4.
    Rooted<PromiseCapability> resultCapability(cx);
    if (rvalUsed) {
        PromiseObject* resultPromise = CreatePromiseObjectWithoutResolutionFunctions(cx);
        if (!resultPromise)
            return false;

        resultCapability.promise().set(resultPromise);
    }

    // Step 5.
    if (!PerformPromiseThen(cx, promise, onFulfilled, onRejected, resultCapability))
        return false;

    if (rvalUsed)
        rval.setObject(*resultCapability.promise());
    else
        rval.setUndefined();
    return true;
}

MOZ_MUST_USE bool
js::RejectPromiseWithPendingError(JSContext* cx, Handle<PromiseObject*> promise)
{
    // Not much we can do about uncatchable exceptions, just bail.
    RootedValue exn(cx);
    if (!GetAndClearException(cx, &exn))
        return false;
    return PromiseObject::reject(cx, promise, exn);
}

static MOZ_MUST_USE bool PerformPromiseThenWithReaction(JSContext* cx,
                                                        Handle<PromiseObject*> promise,
                                                        Handle<PromiseReactionRecord*> reaction);

// Some async/await functions are implemented here instead of
// js/src/builtin/AsyncFunction.cpp, to call Promise internal functions.

// ES 2018 draft 14.6.11 and 14.7.14 step 1.
MOZ_MUST_USE PromiseObject*
js::CreatePromiseObjectForAsync(JSContext* cx, HandleValue generatorVal)
{
    // Step 1.
    Rooted<PromiseObject*> promise(cx, CreatePromiseObjectWithoutResolutionFunctions(cx));
    if (!promise)
        return nullptr;

    AddPromiseFlags(*promise, PROMISE_FLAG_ASYNC);
    promise->setFixedSlot(PromiseSlot_AwaitGenerator, generatorVal);
    return promise;
}

bool
js::IsPromiseForAsync(JSObject* promise)
{
    return promise->is<PromiseObject>() &&
           PromiseHasAnyFlag(promise->as<PromiseObject>(), PROMISE_FLAG_ASYNC);
}

// ES 2018 draft 25.5.5.2 steps 3.f, 3.g.
MOZ_MUST_USE bool
js::AsyncFunctionThrown(JSContext* cx, Handle<PromiseObject*> resultPromise)
{
    // Step 3.f.
    RootedValue exc(cx);
    if (!MaybeGetAndClearException(cx, &exc))
        return false;

    if (!RejectPromiseInternal(cx, resultPromise, exc))
        return false;

    // Step 3.g.
    return true;
}

// ES 2018 draft 25.5.5.2 steps 3.d-e, 3.g.
MOZ_MUST_USE bool
js::AsyncFunctionReturned(JSContext* cx, Handle<PromiseObject*> resultPromise, HandleValue value)
{
    // Steps 3.d-e.
    if (!ResolvePromiseInternal(cx, resultPromise, value))
        return false;

    // Step 3.g.
    return true;
}

// Helper function that performs the equivalent steps as
// Async Iteration proposal 4.1 Await steps 2-3, 6-9 or similar.
template <typename T>
static MOZ_MUST_USE bool
InternalAwait(JSContext* cx, HandleValue value, HandleObject resultPromise,
              HandleValue onFulfilled, HandleValue onRejected, T extraStep)
{
    MOZ_ASSERT(onFulfilled.isInt32());
    MOZ_ASSERT(onRejected.isInt32());

    // Step 2.
    Rooted<PromiseObject*> promise(cx, CreatePromiseObjectWithoutResolutionFunctions(cx));
    if (!promise)
        return false;

    // Step 3.
    if (!ResolvePromiseInternal(cx, promise, value))
        return false;

    // Steps 7-8.
    Rooted<PromiseCapability> resultCapability(cx);
    resultCapability.promise().set(resultPromise);
    Rooted<PromiseReactionRecord*> reaction(cx, NewReactionRecord(cx, resultCapability,
                                                                  onFulfilled, onRejected,
                                                                  IncumbentGlobalObject::Yes));
    if (!reaction)
        return false;

    // Step 6.
    extraStep(reaction);

    // Step 9.
    return PerformPromiseThenWithReaction(cx, promise, reaction);
}

// ES 2018 draft 25.5.5.3 steps 2-10.
MOZ_MUST_USE bool
js::AsyncFunctionAwait(JSContext* cx, Handle<PromiseObject*> resultPromise, HandleValue value)
{
    // Steps 4-5.
    RootedValue onFulfilled(cx, Int32Value(PromiseHandlerAsyncFunctionAwaitedFulfilled));
    RootedValue onRejected(cx, Int32Value(PromiseHandlerAsyncFunctionAwaitedRejected));

    // Steps 2-3, 6-10.
    auto extra = [](Handle<PromiseReactionRecord*> reaction) {
        reaction->setIsAsyncFunction();
    };
    return InternalAwait(cx, value, resultPromise, onFulfilled, onRejected, extra);
}

// Async Iteration proposal 4.1 Await steps 2-9.
MOZ_MUST_USE bool
js::AsyncGeneratorAwait(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                        HandleValue value)
{
    // Steps 4-5.
    RootedValue onFulfilled(cx, Int32Value(PromiseHandlerAsyncGeneratorAwaitedFulfilled));
    RootedValue onRejected(cx, Int32Value(PromiseHandlerAsyncGeneratorAwaitedRejected));

    // Steps 2-3, 6-9.
    auto extra = [&](Handle<PromiseReactionRecord*> reaction) {
        reaction->setIsAsyncGenerator(asyncGenObj);
    };
    return InternalAwait(cx, value, nullptr, onFulfilled, onRejected, extra);
}

// Async Iteration proposal 11.1.3.2.1 %AsyncFromSyncIteratorPrototype%.next
// Async Iteration proposal 11.1.3.2.2 %AsyncFromSyncIteratorPrototype%.return
// Async Iteration proposal 11.1.3.2.3 %AsyncFromSyncIteratorPrototype%.throw
bool
js::AsyncFromSyncIteratorMethod(JSContext* cx, CallArgs& args, CompletionKind completionKind)
{
    // Step 1.
    HandleValue thisVal = args.thisv();

    // Step 2.
    Rooted<PromiseObject*> resultPromise(cx, CreatePromiseObjectWithoutResolutionFunctions(cx));
    if (!resultPromise)
        return false;

    // Step 3.
    if (!thisVal.isObject() || !thisVal.toObject().is<AsyncFromSyncIteratorObject>()) {
        // Step 3.a.
        RootedValue badGeneratorError(cx);
        if (!GetTypeError(cx, JSMSG_NOT_AN_ASYNC_ITERATOR, &badGeneratorError))
            return false;

        // Step 3.b.
        if (!RejectPromiseInternal(cx, resultPromise, badGeneratorError))
            return false;

        // Step 3.c.
        args.rval().setObject(*resultPromise);
        return true;
    }

    Rooted<AsyncFromSyncIteratorObject*> asyncIter(
        cx, &thisVal.toObject().as<AsyncFromSyncIteratorObject>());

    // Step 4.
    RootedObject iter(cx, asyncIter->iterator());

    RootedValue func(cx);
    if (completionKind == CompletionKind::Normal) {
        // 11.1.3.2.1 steps 5-6 (partially).
        if (!GetProperty(cx, iter, iter, cx->names().next, &func))
            return AbruptRejectPromise(cx, args, resultPromise, nullptr);
    } else if (completionKind == CompletionKind::Return) {
        // 11.1.3.2.2 steps 5-6.
        if (!GetProperty(cx, iter, iter, cx->names().return_, &func))
            return AbruptRejectPromise(cx, args, resultPromise, nullptr);

        // Step 7.
        if (func.isNullOrUndefined()) {
            // Step 7.a.
            JSObject* resultObj = CreateIterResultObject(cx, args.get(0), true);
            if (!resultObj)
                return AbruptRejectPromise(cx, args, resultPromise, nullptr);

            RootedValue resultVal(cx, ObjectValue(*resultObj));

            // Step 7.b.
            if (!ResolvePromiseInternal(cx, resultPromise, resultVal))
                return AbruptRejectPromise(cx, args, resultPromise, nullptr);

            // Step 7.c.
            args.rval().setObject(*resultPromise);
            return true;
        }
    } else {
        // 11.1.3.2.3 steps 5-6.
        MOZ_ASSERT(completionKind == CompletionKind::Throw);
        if (!GetProperty(cx, iter, iter, cx->names().throw_, &func))
            return AbruptRejectPromise(cx, args, resultPromise, nullptr);

        // Step 7.
        if (func.isNullOrUndefined()) {
            // Step 7.a.
            if (!RejectPromiseInternal(cx, resultPromise, args.get(0)))
                return AbruptRejectPromise(cx, args, resultPromise, nullptr);

            // Step 7.b.
            args.rval().setObject(*resultPromise);
            return true;
        }
    }

    // 11.1.3.2.1 steps 5-6 (partially).
    // 11.1.3.2.2, 11.1.3.2.3 steps 8-9.
    RootedValue iterVal(cx, ObjectValue(*iter));
    RootedValue resultVal(cx);
    if (!Call(cx, func, iterVal, args.get(0), &resultVal))
        return AbruptRejectPromise(cx, args, resultPromise, nullptr);

    // 11.1.3.2.1 steps 5-6 (partially).
    // 11.1.3.2.2, 11.1.3.2.3 steps 10.
    if (!resultVal.isObject()) {
        CheckIsObjectKind kind;
        switch (completionKind) {
          case CompletionKind::Normal:
            kind = CheckIsObjectKind::IteratorNext;
            break;
          case CompletionKind::Throw:
            kind = CheckIsObjectKind::IteratorThrow;
            break;
          case CompletionKind::Return:
            kind = CheckIsObjectKind::IteratorReturn;
            break;
        }
        MOZ_ALWAYS_FALSE(ThrowCheckIsObject(cx, kind));
        return AbruptRejectPromise(cx, args, resultPromise, nullptr);
    }

    RootedObject resultObj(cx, &resultVal.toObject());

    // Following step numbers are for 11.1.3.2.1.
    // For 11.1.3.2.2 and 11.1.3.2.3, steps 7-16 corresponds to steps 11-20.

    // Steps 7-8.
    RootedValue doneVal(cx);
    if (!GetProperty(cx, resultObj, resultObj, cx->names().done, &doneVal))
        return AbruptRejectPromise(cx, args, resultPromise, nullptr);
    bool done = ToBoolean(doneVal);

    // Steps 9-10.
    RootedValue value(cx);
    if (!GetProperty(cx, resultObj, resultObj, cx->names().value, &value))
        return AbruptRejectPromise(cx, args, resultPromise, nullptr);

    // Steps 13-14.
    RootedValue onFulfilled(cx, Int32Value(done
                                           ? PromiseHandlerAsyncFromSyncIteratorValueUnwrapDone
                                           : PromiseHandlerAsyncFromSyncIteratorValueUnwrapNotDone));
    RootedValue onRejected(cx, Int32Value(PromiseHandlerThrower));

    // Steps 11-12, 15.
    auto extra = [](Handle<PromiseReactionRecord*> reaction) {
    };
    if (!InternalAwait(cx, value, resultPromise, onFulfilled, onRejected, extra))
        return false;

    // Step 16.
    args.rval().setObject(*resultPromise);
    return true;
}

enum class ResumeNextKind {
    Enqueue, Reject, Resolve
};

static MOZ_MUST_USE bool
AsyncGeneratorResumeNext(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                         ResumeNextKind kind, HandleValue valueOrException = UndefinedHandleValue,
                         bool done = false);

// Async Iteration proposal 11.4.3.3.
MOZ_MUST_USE bool
js::AsyncGeneratorResolve(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                          HandleValue value, bool done)
{
    return AsyncGeneratorResumeNext(cx, asyncGenObj, ResumeNextKind::Resolve, value, done);
}

// Async Iteration proposal 11.4.3.4.
MOZ_MUST_USE bool
js::AsyncGeneratorReject(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                         HandleValue exception)
{
    return AsyncGeneratorResumeNext(cx, asyncGenObj, ResumeNextKind::Reject, exception);
}

// Async Iteration proposal 11.4.3.5.
static MOZ_MUST_USE bool
AsyncGeneratorResumeNext(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                         ResumeNextKind kind,
                         HandleValue valueOrException_ /* = UndefinedHandleValue */,
                         bool done /* = false */)
{
    RootedValue valueOrException(cx, valueOrException_);

    while (true) {
        switch (kind) {
          case ResumeNextKind::Enqueue:
            // No further action required.
            break;
          case ResumeNextKind::Reject: {
            // 11.4.3.4 AsyncGeneratorReject ( generator, exception )
            HandleValue exception = valueOrException;

            // Step 1 (implicit).

            // Steps 2-3.
            MOZ_ASSERT(!asyncGenObj->isQueueEmpty());

            // Step 4.
            AsyncGeneratorRequest* request =
                AsyncGeneratorObject::dequeueRequest(cx, asyncGenObj);
            if (!request)
                return false;

            // Step 5.
            Rooted<PromiseObject*> resultPromise(cx, request->promise());

            asyncGenObj->cacheRequest(request);

            // Step 6.
            if (!RejectPromiseInternal(cx, resultPromise, exception))
                return false;

            // Steps 7-8.
            break;
          }
          case ResumeNextKind::Resolve: {
            // 11.4.3.3 AsyncGeneratorResolve ( generator, value, done )
            HandleValue value = valueOrException;

            // Step 1 (implicit).

            // Steps 2-3.
            MOZ_ASSERT(!asyncGenObj->isQueueEmpty());

            // Step 4.
            AsyncGeneratorRequest* request =
                AsyncGeneratorObject::dequeueRequest(cx, asyncGenObj);
            if (!request)
                return false;

            // Step 5.
            Rooted<PromiseObject*> resultPromise(cx, request->promise());

            asyncGenObj->cacheRequest(request);

            // Step 6.
            JSObject* resultObj = CreateIterResultObject(cx, value, done);
            if (!resultObj)
                return false;

            RootedValue resultValue(cx, ObjectValue(*resultObj));

            // Step 7.
            if (!ResolvePromiseInternal(cx, resultPromise, resultValue))
                return false;

            // Steps 8-9.
            break;
          }
        }

        // Step 1 (implicit).

        // Steps 2-3.
        MOZ_ASSERT(!asyncGenObj->isExecuting());

        // Step 4.
        if (asyncGenObj->isAwaitingYieldReturn() || asyncGenObj->isAwaitingReturn())
            return true;

        // Steps 5-6.
        if (asyncGenObj->isQueueEmpty())
            return true;

        // Steps 7-8.
        Rooted<AsyncGeneratorRequest*> request(
            cx, AsyncGeneratorObject::peekRequest(cx, asyncGenObj));
        if (!request)
            return false;

        // Step 9.
        CompletionKind completionKind = request->completionKind();

        // Step 10.
        if (completionKind != CompletionKind::Normal) {
            // Step 10.a.
            if (asyncGenObj->isSuspendedStart())
                asyncGenObj->setCompleted();

            // Step 10.b.
            if (asyncGenObj->isCompleted()) {
                RootedValue value(cx, request->completionValue());

                // Step 10.b.i.
                if (completionKind == CompletionKind::Return) {
                    // Steps 10.b.i.1.
                    asyncGenObj->setAwaitingReturn();

                    // Steps 10.b.i.4-6 (reordered).
                    static constexpr int32_t ResumeNextReturnFulfilled =
                            PromiseHandlerAsyncGeneratorResumeNextReturnFulfilled;
                    static constexpr int32_t ResumeNextReturnRejected =
                            PromiseHandlerAsyncGeneratorResumeNextReturnRejected;

                    RootedValue onFulfilled(cx, Int32Value(ResumeNextReturnFulfilled));
                    RootedValue onRejected(cx, Int32Value(ResumeNextReturnRejected));

                    // Steps 10.b.i.2-3, 7-10.
                    auto extra = [&](Handle<PromiseReactionRecord*> reaction) {
                        reaction->setIsAsyncGenerator(asyncGenObj);
                    };
                    return InternalAwait(cx, value, nullptr, onFulfilled, onRejected, extra);
                }

                // Step 10.b.ii.1.
                MOZ_ASSERT(completionKind == CompletionKind::Throw);

                // Steps 10.b.ii.2-3.
                kind = ResumeNextKind::Reject;
                valueOrException.set(value);
                // |done| is unused for ResumeNextKind::Reject.
                continue;
            }
        } else if (asyncGenObj->isCompleted()) {
            // Step 11.
            kind = ResumeNextKind::Resolve;
            valueOrException.setUndefined();
            done = true;
            continue;
        }

        // Step 12.
        MOZ_ASSERT(asyncGenObj->isSuspendedStart() || asyncGenObj->isSuspendedYield());

        // Step 16 (reordered).
        asyncGenObj->setExecuting();

        RootedValue argument(cx, request->completionValue());

        if (completionKind == CompletionKind::Return) {
            // 11.4.3.7 AsyncGeneratorYield step 8.b-e.
            // Since we don't have the place that handles return from yield
            // inside the generator, handle the case here, with extra state
            // State_AwaitingYieldReturn.
            asyncGenObj->setAwaitingYieldReturn();

            static constexpr int32_t YieldReturnAwaitedFulfilled =
                    PromiseHandlerAsyncGeneratorYieldReturnAwaitedFulfilled;
            static constexpr int32_t YieldReturnAwaitedRejected =
                    PromiseHandlerAsyncGeneratorYieldReturnAwaitedRejected;

            RootedValue onFulfilled(cx, Int32Value(YieldReturnAwaitedFulfilled));
            RootedValue onRejected(cx, Int32Value(YieldReturnAwaitedRejected));

            auto extra = [&](Handle<PromiseReactionRecord*> reaction) {
                reaction->setIsAsyncGenerator(asyncGenObj);
            };
            return InternalAwait(cx, argument, nullptr, onFulfilled, onRejected, extra);
        }

        // Steps 13-15, 17-21.
        return AsyncGeneratorResume(cx, asyncGenObj, completionKind, argument);
    }
}

// Async Iteration proposal 11.4.3.6.
MOZ_MUST_USE bool
js::AsyncGeneratorEnqueue(JSContext* cx, HandleValue asyncGenVal,
                          CompletionKind completionKind, HandleValue completionValue,
                          MutableHandleValue result)
{
    // Step 1 (implicit).

    // Step 2.
    Rooted<PromiseObject*> resultPromise(cx, CreatePromiseObjectWithoutResolutionFunctions(cx));
    if (!resultPromise)
        return false;

    // Step 3.
    if (!asyncGenVal.isObject() || !asyncGenVal.toObject().is<AsyncGeneratorObject>()) {
        // Step 3.a.
        RootedValue badGeneratorError(cx);
        if (!GetTypeError(cx, JSMSG_NOT_AN_ASYNC_GENERATOR, &badGeneratorError))
            return false;

        // Step 3.b.
        if (!RejectPromiseInternal(cx, resultPromise, badGeneratorError))
            return false;

        // Step 3.c.
        result.setObject(*resultPromise);
        return true;
    }

    Rooted<AsyncGeneratorObject*> asyncGenObj(
        cx, &asyncGenVal.toObject().as<AsyncGeneratorObject>());

    // Step 5 (reordered).
    Rooted<AsyncGeneratorRequest*> request(
        cx, AsyncGeneratorObject::createRequest(cx, asyncGenObj, completionKind, completionValue,
                                                resultPromise));
    if (!request)
        return false;

    // Steps 4, 6.
    if (!AsyncGeneratorObject::enqueueRequest(cx, asyncGenObj, request))
        return false;

    // Step 7.
    if (!asyncGenObj->isExecuting()) {
        // Step 8.
        if (!AsyncGeneratorResumeNext(cx, asyncGenObj, ResumeNextKind::Enqueue))
            return false;
    }

    // Step 9.
    result.setObject(*resultPromise);
    return true;
}

static bool
Promise_catch_impl(JSContext* cx, unsigned argc, Value* vp, bool rvalUsed)
{
    CallArgs args = CallArgsFromVp(argc, vp);

    HandleValue thisVal = args.thisv();
    HandleValue onFulfilled = UndefinedHandleValue;
    HandleValue onRejected = args.get(0);

    // Fast path when the default Promise state is intact.
    if (CanCallOriginalPromiseThenBuiltin(cx, thisVal)) {
        return OriginalPromiseThenBuiltin(cx, thisVal, onFulfilled, onRejected, args.rval(),
                                          rvalUsed);
    }

    // Step 1.
    RootedValue thenVal(cx);
    if (!GetProperty(cx, thisVal, cx->names().then, &thenVal))
        return false;

    if (IsNativeFunction(thenVal, &Promise_then))
        return Promise_then_impl(cx, thisVal, onFulfilled, onRejected, args.rval(), rvalUsed);

    return Call(cx, thenVal, thisVal, UndefinedHandleValue, onRejected, args.rval());
}

static MOZ_ALWAYS_INLINE bool
IsPromiseThenOrCatchRetValImplicitlyUsed(JSContext* cx)
{
    // The returned promise of Promise#then and Promise#catch contains
    // stack info if async stack is enabled.  Even if their return value is not
    // used explicitly in the script, the stack info is observable in devtools
    // and profilers.  We shouldn't apply the optimization not to allocate the
    // returned Promise object if the it's implicitly used by them.
    //
    // FIXME: Once bug 1280819 gets fixed, we can use ShouldCaptureDebugInfo.
    if (!cx->options().asyncStack())
        return false;

    // If devtools is opened, the current compartment will become debuggee.
    if (cx->compartment()->isDebuggee())
        return true;

    // The profiler also makes the call stack observable
    if (JS::IsProfileTimelineRecordingEnabled())
        return true;

    // The stack is also observable from Error#stack, but we don't care since
    // it's nonstandard feature.
    return false;
}

// ES2016, 25.4.5.3.
static bool
Promise_catch_noRetVal(JSContext* cx, unsigned argc, Value* vp)
{
    return Promise_catch_impl(cx, argc, vp, IsPromiseThenOrCatchRetValImplicitlyUsed(cx));
}

// ES2016, 25.4.5.3.
static bool
Promise_catch(JSContext* cx, unsigned argc, Value* vp)
{
    return Promise_catch_impl(cx, argc, vp, true);
}

static bool
Promise_then_impl(JSContext* cx, HandleValue promiseVal, HandleValue onFulfilled,
                  HandleValue onRejected, MutableHandleValue rval, bool rvalUsed)
{
    // Step 1 (implicit).
    // Step 2.
    if (!promiseVal.isObject()) {
        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_NOT_NONNULL_OBJECT,
                                  "Receiver of Promise.prototype.then call");
        return false;
    }

    // Fast path when the default Promise state is intact.
    if (CanCallOriginalPromiseThenBuiltin(cx, promiseVal))
        return OriginalPromiseThenBuiltin(cx, promiseVal, onFulfilled, onRejected, rval, rvalUsed);

    RootedObject promiseObj(cx, &promiseVal.toObject());
    Rooted<PromiseObject*> promise(cx);

    if (promiseObj->is<PromiseObject>()) {
        promise = &promiseObj->as<PromiseObject>();
    } else {
        JSObject* unwrappedPromiseObj = CheckedUnwrap(promiseObj);
        if (!unwrappedPromiseObj) {
            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_UNWRAP_DENIED);
            return false;
        }
        if (!unwrappedPromiseObj->is<PromiseObject>()) {
            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_INCOMPATIBLE_PROTO,
                                      "Promise", "then", "value");
            return false;
        }
        promise = &unwrappedPromiseObj->as<PromiseObject>();
    }

    // Steps 3-5.
    CreateDependentPromise createDependent = rvalUsed
                                             ? CreateDependentPromise::Always
                                             : CreateDependentPromise::SkipIfCtorUnobservable;
    RootedObject resultPromise(cx);
    if (!OriginalPromiseThen(cx, promise, onFulfilled, onRejected, &resultPromise,
                             createDependent))
    {
        return false;
    }

    if (rvalUsed)
        rval.setObject(*resultPromise);
    else
        rval.setUndefined();
    return true;
}

// ES2016, 25.4.5.3.
bool
Promise_then_noRetVal(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);
    return Promise_then_impl(cx, args.thisv(), args.get(0), args.get(1), args.rval(),
                             IsPromiseThenOrCatchRetValImplicitlyUsed(cx));
}

// ES2016, 25.4.5.3.
static bool
Promise_then(JSContext* cx, unsigned argc, Value* vp)
{
    CallArgs args = CallArgsFromVp(argc, vp);
    return Promise_then_impl(cx, args.thisv(), args.get(0), args.get(1), args.rval(), true);
}

// ES2016, 25.4.5.3.1.
static MOZ_MUST_USE bool
PerformPromiseThen(JSContext* cx, Handle<PromiseObject*> promise, HandleValue onFulfilled_,
                   HandleValue onRejected_, Handle<PromiseCapability> resultCapability)
{
    // Step 1 (implicit).
    // Step 2 (implicit).

    // Step 3.
    RootedValue onFulfilled(cx, onFulfilled_);
    if (!IsCallable(onFulfilled))
        onFulfilled = Int32Value(PromiseHandlerIdentity);

    // Step 4.
    RootedValue onRejected(cx, onRejected_);
    if (!IsCallable(onRejected))
        onRejected = Int32Value(PromiseHandlerThrower);

    // Step 7.
    Rooted<PromiseReactionRecord*> reaction(cx, NewReactionRecord(cx, resultCapability,
                                                                  onFulfilled, onRejected,
                                                                  IncumbentGlobalObject::Yes));
    if (!reaction)
        return false;

    return PerformPromiseThenWithReaction(cx, promise, reaction);
}

static MOZ_MUST_USE bool
PerformPromiseThenWithoutSettleHandlers(JSContext* cx, Handle<PromiseObject*> promise,
                                        Handle<PromiseObject*> promiseToResolve,
                                        Handle<PromiseCapability> resultCapability)
{
    // Step 1 (implicit).
    // Step 2 (implicit).

    // Step 3.
    HandleValue onFulfilled = NullHandleValue;

    // Step 4.
    HandleValue onRejected = NullHandleValue;

    // Step 7.
    Rooted<PromiseReactionRecord*> reaction(cx, NewReactionRecord(cx, resultCapability,
                                                                  onFulfilled, onRejected,
                                                                  IncumbentGlobalObject::Yes));
    if (!reaction)
        return false;

    reaction->setIsDefaultResolvingHandler(promiseToResolve);

    return PerformPromiseThenWithReaction(cx, promise, reaction);
}

static MOZ_MUST_USE bool
PerformPromiseThenWithReaction(JSContext* cx, Handle<PromiseObject*> promise,
                               Handle<PromiseReactionRecord*> reaction)
{
    JS::PromiseState state = promise->state();
    int32_t flags = promise->flags();
    if (state == JS::PromiseState::Pending) {
        // Steps 5,6 (reordered).
        // Instead of creating separate reaction records for fulfillment and
        // rejection, we create a combined record. All places we use the record
        // can handle that.
        if (!AddPromiseReaction(cx, promise, reaction))
            return false;
    }

    // Steps 8,9.
    else {
        // Step 9.a.
        MOZ_ASSERT_IF(state != JS::PromiseState::Fulfilled, state == JS::PromiseState::Rejected);

        // Step 8.a. / 9.b.
        RootedValue valueOrReason(cx, promise->valueOrReason());

        // We might be operating on a promise from another compartment. In
        // that case, we need to wrap the result/reason value before using it.
        if (!cx->compartment()->wrap(cx, &valueOrReason))
            return false;

        // Step 9.c.
        if (state == JS::PromiseState::Rejected && !(flags & PROMISE_FLAG_HANDLED))
            cx->runtime()->removeUnhandledRejectedPromise(cx, promise);

        // Step 8.b. / 9.d.
        if (!EnqueuePromiseReactionJob(cx, reaction, valueOrReason, state))
            return false;
    }

    // Step 10.
    promise->setFixedSlot(PromiseSlot_Flags, Int32Value(flags | PROMISE_FLAG_HANDLED));

    // Step 11.
    return true;
}

static MOZ_MUST_USE bool
AddPromiseReaction(JSContext* cx, Handle<PromiseObject*> promise,
                   Handle<PromiseReactionRecord*> reaction)
{
    RootedValue reactionVal(cx, ObjectValue(*reaction));

    // The code that creates Promise reactions can handle wrapped Promises,
    // unwrapping them as needed. That means that the `promise` and `reaction`
    // objects we have here aren't necessarily from the same compartment. In
    // order to store the reaction on the promise, we have to ensure that it
    // is properly wrapped.
    mozilla::Maybe<AutoCompartment> ac;
    if (promise->compartment() != cx->compartment()) {
        ac.emplace(cx, promise);
        if (!cx->compartment()->wrap(cx, &reactionVal))
            return false;
    }

    // 25.4.5.3.1 steps 7.a,b.
    RootedValue reactionsVal(cx, promise->reactions());

    if (reactionsVal.isUndefined()) {
        // If no reactions existed so far, just store the reaction record directly.
        promise->setFixedSlot(PromiseSlot_ReactionsOrResult, reactionVal);
        return true;
    }

    RootedObject reactionsObj(cx, &reactionsVal.toObject());

    // If only a single reaction exists, it's stored directly instead of in a
    // list. In that case, `reactionsObj` might be a wrapper, which we can
    // always safely unwrap.
    if (IsProxy(reactionsObj)) {
        reactionsObj = UncheckedUnwrap(reactionsObj);
        if (JS_IsDeadWrapper(reactionsObj)) {
            JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_DEAD_OBJECT);
            return false;
        }
        MOZ_ASSERT(reactionsObj->is<PromiseReactionRecord>());
    }

    if (reactionsObj->is<PromiseReactionRecord>()) {
        // If a single reaction existed so far, create a list and store the
        // old and the new reaction in it.
        ArrayObject* reactions = NewDenseFullyAllocatedArray(cx, 2);
        if (!reactions)
            return false;

        reactions->setDenseInitializedLength(2);
        reactions->initDenseElement(0, reactionsVal);
        reactions->initDenseElement(1, reactionVal);

        promise->setFixedSlot(PromiseSlot_ReactionsOrResult, ObjectValue(*reactions));
    } else {
        // Otherwise, just store the new reaction.
        HandleNativeObject reactions = reactionsObj.as<NativeObject>();
        uint32_t len = reactions->getDenseInitializedLength();
        DenseElementResult result = reactions->ensureDenseElements(cx, len, 1);
        if (result != DenseElementResult::Success) {
            MOZ_ASSERT(result == DenseElementResult::Failure);
            return false;
        }
        reactions->setDenseElement(len, reactionVal);
    }

    return true;
}

static MOZ_MUST_USE bool
AddDummyPromiseReactionForDebugger(JSContext* cx, Handle<PromiseObject*> promise,
                                   HandleObject dependentPromise)
{
    if (promise->state() != JS::PromiseState::Pending)
        return true;

    // Leave resolve and reject as null.
    Rooted<PromiseCapability> capability(cx);
    capability.promise().set(dependentPromise);

    Rooted<PromiseReactionRecord*> reaction(cx, NewReactionRecord(cx, capability,
                                                                  NullHandleValue, NullHandleValue,
                                                                  IncumbentGlobalObject::No));
    if (!reaction)
        return false;

    reaction->setIsDebuggerDummy();

    return AddPromiseReaction(cx, promise, reaction);
}

uint64_t
PromiseObject::getID()
{
    return PromiseDebugInfo::id(this);
}

double
PromiseObject::lifetime()
{
    return MillisecondsSinceStartup() - allocationTime();
}

/**
 * Returns all promises that directly depend on this one. That means those
 * created by calling `then` on this promise, or the promise returned by
 * `Promise.all(iterable)` or `Promise.race(iterable)`, with this promise
 * being a member of the passed-in `iterable`.
 *
 * Per spec, we should have separate lists of reaction records for the
 * fulfill and reject cases. As an optimization, we have only one of those,
 * containing the required data for both cases. So we just walk that list
 * and extract the dependent promises from all reaction records.
 */
bool
PromiseObject::dependentPromises(JSContext* cx, MutableHandle<GCVector<Value>> values)
{
    if (state() != JS::PromiseState::Pending)
        return true;

    RootedValue reactionsVal(cx, reactions());

    // If no reactions are pending, we don't have list and are done.
    if (reactionsVal.isNullOrUndefined())
        return true;

    RootedNativeObject reactions(cx, &reactionsVal.toObject().as<NativeObject>());

    // If only a single reaction is pending, it's stored directly.
    if (reactions->is<PromiseReactionRecord>()) {
        // Not all reactions have a Promise on them.
        RootedObject promiseObj(cx, reactions->as<PromiseReactionRecord>().promise());
        if (!promiseObj)
            return true;

        if (!values.growBy(1))
            return false;

        values[0].setObject(*promiseObj);
        return true;
    }

    uint32_t len = reactions->getDenseInitializedLength();
    MOZ_ASSERT(len >= 2);

    uint32_t valuesIndex = 0;
    for (uint32_t i = 0; i < len; i++) {
        const Value& element = reactions->getDenseElement(i);
        PromiseReactionRecord* reaction = &element.toObject().as<PromiseReactionRecord>();

        // Not all reactions have a Promise on them.
        RootedObject promiseObj(cx, reaction->promise());
        if (!promiseObj)
            continue;
        if (!values.growBy(1))
            return false;

        values[valuesIndex++].setObject(*promiseObj);
    }

    return true;
}

/* static */ bool
PromiseObject::resolve(JSContext* cx, Handle<PromiseObject*> promise, HandleValue resolutionValue)
{
    MOZ_ASSERT(!PromiseHasAnyFlag(*promise, PROMISE_FLAG_ASYNC));
    if (promise->state() != JS::PromiseState::Pending)
        return true;

    if (PromiseHasAnyFlag(*promise, PROMISE_FLAG_DEFAULT_RESOLVING_FUNCTIONS))
        return ResolvePromiseInternal(cx, promise, resolutionValue);

    JSFunction* resolveFun = GetResolveFunctionFromPromise(promise);
    if (!resolveFun)
        return true;

    RootedValue funVal(cx, ObjectValue(*resolveFun));

    // For xray'd Promises, the resolve fun may have been created in another
    // compartment. For the call below to work in that case, wrap the
    // function into the current compartment.
    if (!cx->compartment()->wrap(cx, &funVal))
        return false;

    RootedValue dummy(cx);
    return Call(cx, funVal, UndefinedHandleValue, resolutionValue, &dummy);
}

/* static */ bool
PromiseObject::reject(JSContext* cx, Handle<PromiseObject*> promise, HandleValue rejectionValue)
{
    MOZ_ASSERT(!PromiseHasAnyFlag(*promise, PROMISE_FLAG_ASYNC));
    if (promise->state() != JS::PromiseState::Pending)
        return true;

    if (PromiseHasAnyFlag(*promise, PROMISE_FLAG_DEFAULT_RESOLVING_FUNCTIONS))
        return ResolvePromise(cx, promise, rejectionValue, JS::PromiseState::Rejected);

    RootedValue funVal(cx, promise->getFixedSlot(PromiseSlot_RejectFunction));
    MOZ_ASSERT(IsCallable(funVal));

    RootedValue dummy(cx);
    return Call(cx, funVal, UndefinedHandleValue, rejectionValue, &dummy);
}

/* static */ void
PromiseObject::onSettled(JSContext* cx, Handle<PromiseObject*> promise)
{
    PromiseDebugInfo::setResolutionInfo(cx, promise);

    if (promise->state() == JS::PromiseState::Rejected && promise->isUnhandled())
        cx->runtime()->addUnhandledRejectedPromise(cx, promise);

    JS::dbg::onPromiseSettled(cx, promise);
}

JSFunction*
js::PromiseLookup::getPromiseConstructor(JSContext* cx)
{
    const Value& val = cx->global()->getConstructor(JSProto_Promise);
    return val.isObject() ? &val.toObject().as<JSFunction>() : nullptr;
}

NativeObject*
js::PromiseLookup::getPromisePrototype(JSContext* cx)
{
    const Value& val = cx->global()->getPrototype(JSProto_Promise);
    return val.isObject() ? &val.toObject().as<NativeObject>() : nullptr;
}

bool
js::PromiseLookup::isDataPropertyNative(JSContext* cx, NativeObject* obj, uint32_t slot,
                                        JSNative native)
{
    JSFunction* fun;
    if (!IsFunctionObject(obj->getSlot(slot), &fun))
        return false;
    return fun->maybeNative() == native && fun->compartment() == cx->compartment();
}

bool
js::PromiseLookup::isAccessorPropertyNative(JSContext* cx, Shape* shape, JSNative native)
{
    JSObject* getter = shape->getterObject();
    return getter && IsNativeFunction(getter, native) &&
           getter->as<JSFunction>().compartment() == cx->compartment();
}

void
js::PromiseLookup::initialize(JSContext* cx)
{
    MOZ_ASSERT(state_ == State::Uninitialized);

    // Get the canonical Promise.prototype.
    NativeObject* promiseProto = getPromisePrototype(cx);

    // Check condition 1:
    // Leave the cache uninitialized if the Promise class itself is not yet
    // initialized.
    if (!promiseProto)
        return;

    // Get the canonical Promise constructor.
    JSFunction* promiseCtor = getPromiseConstructor(cx);
    MOZ_ASSERT(promiseCtor,
               "The Promise constructor is initialized iff Promise.prototype is initialized");

    // Shortcut returns below means Promise[@@species] will never be
    // optimizable, set to disabled now, and clear it later when we succeed.
    state_ = State::Disabled;

    // Check condition 2:
    // Look up Promise.prototype.constructor and ensure it's a data property.
    Shape* ctorShape = promiseProto->lookup(cx, cx->names().constructor);
    if (!ctorShape || !ctorShape->hasSlot())
        return;

    // Get the referred value, and ensure it holds the canonical Promise
    // constructor.
    JSFunction* ctorFun;
    if (!IsFunctionObject(promiseProto->getSlot(ctorShape->slot()), &ctorFun))
        return;
    if (ctorFun != promiseCtor)
        return;

    // Check condition 3:
    // Look up Promise.prototype.then and ensure it's a data property.
    Shape* thenShape = promiseProto->lookup(cx, cx->names().then);
    if (!thenShape || !thenShape->hasSlot())
        return;

    // Get the referred value, and ensure it holds the canonical "then"
    // function.
    if (!isDataPropertyNative(cx, promiseProto, thenShape->slot(), Promise_then))
        return;

    // Check condition 4:
    // Look up the '@@species' value on Promise.
    Shape* speciesShape = promiseCtor->lookup(cx, SYMBOL_TO_JSID(cx->wellKnownSymbols().species));
    if (!speciesShape || !speciesShape->hasGetterObject())
        return;

    // Get the referred value, ensure it holds the canonical Promise[@@species]
    // function.
    if (!isAccessorPropertyNative(cx, speciesShape, Promise_static_species))
        return;

    // Check condition 5:
    // Look up Promise.resolve and ensure it's a data property.
    Shape* resolveShape = promiseCtor->lookup(cx, cx->names().resolve);
    if (!resolveShape || !resolveShape->hasSlot())
        return;

    // Get the referred value, and ensure it holds the canonical "resolve"
    // function.
    if (!isDataPropertyNative(cx, promiseCtor, resolveShape->slot(), Promise_static_resolve))
        return;

    // Store raw pointers below. This is okay to do here, because all objects
    // are in the tenured heap.
    MOZ_ASSERT(!IsInsideNursery(promiseCtor->lastProperty()));
    MOZ_ASSERT(!IsInsideNursery(speciesShape));
    MOZ_ASSERT(!IsInsideNursery(promiseProto->lastProperty()));

    state_ = State::Initialized;
    promiseConstructorShape_ = promiseCtor->lastProperty();
#ifdef DEBUG
    promiseSpeciesShape_ = speciesShape;
#endif
    promiseProtoShape_ = promiseProto->lastProperty();
    promiseResolveSlot_ = resolveShape->slot();
    promiseProtoConstructorSlot_ = ctorShape->slot();
    promiseProtoThenSlot_ = thenShape->slot();
}

void
js::PromiseLookup::reset()
{
    JS_POISON(this, 0xBB, sizeof(this));
    state_ = State::Uninitialized;
}

bool
js::PromiseLookup::isPromiseStateStillSane(JSContext* cx)
{
    MOZ_ASSERT(state_ == State::Initialized);

    NativeObject* promiseProto = getPromisePrototype(cx);
    MOZ_ASSERT(promiseProto);

    NativeObject* promiseCtor = getPromiseConstructor(cx);
    MOZ_ASSERT(promiseCtor);

    // Ensure that Promise.prototype still has the expected shape.
    if (promiseProto->lastProperty() != promiseProtoShape_)
        return false;

    // Ensure that Promise still has the expected shape.
    if (promiseCtor->lastProperty() != promiseConstructorShape_)
        return false;

    // Ensure that Promise.prototype.constructor is the canonical constructor.
    if (promiseProto->getSlot(promiseProtoConstructorSlot_) != ObjectValue(*promiseCtor))
        return false;

    // Ensure that Promise.prototype.then is the canonical "then" function.
    if (!isDataPropertyNative(cx, promiseProto, promiseProtoThenSlot_, Promise_then))
        return false;

    // Ensure the species getter contains the canonical @@species function.
    // Note: This is currently guaranteed to be always true, because modifying
    // the getter property implies a new shape is generated. If this ever
    // changes, convert this assertion into an if-statement.
#ifdef DEBUG
    MOZ_ASSERT(isAccessorPropertyNative(cx, promiseSpeciesShape_, Promise_static_species));
#endif

    // Ensure that Promise.resolve is the canonical "resolve" function.
    if (!isDataPropertyNative(cx, promiseCtor, promiseResolveSlot_, Promise_static_resolve))
        return false;

    return true;
}

bool
js::PromiseLookup::ensureInitialized(JSContext* cx, Reinitialize reinitialize)
{
    if (state_ == State::Uninitialized) {
        // If the cache is not initialized, initialize it.
        initialize(cx);
    } else if (state_ == State::Initialized) {
        if (reinitialize == Reinitialize::Allowed) {
            if (!isPromiseStateStillSane(cx)) {
                // If the promise state is no longer sane, reinitialize.
                reset();
                initialize(cx);
            }
        } else {
            // When we're not allowed to reinitialize, the promise state must
            // still be sane if the cache is already initialized.
            MOZ_ASSERT(isPromiseStateStillSane(cx));
        }
    }

    // If the cache is disabled or still uninitialized, don't bother trying to
    // optimize.
    if (state_ != State::Initialized)
        return false;

    // By the time we get here, we should have a sane promise state.
    MOZ_ASSERT(isPromiseStateStillSane(cx));

    return true;
}

bool
js::PromiseLookup::isDefaultPromiseState(JSContext* cx)
{
    // Promise and Promise.prototype are in their default states iff the
    // lookup cache was successfully initialized.
    return ensureInitialized(cx, Reinitialize::Allowed);
}

bool
js::PromiseLookup::hasDefaultProtoAndNoShadowedProperties(JSContext* cx, PromiseObject* promise)
{
    // Ensure |promise|'s prototype is the actual Promise.prototype.
    if (promise->staticPrototype() != getPromisePrototype(cx))
        return false;

    // Ensure |promise| doesn't define any own properties. This serves as a
    // quick check to make sure |promise| doesn't define an own "constructor"
    // or "then" property which may shadow Promise.prototype.constructor or
    // Promise.prototype.then.
    return promise->lastProperty()->isEmptyShape();
}

bool
js::PromiseLookup::isDefaultInstance(JSContext* cx, PromiseObject* promise,
                                     Reinitialize reinitialize)
{
    // Promise and Promise.prototype must be in their default states.
    if (!ensureInitialized(cx, reinitialize))
        return false;

    // The object uses the default properties from Promise.prototype.
    return hasDefaultProtoAndNoShadowedProperties(cx, promise);
}

PromiseTask::PromiseTask(JSContext* cx, Handle<PromiseObject*> promise)
  : runtime_(cx),
    promise_(cx, promise)
{}

PromiseTask::~PromiseTask()
{
    MOZ_ASSERT(CurrentThreadCanAccessZone(promise_->zone()));
}

void
PromiseTask::finish(JSContext* cx)
{
    MOZ_ASSERT(cx == runtime_);
    {
        // We can't leave a pending exception when returning to the caller so do
        // the same thing as Gecko, which is to ignore the error. This should
        // only happen due to OOM or interruption.
        AutoCompartment ac(cx, promise_);
        if (!finishPromise(cx, promise_))
            cx->clearPendingException();
    }
    js_delete(this);
}

void
PromiseTask::cancel(JSContext* cx)
{
    MOZ_ASSERT(cx == runtime_);
    js_delete(this);
}

bool
PromiseTask::executeAndFinish(JSContext* cx)
{
    MOZ_ASSERT(!CanUseExtraThreads());
    execute();
    return finishPromise(cx, promise_);
}

static JSObject*
CreatePromisePrototype(JSContext* cx, JSProtoKey key)
{
    return GlobalObject::createBlankPrototype(cx, cx->global(), &PromiseObject::protoClass_);
}

const JSJitInfo promise_then_info = {
  { (JSJitGetterOp)Promise_then_noRetVal },
  { 0 }, /* unused */
  { 0 }, /* unused */
  JSJitInfo::IgnoresReturnValueNative,
  JSJitInfo::AliasEverything,
  JSVAL_TYPE_UNDEFINED,
};

const JSJitInfo promise_catch_info = {
  { (JSJitGetterOp)Promise_catch_noRetVal },
  { 0 }, /* unused */
  { 0 }, /* unused */
  JSJitInfo::IgnoresReturnValueNative,
  JSJitInfo::AliasEverything,
  JSVAL_TYPE_UNDEFINED,
};

static const JSFunctionSpec promise_methods[] = {
    JS_FNINFO("then", Promise_then, &promise_then_info, 2, 0),
    JS_FNINFO("catch", Promise_catch, &promise_catch_info, 1, 0),
    JS_SELF_HOSTED_FN("finally", "Promise_finally", 1, 0),
    JS_FS_END
};

static const JSPropertySpec promise_properties[] = {
    JS_STRING_SYM_PS(toStringTag, "Promise", JSPROP_READONLY),
    JS_PS_END
};

static const JSFunctionSpec promise_static_methods[] = {
    JS_FN("all", Promise_static_all, 1, 0),
    JS_FN("allSettled", Promise_static_allSettled, 1, 0),
    JS_FN("any", Promise_static_any, 1, 0),
    JS_FN("race", Promise_static_race, 1, 0),
    JS_FN("reject", Promise_reject, 1, 0),
    JS_FN("resolve", Promise_static_resolve, 1, 0),
    JS_FS_END
};

static const JSPropertySpec promise_static_properties[] = {
    JS_SYM_GET(species, Promise_static_species, 0),
    JS_PS_END
};

static const ClassSpec PromiseObjectClassSpec = {
    GenericCreateConstructor<PromiseConstructor, 1, gc::AllocKind::FUNCTION>,
    CreatePromisePrototype,
    promise_static_methods,
    promise_static_properties,
    promise_methods,
    promise_properties
};

const Class PromiseObject::class_ = {
    "Promise",
    JSCLASS_HAS_RESERVED_SLOTS(RESERVED_SLOTS) | JSCLASS_HAS_CACHED_PROTO(JSProto_Promise) |
    JSCLASS_HAS_XRAYED_CONSTRUCTOR,
    JS_NULL_CLASS_OPS,
    &PromiseObjectClassSpec
};

static const ClassSpec PromiseObjectProtoClassSpec = {
    DELEGATED_CLASSSPEC(PromiseObject::class_.spec),
    nullptr,
    nullptr,
    nullptr,
    nullptr,
    nullptr,
    nullptr,
    ClassSpec::IsDelegated
};

const Class PromiseObject::protoClass_ = {
    "PromiseProto",
    JSCLASS_HAS_CACHED_PROTO(JSProto_Promise),
    JS_NULL_CLASS_OPS,
    &PromiseObjectProtoClassSpec
};