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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
* A common base class for representing WebIDL callback function and
* callback interface types in C++.
* This class implements common functionality like lifetime
* management, initialization with the JS object, and setup of the
* call environment. Subclasses are responsible for providing methods
* that do the call into JS as needed.
#ifndef mozilla_dom_CallbackObject_h
#define mozilla_dom_CallbackObject_h
#include <cstddef>
#include <cstdint>
#include <utility>
#include "js/Exception.h"
#include "js/RootingAPI.h"
#include "js/Wrapper.h"
#include "jsapi.h"
#include "mozilla/AlreadyAddRefed.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/HoldDropJSObjects.h"
#include "mozilla/Maybe.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/RefPtr.h"
#include "mozilla/dom/AutoEntryScript.h"
#include "mozilla/dom/BindingCallContext.h"
#include "mozilla/dom/ScriptSettings.h"
#include "nsCOMPtr.h"
#include "nsCycleCollectionParticipant.h"
#include "nsID.h"
#include "nsIGlobalObject.h"
#include "nsISupports.h"
#include "nsISupportsUtils.h"
#include "nsStringFwd.h"
class JSAutoRealm;
class JSObject;
class JSTracer;
class nsCycleCollectionTraversalCallback;
struct JSContext;
namespace JS {
class AutoSetAsyncStackForNewCalls;
class Realm;
class Value;
} // namespace JS
namespace mozilla {
class ErrorResult;
class PromiseJobRunnable;
template <class T>
class OwningNonNull;
namespace dom {
{ \
0xbe74c190, 0x6d76, 0x4991, { \
0x84, 0xb9, 0x65, 0x06, 0x99, 0xe6, 0x93, 0x2b \
} \
class CallbackObject : public nsISupports {
// The caller may pass a global object which will act as an override for the
// incumbent script settings object when the callback is invoked (overriding
// the entry point computed from aCallback). If no override is required, the
// caller should pass null. |aCx| is used to capture the current
// stack, which is later used as an async parent when the callback
// is invoked. aCx can be nullptr, in which case no stack is
// captured.
explicit CallbackObject(JSContext* aCx, JS::Handle<JSObject*> aCallback,
JS::Handle<JSObject*> aCallbackGlobal,
nsIGlobalObject* aIncumbentGlobal) {
if (aCx && JS::IsAsyncStackCaptureEnabledForRealm(aCx)) {
JS::Rooted<JSObject*> stack(aCx);
if (!JS::CaptureCurrentStack(aCx, &stack)) {
Init(aCallback, aCallbackGlobal, stack, aIncumbentGlobal);
} else {
Init(aCallback, aCallbackGlobal, nullptr, aIncumbentGlobal);
// Instead of capturing the current stack to use as an async parent when the
// callback is invoked, the caller can use this overload to pass in a stack
// for that purpose.
explicit CallbackObject(JSObject* aCallback, JSObject* aCallbackGlobal,
JSObject* aAsyncStack,
nsIGlobalObject* aIncumbentGlobal) {
Init(aCallback, aCallbackGlobal, aAsyncStack, aIncumbentGlobal);
// This is guaranteed to be non-null from the time the CallbackObject is
// created until JavaScript has had a chance to run. It will only return null
// after a JavaScript caller has called nukeSandbox on a Sandbox object and
// the cycle collector has had a chance to run, unless Reset() is explicitly
// called (see below).
// This means that any native callee which receives a CallbackObject as an
// argument can safely rely on the callback being non-null so long as it
// doesn't trigger any scripts before it accesses it.
JSObject* CallbackOrNull() const {
return CallbackPreserveColor();
JSObject* CallbackGlobalOrNull() const {
return mCallbackGlobal;
// Like CallbackOrNull(), but will return a new dead proxy object in the
// caller's realm if the callback is null.
JSObject* Callback(JSContext* aCx);
JSObject* GetCreationStack() const { return mCreationStack; }
void MarkForCC() {
* This getter does not change the color of the JSObject meaning that the
* object returned is not guaranteed to be kept alive past the next CC.
JSObject* CallbackPreserveColor() const { return mCallback.unbarrieredGet(); }
JSObject* CallbackGlobalPreserveColor() const {
return mCallbackGlobal.unbarrieredGet();
* If the callback is known to be non-gray, then this method can be
* used instead of CallbackOrNull() to avoid the overhead of
* ExposeObjectToActiveJS().
JSObject* CallbackKnownNotGray() const {
return CallbackPreserveColor();
nsIGlobalObject* IncumbentGlobalOrNull() const { return mIncumbentGlobal; }
enum ExceptionHandling {
// Report any exception and don't throw it to the caller code.
// Throw any exception to the caller code and don't report it.
// Throw an exception to the caller code if the thrown exception is a
// binding object for a DOMException from the caller's scope, otherwise
// report it.
// Append a UTF-8 string to aOutString that describes the callback function,
// for use in logging or profiler markers.
// The string contains the function name and its source location, if
// available, in the following format:
// "<functionName> (<sourceURL>:<lineNumber>)"
void GetDescription(nsACString& aOutString);
size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
return aMallocSizeOf(this);
// Used for cycle collection optimization. Should return true only if all our
// outgoing edges are to known-live objects. In that case, there's no point
// traversing our edges to them, because we know they can't be collected
// anyway.
bool IsBlackForCC() const {
// Play it safe in case this gets called after unlink.
return (!mCallback || !JS::ObjectIsMarkedGray(mCallback)) &&
(!mCallbackGlobal || !JS::ObjectIsMarkedGray(mCallbackGlobal)) &&
(!mCreationStack || !JS::ObjectIsMarkedGray(mCreationStack)) &&
(!mIncumbentJSGlobal ||
!JS::ObjectIsMarkedGray(mIncumbentJSGlobal)) &&
// mIncumbentGlobal is known-live if we have a known-live
// mIncumbentJSGlobal, since mIncumbentJSGlobal will keep a ref to
// it. At this point if mIncumbentJSGlobal is not null, it's
// known-live.
(!mIncumbentGlobal || mIncumbentJSGlobal);
virtual ~CallbackObject() { mozilla::DropJSObjects(this); }
explicit CallbackObject(CallbackObject* aCallbackObject) {
Init(aCallbackObject->mCallback, aCallbackObject->mCallbackGlobal,
aCallbackObject->mCreationStack, aCallbackObject->mIncumbentGlobal);
bool operator==(const CallbackObject& aOther) const {
JSObject* wrappedThis = CallbackPreserveColor();
JSObject* wrappedOther = aOther.CallbackPreserveColor();
if (!wrappedThis || !wrappedOther) {
return this == &aOther;
JSObject* thisObj = js::UncheckedUnwrap(wrappedThis);
JSObject* otherObj = js::UncheckedUnwrap(wrappedOther);
return thisObj == otherObj;
class JSObjectsDropper final {
explicit JSObjectsDropper(CallbackObject* aHolder) : mHolder(aHolder) {}
~JSObjectsDropper() { mHolder->ClearJSObjects(); }
RefPtr<CallbackObject> mHolder;
inline void InitNoHold(JSObject* aCallback, JSObject* aCallbackGlobal,
JSObject* aCreationStack,
nsIGlobalObject* aIncumbentGlobal) {
MOZ_ASSERT(aCallback && !mCallback);
MOZ_DIAGNOSTIC_ASSERT(JS::GetCompartment(aCallback) ==
mCallback = aCallback;
mCallbackGlobal = aCallbackGlobal;
mCreationStack = aCreationStack;
if (aIncumbentGlobal) {
mIncumbentGlobal = aIncumbentGlobal;
// We don't want to expose to JS here (change the color). If someone ever
// reads mIncumbentJSGlobal, that will expose. If not, no need to expose
// here.
mIncumbentJSGlobal = aIncumbentGlobal->GetGlobalJSObjectPreserveColor();
inline void Init(JSObject* aCallback, JSObject* aCallbackGlobal,
JSObject* aCreationStack,
nsIGlobalObject* aIncumbentGlobal) {
// Set script objects before we hold, on the off chance that a GC could
// somehow happen in there... (which would be pretty odd, granted).
InitNoHold(aCallback, aCallbackGlobal, aCreationStack, aIncumbentGlobal);
// Provide a way to clear this object's pointers to GC things after the
// callback has been run. Note that CallbackOrNull() will return null after
// this point. This should only be called if the object is known not to be
// used again, and no handles (e.g. those returned by CallbackPreserveColor)
// are in use.
void Reset() { ClearJSReferences(); }
friend class mozilla::PromiseJobRunnable;
inline void ClearJSReferences() {
mCallback = nullptr;
mCallbackGlobal = nullptr;
mCreationStack = nullptr;
mIncumbentJSGlobal = nullptr;
CallbackObject(const CallbackObject&) = delete;
CallbackObject& operator=(const CallbackObject&) = delete;
void ClearJSObjects() {
MOZ_ASSERT_IF(mIncumbentJSGlobal, mCallback);
if (mCallback) {
// For use from subclasses that want to be usable with Rooted.
void Trace(JSTracer* aTracer);
// For use from subclasses that want to be traced for a bit then possibly
// switch to HoldJSObjects and do other slow JS-related init work we might do.
// If we have more than one owner, this will HoldJSObjects and do said slow
// init work; otherwise it will just forget all our JS references.
void FinishSlowJSInitIfMoreThanOneOwner(JSContext* aCx);
// Struct used as a way to force a CallbackObject constructor to not call
// HoldJSObjects. We're putting it here so that CallbackObject subclasses will
// have access to it, but outside code will not.
// Places that use this need to ensure that the callback is traced (e.g. via a
// Rooted) until the HoldJSObjects call happens.
struct FastCallbackConstructor {};
// Just like the public version without the FastCallbackConstructor argument,
// except for not calling HoldJSObjects and not capturing async stacks (on the
// assumption that we will do that last whenever we decide to actually
// HoldJSObjects; see FinishSlowJSInitIfMoreThanOneOwner). If you use this,
// you MUST ensure that the object is traced until the HoldJSObjects happens!
CallbackObject(JSObject* aCallback, JSObject* aCallbackGlobal,
const FastCallbackConstructor&) {
InitNoHold(aCallback, aCallbackGlobal, nullptr, nullptr);
// mCallback is not unwrapped, so it can be a cross-compartment-wrapper.
// This is done to ensure that, if JS code can't call a callback f(), or get
// its members, directly itself, this code won't call f(), or get its members,
// on the code's behalf.
JS::Heap<JSObject*> mCallback;
// mCallbackGlobal is the global that we were in when we created the
// callback. In particular, it is guaranteed to be same-compartment with
// aCallback. We store it separately, because we have no way to recover the
// global if mCallback is a cross-compartment wrapper.
JS::Heap<JSObject*> mCallbackGlobal;
JS::Heap<JSObject*> mCreationStack;
// Ideally, we'd just hold a reference to the nsIGlobalObject, since that's
// what we need to pass to AutoIncumbentScript. Unfortunately, that doesn't
// hold the actual JS global alive. So we maintain an additional pointer to
// the JS global itself so that we can trace it.
// At some point we should consider trying to make native globals hold their
// scripted global alive, at which point we can get rid of the duplication
// here.
nsCOMPtr<nsIGlobalObject> mIncumbentGlobal;
JS::TenuredHeap<JSObject*> mIncumbentJSGlobal;
class MOZ_STACK_CLASS CallSetup {
* A class that performs whatever setup we need to safely make a
* call while this class is on the stack, After the constructor
* returns, the call is safe to make if GetContext() returns
* non-null.
// If aExceptionHandling == eRethrowContentExceptions then aRealm
// needs to be set to the realm in which exceptions will be rethrown.
// If aExceptionHandling == eRethrowExceptions then aRealm may be set
// to the realm in which exceptions will be rethrown. In that case
// they will only be rethrown if that realm's principal subsumes the
// principal of our (unwrapped) callback.
CallSetup(CallbackObject* aCallback, ErrorResult& aRv,
const char* aExecutionReason,
ExceptionHandling aExceptionHandling, JS::Realm* aRealm = nullptr,
bool aIsJSImplementedWebIDL = false);
JSContext* GetContext() const { return mCx; }
// Safe to call this after the constructor has run without throwing on the
// ErrorResult it was handed.
BindingCallContext& GetCallContext() { return *mCallContext; }
// We better not get copy-constructed
CallSetup(const CallSetup&) = delete;
bool ShouldRethrowException(JS::Handle<JS::Value> aException);
// Members which can go away whenever
JSContext* mCx;
// Caller's realm. This will only have a sensible value if
// mExceptionHandling == eRethrowContentExceptions.
JS::Realm* mRealm;
// And now members whose construction/destruction order we need to control.
Maybe<AutoEntryScript> mAutoEntryScript;
Maybe<AutoIncumbentScript> mAutoIncumbentScript;
Maybe<JS::Rooted<JSObject*>> mRootedCallable;
// The global of mRootedCallable.
Maybe<JS::Rooted<JSObject*>> mRootedCallableGlobal;
// Members which are used to set the async stack.
Maybe<JS::Rooted<JSObject*>> mAsyncStack;
Maybe<JS::AutoSetAsyncStackForNewCalls> mAsyncStackSetter;
// Can't construct a JSAutoRealm without a JSContext either. Also,
// Put mAr after mAutoEntryScript so that we exit the realm before we
// pop the script settings stack. Though in practice we'll often manually
// order those two things.
Maybe<JSAutoRealm> mAr;
// Our BindingCallContext. This is a Maybe so we can avoid constructing it
// until after we have a JSContext to construct it with.
Maybe<BindingCallContext> mCallContext;
// An ErrorResult to possibly re-throw exceptions on and whether
// we should re-throw them.
ErrorResult& mErrorResult;
const ExceptionHandling mExceptionHandling;
const bool mIsMainThread;
template <class WebIDLCallbackT, class XPCOMCallbackT>
class CallbackObjectHolder;
template <class T, class U>
void ImplCycleCollectionUnlink(CallbackObjectHolder<T, U>& aField);
class CallbackObjectHolderBase {
// Returns null on all failures
already_AddRefed<nsISupports> ToXPCOMCallback(CallbackObject* aCallback,
const nsIID& aIID) const;
template <class WebIDLCallbackT, class XPCOMCallbackT>
class CallbackObjectHolder : CallbackObjectHolderBase {
* A class which stores either a WebIDLCallbackT* or an XPCOMCallbackT*. Both
* types must inherit from nsISupports. The pointer that's stored can be
* null.
* When storing a WebIDLCallbackT*, mPtrBits is set to the pointer value.
* When storing an XPCOMCallbackT*, mPtrBits is the pointer value with low bit
* set.
explicit CallbackObjectHolder(WebIDLCallbackT* aCallback)
: mPtrBits(reinterpret_cast<uintptr_t>(aCallback)) {
explicit CallbackObjectHolder(XPCOMCallbackT* aCallback)
: mPtrBits(reinterpret_cast<uintptr_t>(aCallback) | XPCOMCallbackFlag) {
CallbackObjectHolder(CallbackObjectHolder&& aOther)
: mPtrBits(aOther.mPtrBits) {
aOther.mPtrBits = 0;
static_assert(sizeof(CallbackObjectHolder) == sizeof(void*),
"This object is expected to be as small as a pointer, and it "
"is currently passed by value in various places. If it is "
"bloating, we may want to pass it by reference then.");
CallbackObjectHolder(const CallbackObjectHolder& aOther) = delete;
CallbackObjectHolder() : mPtrBits(0) {}
~CallbackObjectHolder() { UnlinkSelf(); }
void operator=(WebIDLCallbackT* aCallback) {
mPtrBits = reinterpret_cast<uintptr_t>(aCallback);
void operator=(XPCOMCallbackT* aCallback) {
mPtrBits = reinterpret_cast<uintptr_t>(aCallback) | XPCOMCallbackFlag;
void operator=(CallbackObjectHolder&& aOther) {
mPtrBits = aOther.mPtrBits;
aOther.mPtrBits = 0;
void operator=(const CallbackObjectHolder& aOther) = delete;
void Reset() { UnlinkSelf(); }
nsISupports* GetISupports() const {
return reinterpret_cast<nsISupports*>(mPtrBits & ~XPCOMCallbackFlag);
already_AddRefed<nsISupports> Forget() {
// This can be called from random threads. Make sure to not refcount things
// in here!
nsISupports* supp = GetISupports();
mPtrBits = 0;
return dont_AddRef(supp);
// Boolean conversion operator so people can use this in boolean tests
explicit operator bool() const { return GetISupports(); }
CallbackObjectHolder Clone() const {
CallbackObjectHolder result;
result.mPtrBits = mPtrBits;
return result;
// Even if HasWebIDLCallback returns true, GetWebIDLCallback() might still
// return null.
bool HasWebIDLCallback() const { return !(mPtrBits & XPCOMCallbackFlag); }
WebIDLCallbackT* GetWebIDLCallback() const {
return reinterpret_cast<WebIDLCallbackT*>(mPtrBits);
XPCOMCallbackT* GetXPCOMCallback() const {
return reinterpret_cast<XPCOMCallbackT*>(mPtrBits & ~XPCOMCallbackFlag);
bool operator==(WebIDLCallbackT* aOtherCallback) const {
if (!aOtherCallback) {
// If other is null, then we must be null to be equal.
return !GetISupports();
if (!HasWebIDLCallback() || !GetWebIDLCallback()) {
// If other is non-null, then we can't be equal if we have a
// non-WebIDL callback or a null callback.
return false;
return *GetWebIDLCallback() == *aOtherCallback;
bool operator==(XPCOMCallbackT* aOtherCallback) const {
return (!aOtherCallback && !GetISupports()) ||
(!HasWebIDLCallback() && GetXPCOMCallback() == aOtherCallback);
bool operator==(const CallbackObjectHolder& aOtherCallback) const {
if (aOtherCallback.HasWebIDLCallback()) {
return *this == aOtherCallback.GetWebIDLCallback();
return *this == aOtherCallback.GetXPCOMCallback();
// Try to return an XPCOMCallbackT version of this object.
already_AddRefed<XPCOMCallbackT> ToXPCOMCallback() const {
if (!HasWebIDLCallback()) {
RefPtr<XPCOMCallbackT> callback = GetXPCOMCallback();
return callback.forget();
nsCOMPtr<nsISupports> supp = CallbackObjectHolderBase::ToXPCOMCallback(
GetWebIDLCallback(), NS_GET_TEMPLATE_IID(XPCOMCallbackT));
if (supp) {
// ToXPCOMCallback already did the right QI for us.
return supp.forget().downcast<XPCOMCallbackT>();
return nullptr;
// Try to return a WebIDLCallbackT version of this object.
already_AddRefed<WebIDLCallbackT> ToWebIDLCallback() const {
if (HasWebIDLCallback()) {
RefPtr<WebIDLCallbackT> callback = GetWebIDLCallback();
return callback.forget();
return nullptr;
static const uintptr_t XPCOMCallbackFlag = 1u;
friend void ImplCycleCollectionUnlink<WebIDLCallbackT, XPCOMCallbackT>(
CallbackObjectHolder& aField);
void UnlinkSelf() {
// NS_IF_RELEASE because we might have been unlinked before
nsISupports* ptr = GetISupports();
// Clear mPtrBits before the release to prevent reentrance.
mPtrBits = 0;
uintptr_t mPtrBits;
template <class T, class U>
inline void ImplCycleCollectionTraverse(
nsCycleCollectionTraversalCallback& aCallback,
CallbackObjectHolder<T, U>& aField, const char* aName,
uint32_t aFlags = 0) {
if (aField) {
CycleCollectionNoteChild(aCallback, aField.GetISupports(), aName, aFlags);
template <class T, class U>
void ImplCycleCollectionUnlink(CallbackObjectHolder<T, U>& aField) {
// T is expected to be a RefPtr or OwningNonNull around a CallbackObject
// subclass. This class is used in bindings to safely handle Fast* callbacks;
// it ensures that the callback is traced, and that if something is holding onto
// the callback when we're done with it HoldJSObjects is called.
// Since we effectively hold a ref to a refcounted thing (like RefPtr or
// OwningNonNull), we are also MOZ_IS_SMARTPTR_TO_REFCOUNTED for static analysis
// purposes.
template <typename T>
: public JS::Rooted<T> {
explicit RootedCallback(JSContext* cx) : JS::Rooted<T>(cx), mCx(cx) {}
// We need a way to make assignment from pointers (how we're normally used)
// work.
template <typename S>
void operator=(S* arg) {
// But nullptr can't use the above template, because it doesn't know which S
// to select. So we need a special overload for nullptr.
void operator=(decltype(nullptr) arg) { this->get().operator=(arg); }
// Codegen relies on being able to do CallbackOrNull() and Callback() on us.
JSObject* CallbackOrNull() const { return this->get()->CallbackOrNull(); }
JSObject* Callback(JSContext* aCx) const {
return this->get()->Callback(aCx);
~RootedCallback() {
// Ensure that our callback starts holding on to its own JS objects as
// needed. We really do need to check that things are initialized even when
// T is OwningNonNull, because we might be running before the OwningNonNull
// ever got assigned to!
if (IsInitialized(this->get())) {
template <typename U>
static bool IsInitialized(U& aArg); // Not implemented
template <typename U>
static bool IsInitialized(RefPtr<U>& aRefPtr) {
return aRefPtr;
template <typename U>
static bool IsInitialized(OwningNonNull<U>& aOwningNonNull) {
return aOwningNonNull.isInitialized();
JSContext* mCx;
} // namespace dom
} // namespace mozilla
#endif // mozilla_dom_CallbackObject_h