dom/bindings/CallbackObject.h
author Markus Stange <mstange@themasta.com>
Wed, 10 Jul 2019 20:48:14 +0000
changeset 482276 a6ad66ea6ddd7dd14eafc95f5128ee15c40cae18
parent 481955 e4a2b09bc49c736cdbaaaed9730f20190fb4ce08
child 517347 29a9227d08acce58e23e1bccd384d459f9276f92
permissions -rw-r--r--
Bug 1515214 - Add CallbackObject::GetDescription and TimeoutHandler::GetDescription. r=bzbarsky This allows us to create profiler markers whose description contains the name of the function and its file / line number. This allows the profiler UI to match up setTimeout callbacks for multiple instances of the same page load, in order to create meaningful profile comparisons based on markers. Differential Revision: https://phabricator.services.mozilla.com/D19192

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/**
 * 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 "nsISupports.h"
#include "nsISupportsImpl.h"
#include "nsCycleCollectionParticipant.h"
#include "js/Wrapper.h"
#include "mozilla/Assertions.h"
#include "mozilla/ErrorResult.h"
#include "mozilla/HoldDropJSObjects.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/OwningNonNull.h"
#include "mozilla/dom/ScriptSettings.h"
#include "nsWrapperCache.h"
#include "nsJSEnvironment.h"
#include "xpcpublic.h"
#include "jsapi.h"
#include "js/ContextOptions.h"
#include "js/TracingAPI.h"

namespace mozilla {

class PromiseJobRunnable;

namespace dom {

#define DOM_CALLBACKOBJECT_IID                       \
  {                                                  \
    0xbe74c190, 0x6d76, 0x4991, {                    \
      0x84, 0xb9, 0x65, 0x06, 0x99, 0xe6, 0x93, 0x2b \
    }                                                \
  }

class CallbackObject : public nsISupports {
 public:
  NS_DECLARE_STATIC_IID_ACCESSOR(DOM_CALLBACKOBJECT_IID)

  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
  NS_DECL_CYCLE_COLLECTION_SKIPPABLE_SCRIPT_HOLDER_CLASS(CallbackObject)

  // 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::ContextOptionsRef(aCx).asyncStack()) {
      JS::RootedObject stack(aCx);
      if (!JS::CaptureCurrentStack(aCx, &stack)) {
        JS_ClearPendingException(aCx);
      }
      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.
  JS::Handle<JSObject*> CallbackOrNull() const {
    mCallback.exposeToActiveJS();
    return CallbackPreserveColor();
  }

  JSObject* CallbackGlobalOrNull() const {
    mCallbackGlobal.exposeToActiveJS();
    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() {
    mCallback.exposeToActiveJS();
    mCallbackGlobal.exposeToActiveJS();
    mCreationStack.exposeToActiveJS();
  }

  /*
   * 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.
   *
   * This should only be called if you are certain that the return value won't
   * be passed into a JS API function and that it won't be stored without being
   * rooted (or otherwise signaling the stored value to the CC).
   *
   * Note that calling Reset() will also affect the value of any handle
   * previously returned here. Don't call Reset() if a handle is still in use.
   */
  JS::Handle<JSObject*> CallbackPreserveColor() const {
    // Calling fromMarkedLocation() is safe because we trace our mCallback, and
    // because the value of mCallback cannot change after if has been set
    // (except for calling Reset() as described above).
    return JS::Handle<JSObject*>::fromMarkedLocation(mCallback.address());
  }
  JS::Handle<JSObject*> CallbackGlobalPreserveColor() const {
    // The comment in CallbackPreserveColor applies here as well.
    return JS::Handle<JSObject*>::fromMarkedLocation(mCallbackGlobal.address());
  }

  /*
   * If the callback is known to be non-gray, then this method can be
   * used instead of CallbackOrNull() to avoid the overhead of
   * ExposeObjectToActiveJS().
   */
  JS::Handle<JSObject*> CallbackKnownNotGray() const {
    JS::AssertObjectIsNotGray(mCallback);
    return CallbackPreserveColor();
  }

  nsIGlobalObject* IncumbentGlobalOrNull() const { return mIncumbentGlobal; }

  enum ExceptionHandling {
    // Report any exception and don't throw it to the caller code.
    eReportExceptions,
    // 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.
    eRethrowContentExceptions,
    // Throw exceptions to the caller code, unless the caller realm is
    // provided, the exception is not a DOMException from the caller
    // realm, and the caller realm does not subsume our unwrapped callback.
    eRethrowExceptions
  };

  // 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);
  }

 protected:
  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 {
   public:
    explicit JSObjectsDropper(CallbackObject* aHolder) : mHolder(aHolder) {}

    ~JSObjectsDropper() { mHolder->ClearJSObjects(); }

   private:
    RefPtr<CallbackObject> mHolder;
  };

 private:
  inline void InitNoHold(JSObject* aCallback, JSObject* aCallbackGlobal,
                         JSObject* aCreationStack,
                         nsIGlobalObject* aIncumbentGlobal) {
    MOZ_ASSERT(aCallback && !mCallback);
    MOZ_ASSERT(aCallbackGlobal);
    MOZ_DIAGNOSTIC_ASSERT(js::GetObjectCompartment(aCallback) ==
                          js::GetObjectCompartment(aCallbackGlobal));
    MOZ_ASSERT(JS_IsGlobalObject(aCallbackGlobal));
    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);
    mozilla::HoldJSObjects(this);
  }

  // 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;

 protected:
  void ClearJSObjects() {
    MOZ_ASSERT_IF(mIncumbentJSGlobal, mCallback);
    if (mCallback) {
      ClearJSReferences();
    }
  }

  // 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.
     */
   public:
    // 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);
    MOZ_CAN_RUN_SCRIPT ~CallSetup();

    JSContext* GetContext() const { return mCx; }

   private:
    // 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 or eRethrowExceptions.
    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;

    // 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 {
 protected:
  // 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.
   */
 public:
  explicit CallbackObjectHolder(WebIDLCallbackT* aCallback)
      : mPtrBits(reinterpret_cast<uintptr_t>(aCallback)) {
    NS_IF_ADDREF(aCallback);
  }

  explicit CallbackObjectHolder(XPCOMCallbackT* aCallback)
      : mPtrBits(reinterpret_cast<uintptr_t>(aCallback) | XPCOMCallbackFlag) {
    NS_IF_ADDREF(aCallback);
  }

  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) {
    UnlinkSelf();
    mPtrBits = reinterpret_cast<uintptr_t>(aCallback);
    NS_IF_ADDREF(aCallback);
  }

  void operator=(XPCOMCallbackT* aCallback) {
    UnlinkSelf();
    mPtrBits = reinterpret_cast<uintptr_t>(aCallback) | XPCOMCallbackFlag;
    NS_IF_ADDREF(aCallback);
  }

  void operator=(CallbackObjectHolder&& aOther) {
    UnlinkSelf();
    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;
    NS_IF_ADDREF(GetISupports());
    return result;
  }

  // Even if HasWebIDLCallback returns true, GetWebIDLCallback() might still
  // return null.
  bool HasWebIDLCallback() const { return !(mPtrBits & XPCOMCallbackFlag); }

  WebIDLCallbackT* GetWebIDLCallback() const {
    MOZ_ASSERT(HasWebIDLCallback());
    return reinterpret_cast<WebIDLCallbackT*>(mPtrBits);
  }

  XPCOMCallbackT* GetXPCOMCallback() const {
    MOZ_ASSERT(!HasWebIDLCallback());
    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;
  }

 private:
  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;
    NS_IF_RELEASE(ptr);
  }

  uintptr_t mPtrBits;
};

NS_DEFINE_STATIC_IID_ACCESSOR(CallbackObject, DOM_CALLBACKOBJECT_IID)

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) {
  aField.UnlinkSelf();
}

// 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>
class MOZ_RAII MOZ_IS_SMARTPTR_TO_REFCOUNTED RootedCallback
    : public JS::Rooted<T> {
 public:
  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) {
    this->get().operator=(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.
  JS::Handle<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())) {
      this->get()->FinishSlowJSInitIfMoreThanOneOwner(mCx);
    }
  }

 private:
  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