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Diffstat (limited to '')
| -rw-r--r-- | xpcom/threads/nsProxyRelease.h | 390 |
1 files changed, 390 insertions, 0 deletions
diff --git a/xpcom/threads/nsProxyRelease.h b/xpcom/threads/nsProxyRelease.h new file mode 100644 index 0000000000..00f69e3287 --- /dev/null +++ b/xpcom/threads/nsProxyRelease.h @@ -0,0 +1,390 @@ +/* -*- 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/. */ + +#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. + * For various reasons (see bug 771074), this restriction applies to + * 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, + nsIEventTarget* aMainThreadEventTarget = nullptr) + : mRawPtr(aPtr), + mStrict(aStrict), + mMainThreadEventTarget(aMainThreadEventTarget) +#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, + nsIEventTarget* aMainThreadEventTarget = nullptr) + : mRawPtr(aPtr.take()), + mStrict(aStrict), + mMainThreadEventTarget(aMainThreadEventTarget) +#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) { + if (!mMainThreadEventTarget) { + mMainThreadEventTarget = do_GetMainThread(); + } + MOZ_ASSERT(mMainThreadEventTarget); + NS_ProxyRelease( +#ifdef RELEASE_OR_BETA + nullptr, +#else + mName, +#endif + mMainThreadEventTarget, 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; + + nsCOMPtr<nsIEventTarget> mMainThreadEventTarget; + +#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 |