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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
#ifndef nsProxyRelease_h__
#define nsProxyRelease_h__
#include <utility>
#include "MainThreadUtils.h"
#include "mozilla/Likely.h"
#include "mozilla/Unused.h"
#include "nsCOMPtr.h"
#include "nsIEventTarget.h"
#include "nsISerialEventTarget.h"
#include "nsIThread.h"
#include "nsPrintfCString.h"
#include "nsThreadUtils.h"
#ifdef XPCOM_GLUE_AVOID_NSPR
# error NS_ProxyRelease implementation depends on NSPR.
#endif
class nsIRunnable;
namespace detail {
template <typename T>
class ProxyReleaseEvent : public mozilla::CancelableRunnable {
public:
ProxyReleaseEvent(const char* aName, already_AddRefed<T> aDoomed)
: CancelableRunnable(aName), mDoomed(aDoomed.take()) {}
NS_IMETHOD Run() override {
NS_IF_RELEASE(mDoomed);
return NS_OK;
}
nsresult Cancel() override { return Run(); }
#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
NS_IMETHOD GetName(nsACString& aName) override {
if (mName) {
aName.Append(nsPrintfCString("ProxyReleaseEvent for %s", mName));
} else {
aName.AssignLiteral("ProxyReleaseEvent");
}
return NS_OK;
}
#endif
private:
T* MOZ_OWNING_REF mDoomed;
};
template <typename T>
nsresult ProxyRelease(const char* aName, nsIEventTarget* aTarget,
already_AddRefed<T> aDoomed, bool aAlwaysProxy) {
// Auto-managing release of the pointer.
RefPtr<T> doomed = aDoomed;
nsresult rv;
if (!doomed || !aTarget) {
return NS_ERROR_INVALID_ARG;
}
if (!aAlwaysProxy) {
bool onCurrentThread = false;
rv = aTarget->IsOnCurrentThread(&onCurrentThread);
if (NS_SUCCEEDED(rv) && onCurrentThread) {
return NS_OK;
}
}
nsCOMPtr<nsIRunnable> ev = new ProxyReleaseEvent<T>(aName, doomed.forget());
rv = aTarget->Dispatch(ev, NS_DISPATCH_NORMAL);
if (NS_FAILED(rv)) {
NS_WARNING(nsPrintfCString(
"failed to post proxy release event for %s, leaking!", aName)
.get());
// It is better to leak the aDoomed object than risk crashing as
// a result of deleting it on the wrong thread.
}
return rv;
}
template <bool nsISupportsBased>
struct ProxyReleaseChooser {
template <typename T>
static nsresult ProxyRelease(const char* aName, nsIEventTarget* aTarget,
already_AddRefed<T> aDoomed, bool aAlwaysProxy) {
return ::detail::ProxyRelease(aName, aTarget, std::move(aDoomed),
aAlwaysProxy);
}
};
template <>
struct ProxyReleaseChooser<true> {
// We need an intermediate step for handling classes with ambiguous
// inheritance to nsISupports.
template <typename T>
static nsresult ProxyRelease(const char* aName, nsIEventTarget* aTarget,
already_AddRefed<T> aDoomed, bool aAlwaysProxy) {
return ProxyReleaseISupports(aName, aTarget, ToSupports(aDoomed.take()),
aAlwaysProxy);
}
static nsresult ProxyReleaseISupports(const char* aName,
nsIEventTarget* aTarget,
nsISupports* aDoomed,
bool aAlwaysProxy);
};
} // namespace detail
/**
* Ensures that the delete of a smart pointer occurs on the target thread.
* Note: The doomed object will be leaked if dispatch to the target thread
* fails, as releasing it on the current thread may be unsafe
*
* @param aName
* the labelling name of the runnable involved in the releasing.
* @param aTarget
* the target thread where the doomed object should be released.
* @param aDoomed
* the doomed object; the object to be released on the target thread.
* @param aAlwaysProxy
* normally, if NS_ProxyRelease is called on the target thread, then the
* doomed object will be released directly. However, if this parameter is
* true, then an event will always be posted to the target thread for
* asynchronous release.
* @return result of the task which is dispatched to delete the smart pointer
* on the target thread.
* Note: The caller should not attempt to recover from an
* error code returned by trying to perform the final ->Release()
* manually.
*/
template <class T>
inline NS_HIDDEN_(nsresult)
NS_ProxyRelease(const char* aName, nsIEventTarget* aTarget,
already_AddRefed<T> aDoomed, bool aAlwaysProxy = false) {
return ::detail::ProxyReleaseChooser<
std::is_base_of<nsISupports, T>::value>::ProxyRelease(aName, aTarget,
std::move(aDoomed),
aAlwaysProxy);
}
/**
* Ensures that the delete of a smart pointer occurs on the main thread.
*
* @param aName
* the labelling name of the runnable involved in the releasing
* @param aDoomed
* the doomed object; the object to be released on the main thread.
* @param aAlwaysProxy
* normally, if NS_ReleaseOnMainThread is called on the main
* thread, then the doomed object will be released directly. However, if
* this parameter is true, then an event will always be posted to the
* main thread for asynchronous release.
*/
template <class T>
inline NS_HIDDEN_(void)
NS_ReleaseOnMainThread(const char* aName, already_AddRefed<T> aDoomed,
bool aAlwaysProxy = false) {
RefPtr<T> doomed = aDoomed;
if (!doomed) {
return; // Nothing to do.
}
// NS_ProxyRelease treats a null event target as "the current thread". So a
// handle on the main thread is only necessary when we're not already on the
// main thread or the release must happen asynchronously.
nsCOMPtr<nsIEventTarget> target;
if (!NS_IsMainThread() || aAlwaysProxy) {
target = mozilla::GetMainThreadSerialEventTarget();
if (!target) {
MOZ_ASSERT_UNREACHABLE("Could not get main thread; leaking an object!");
mozilla::Unused << doomed.forget().take();
return;
}
}
NS_ProxyRelease(aName, target, doomed.forget(), aAlwaysProxy);
}
/**
* Class to safely handle main-thread-only pointers off the main thread.
*
* Classes like XPCWrappedJS are main-thread-only, which means that it is
* forbidden to call methods on instances of these classes off the main thread.
* AddRef/Release as well.
*
* This presents a problem for consumers that wish to hold a callback alive
* on non-main-thread code. A common example of this is the proxy callback
* pattern, where non-main-thread code holds a strong-reference to the callback
* object, and dispatches new Runnables (also with a strong reference) to the
* main thread in order to execute the callback. This involves several AddRef
* and Release calls on the other thread, which is verboten.
*
* The basic idea of this class is to introduce a layer of indirection.
* nsMainThreadPtrHolder is a threadsafe reference-counted class that internally
* maintains one strong reference to the main-thread-only object. It must be
* instantiated on the main thread (so that the AddRef of the underlying object
* happens on the main thread), but consumers may subsequently pass references
* to the holder anywhere they please. These references are meant to be opaque
* when accessed off-main-thread (assertions enforce this).
*
* The semantics of RefPtr<nsMainThreadPtrHolder<T>> would be cumbersome, so we
* also introduce nsMainThreadPtrHandle<T>, which is conceptually identical to
* the above (though it includes various convenience methods). The basic pattern
* is as follows.
*
* // On the main thread:
* nsCOMPtr<nsIFooCallback> callback = ...;
* nsMainThreadPtrHandle<nsIFooCallback> callbackHandle =
* new nsMainThreadPtrHolder<nsIFooCallback>(callback);
* // Pass callbackHandle to structs/classes that might be accessed on other
* // threads.
*
* All structs and classes that might be accessed on other threads should store
* an nsMainThreadPtrHandle<T> rather than an nsCOMPtr<T>.
*/
template <class T>
class MOZ_IS_SMARTPTR_TO_REFCOUNTED nsMainThreadPtrHolder final {
public:
// We can only acquire a pointer on the main thread. We want to fail fast for
// threading bugs, so by default we assert if our pointer is used or acquired
// off-main-thread. But some consumers need to use the same pointer for
// multiple classes, some of which are main-thread-only and some of which
// aren't. So we allow them to explicitly disable this strict checking.
nsMainThreadPtrHolder(const char* aName, T* aPtr, bool aStrict = true)
: mRawPtr(aPtr),
mStrict(aStrict)
#ifndef RELEASE_OR_BETA
,
mName(aName)
#endif
{
// We can only AddRef our pointer on the main thread, which means that the
// holder must be constructed on the main thread.
MOZ_ASSERT(!mStrict || NS_IsMainThread());
NS_IF_ADDREF(mRawPtr);
}
nsMainThreadPtrHolder(const char* aName, already_AddRefed<T> aPtr,
bool aStrict = true)
: mRawPtr(aPtr.take()),
mStrict(aStrict)
#ifndef RELEASE_OR_BETA
,
mName(aName)
#endif
{
// Since we don't need to AddRef the pointer, this constructor is safe to
// call on any thread.
}
// Copy constructor and operator= deleted. Once constructed, the holder is
// immutable.
T& operator=(nsMainThreadPtrHolder& aOther) = delete;
nsMainThreadPtrHolder(const nsMainThreadPtrHolder& aOther) = delete;
private:
// We can be released on any thread.
~nsMainThreadPtrHolder() {
if (NS_IsMainThread()) {
NS_IF_RELEASE(mRawPtr);
} else if (mRawPtr) {
NS_ReleaseOnMainThread(
#ifdef RELEASE_OR_BETA
nullptr,
#else
mName,
#endif
dont_AddRef(mRawPtr));
}
}
public:
T* get() const {
// Nobody should be touching the raw pointer off-main-thread.
if (mStrict && MOZ_UNLIKELY(!NS_IsMainThread())) {
NS_ERROR("Can't dereference nsMainThreadPtrHolder off main thread");
MOZ_CRASH();
}
return mRawPtr;
}
bool operator==(const nsMainThreadPtrHolder<T>& aOther) const {
return mRawPtr == aOther.mRawPtr;
}
bool operator!() const { return !mRawPtr; }
NS_INLINE_DECL_THREADSAFE_REFCOUNTING(nsMainThreadPtrHolder<T>)
private:
// Our wrapped pointer.
T* mRawPtr = nullptr;
// Whether to strictly enforce thread invariants in this class.
bool mStrict = true;
#ifndef RELEASE_OR_BETA
const char* mName = nullptr;
#endif
};
template <class T>
class MOZ_IS_SMARTPTR_TO_REFCOUNTED nsMainThreadPtrHandle {
public:
nsMainThreadPtrHandle() : mPtr(nullptr) {}
MOZ_IMPLICIT nsMainThreadPtrHandle(decltype(nullptr)) : mPtr(nullptr) {}
explicit nsMainThreadPtrHandle(nsMainThreadPtrHolder<T>* aHolder)
: mPtr(aHolder) {}
explicit nsMainThreadPtrHandle(
already_AddRefed<nsMainThreadPtrHolder<T>> aHolder)
: mPtr(aHolder) {}
nsMainThreadPtrHandle(const nsMainThreadPtrHandle& aOther) = default;
nsMainThreadPtrHandle(nsMainThreadPtrHandle&& aOther) = default;
nsMainThreadPtrHandle& operator=(const nsMainThreadPtrHandle& aOther) =
default;
nsMainThreadPtrHandle& operator=(nsMainThreadPtrHandle&& aOther) = default;
nsMainThreadPtrHandle& operator=(nsMainThreadPtrHolder<T>* aHolder) {
mPtr = aHolder;
return *this;
}
// These all call through to nsMainThreadPtrHolder, and thus implicitly
// assert that we're on the main thread (if strict). Off-main-thread consumers
// must treat these handles as opaque.
T* get() const {
if (mPtr) {
return mPtr.get()->get();
}
return nullptr;
}
operator T*() const { return get(); }
T* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN { return get(); }
// These are safe to call on other threads with appropriate external locking.
bool operator==(const nsMainThreadPtrHandle<T>& aOther) const {
if (!mPtr || !aOther.mPtr) {
return mPtr == aOther.mPtr;
}
return *mPtr == *aOther.mPtr;
}
bool operator!=(const nsMainThreadPtrHandle<T>& aOther) const {
return !operator==(aOther);
}
bool operator==(decltype(nullptr)) const { return mPtr == nullptr; }
bool operator!=(decltype(nullptr)) const { return mPtr != nullptr; }
bool operator!() const { return !mPtr || !*mPtr; }
private:
RefPtr<nsMainThreadPtrHolder<T>> mPtr;
};
class nsCycleCollectionTraversalCallback;
template <typename T>
void CycleCollectionNoteChild(nsCycleCollectionTraversalCallback& aCallback,
T* aChild, const char* aName, uint32_t aFlags);
template <typename T>
inline void ImplCycleCollectionTraverse(
nsCycleCollectionTraversalCallback& aCallback,
nsMainThreadPtrHandle<T>& aField, const char* aName, uint32_t aFlags = 0) {
CycleCollectionNoteChild(aCallback, aField.get(), aName, aFlags);
}
template <typename T>
inline void ImplCycleCollectionUnlink(nsMainThreadPtrHandle<T>& aField) {
aField = nullptr;
}
#endif