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effc2ca999
All event targets should be threadsafe and implemented in C++, and so should be able to be used in `Sync` types in Rust code. This also required annotating all interfaces deriving from `nsIEventTarget`, as well as adding some associated constants to specific types to indicate to the static assertion that they have threadsafe reference counts. Differential Revision: https://phabricator.services.mozilla.com/D183592
328 lines
11 KiB
C++
328 lines
11 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* CRTP refcounting templates. Do not use unless you are an Expert. */
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#ifndef mozilla_RefCounted_h
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#define mozilla_RefCounted_h
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#include <utility>
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#include <type_traits>
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#include "mozilla/AlreadyAddRefed.h"
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#include "mozilla/Assertions.h"
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#include "mozilla/Atomics.h"
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#include "mozilla/Attributes.h"
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#include "mozilla/RefCountType.h"
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#ifdef __wasi__
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# include "mozilla/WasiAtomic.h"
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#else
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# include <atomic>
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#endif // __wasi__
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#if defined(MOZILLA_INTERNAL_API)
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# include "nsXPCOM.h"
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#endif
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#if defined(MOZILLA_INTERNAL_API) && defined(NS_BUILD_REFCNT_LOGGING)
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# define MOZ_REFCOUNTED_LEAK_CHECKING
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#endif
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namespace mozilla {
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/**
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* RefCounted<T> is a sort of a "mixin" for a class T. RefCounted
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* manages, well, refcounting for T, and because RefCounted is
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* parameterized on T, RefCounted<T> can call T's destructor directly.
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* This means T doesn't need to have a virtual dtor and so doesn't
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* need a vtable.
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*
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* RefCounted<T> is created with refcount == 0. Newly-allocated
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* RefCounted<T> must immediately be assigned to a RefPtr to make the
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* refcount > 0. It's an error to allocate and free a bare
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* RefCounted<T>, i.e. outside of the RefPtr machinery. Attempts to
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* do so will abort DEBUG builds.
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*
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* Live RefCounted<T> have refcount > 0. The lifetime (refcounts) of
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* live RefCounted<T> are controlled by RefPtr<T> and
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* RefPtr<super/subclass of T>. Upon a transition from refcounted==1
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* to 0, the RefCounted<T> "dies" and is destroyed. The "destroyed"
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* state is represented in DEBUG builds by refcount==0xffffdead. This
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* state distinguishes use-before-ref (refcount==0) from
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* use-after-destroy (refcount==0xffffdead).
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*
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* Note that when deriving from RefCounted or AtomicRefCounted, you
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* should add MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public
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* section of your class, where ClassName is the name of your class.
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*
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* Note: SpiderMonkey should use js::RefCounted instead since that type
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* will use appropriate js_delete and also not break ref-count logging.
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*/
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namespace detail {
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const MozRefCountType DEAD = 0xffffdead;
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// When building code that gets compiled into Gecko, try to use the
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// trace-refcount leak logging facilities.
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class RefCountLogger {
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public:
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// Called by `RefCounted`-like classes to log a successful AddRef call in the
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// Gecko leak-logging system. This call is a no-op outside of Gecko. Should be
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// called afer incrementing the reference count.
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template <class T>
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static void logAddRef(const T* aPointer, MozRefCountType aRefCount) {
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#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
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const void* pointer = aPointer;
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const char* typeName = aPointer->typeName();
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uint32_t typeSize = aPointer->typeSize();
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NS_LogAddRef(const_cast<void*>(pointer), aRefCount, typeName, typeSize);
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#endif
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}
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// Created by `RefCounted`-like classes to log a successful Release call in
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// the Gecko leak-logging system. The constructor should be invoked before the
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// refcount is decremented to avoid invoking `typeName()` with a zero
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// reference count. This call is a no-op outside of Gecko.
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class MOZ_STACK_CLASS ReleaseLogger final {
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public:
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template <class T>
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explicit ReleaseLogger(const T* aPointer)
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#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
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: mPointer(aPointer),
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mTypeName(aPointer->typeName())
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#endif
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{
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}
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void logRelease(MozRefCountType aRefCount) {
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#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
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MOZ_ASSERT(aRefCount != DEAD);
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NS_LogRelease(const_cast<void*>(mPointer), aRefCount, mTypeName);
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#endif
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}
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#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
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const void* mPointer;
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const char* mTypeName;
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#endif
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};
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};
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// This is used WeakPtr.h as well as this file.
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enum RefCountAtomicity { AtomicRefCount, NonAtomicRefCount };
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template <typename T, RefCountAtomicity Atomicity>
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class RC {
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public:
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explicit RC(T aCount) : mValue(aCount) {}
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RC(const RC&) = delete;
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RC& operator=(const RC&) = delete;
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RC(RC&&) = delete;
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RC& operator=(RC&&) = delete;
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T operator++() { return ++mValue; }
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T operator--() { return --mValue; }
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#ifdef DEBUG
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void operator=(const T& aValue) { mValue = aValue; }
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#endif
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operator T() const { return mValue; }
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private:
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T mValue;
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};
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template <typename T>
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class RC<T, AtomicRefCount> {
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public:
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explicit RC(T aCount) : mValue(aCount) {}
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RC(const RC&) = delete;
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RC& operator=(const RC&) = delete;
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RC(RC&&) = delete;
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RC& operator=(RC&&) = delete;
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T operator++() {
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// Memory synchronization is not required when incrementing a
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// reference count. The first increment of a reference count on a
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// thread is not important, since the first use of the object on a
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// thread can happen before it. What is important is the transfer
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// of the pointer to that thread, which may happen prior to the
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// first increment on that thread. The necessary memory
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// synchronization is done by the mechanism that transfers the
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// pointer between threads.
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return mValue.fetch_add(1, std::memory_order_relaxed) + 1;
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}
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T operator--() {
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// Since this may be the last release on this thread, we need
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// release semantics so that prior writes on this thread are visible
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// to the thread that destroys the object when it reads mValue with
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// acquire semantics.
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T result = mValue.fetch_sub(1, std::memory_order_release) - 1;
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if (result == 0) {
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// We're going to destroy the object on this thread, so we need
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// acquire semantics to synchronize with the memory released by
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// the last release on other threads, that is, to ensure that
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// writes prior to that release are now visible on this thread.
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#if defined(MOZ_TSAN) || defined(__wasi__)
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// TSan doesn't understand std::atomic_thread_fence, so in order
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// to avoid a false positive for every time a refcounted object
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// is deleted, we replace the fence with an atomic operation.
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mValue.load(std::memory_order_acquire);
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#else
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std::atomic_thread_fence(std::memory_order_acquire);
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#endif
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}
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return result;
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}
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#ifdef DEBUG
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// This method is only called in debug builds, so we're not too concerned
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// about its performance.
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void operator=(const T& aValue) {
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mValue.store(aValue, std::memory_order_seq_cst);
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}
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#endif
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operator T() const {
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// Use acquire semantics since we're not sure what the caller is
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// doing.
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return mValue.load(std::memory_order_acquire);
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}
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T IncrementIfNonzero() {
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// This can be a relaxed load as any write of 0 that we observe will leave
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// the field in a permanently zero (or `DEAD`) state (so a "stale" read of 0
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// is fine), and any other value is confirmed by the CAS below.
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//
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// This roughly matches rust's Arc::upgrade implementation as of rust 1.49.0
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T prev = mValue.load(std::memory_order_relaxed);
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while (prev != 0) {
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MOZ_ASSERT(prev != detail::DEAD,
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"Cannot IncrementIfNonzero if marked as dead!");
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// TODO: It may be possible to use relaxed success ordering here?
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if (mValue.compare_exchange_weak(prev, prev + 1,
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std::memory_order_acquire,
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std::memory_order_relaxed)) {
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return prev + 1;
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}
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}
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return 0;
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}
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private:
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std::atomic<T> mValue;
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};
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template <typename T, RefCountAtomicity Atomicity>
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class RefCounted {
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protected:
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RefCounted() : mRefCnt(0) {}
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#ifdef DEBUG
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~RefCounted() { MOZ_ASSERT(mRefCnt == detail::DEAD); }
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#endif
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public:
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// Compatibility with RefPtr.
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void AddRef() const {
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// Note: this method must be thread safe for AtomicRefCounted.
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MOZ_ASSERT(int32_t(mRefCnt) >= 0);
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MozRefCountType cnt = ++mRefCnt;
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detail::RefCountLogger::logAddRef(static_cast<const T*>(this), cnt);
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}
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void Release() const {
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// Note: this method must be thread safe for AtomicRefCounted.
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MOZ_ASSERT(int32_t(mRefCnt) > 0);
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detail::RefCountLogger::ReleaseLogger logger(static_cast<const T*>(this));
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MozRefCountType cnt = --mRefCnt;
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// Note: it's not safe to touch |this| after decrementing the refcount,
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// except for below.
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logger.logRelease(cnt);
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if (0 == cnt) {
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// Because we have atomically decremented the refcount above, only
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// one thread can get a 0 count here, so as long as we can assume that
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// everything else in the system is accessing this object through
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// RefPtrs, it's safe to access |this| here.
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#ifdef DEBUG
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mRefCnt = detail::DEAD;
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#endif
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delete static_cast<const T*>(this);
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}
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}
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using HasThreadSafeRefCnt =
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std::integral_constant<bool, Atomicity == AtomicRefCount>;
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// Compatibility with wtf::RefPtr.
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void ref() { AddRef(); }
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void deref() { Release(); }
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MozRefCountType refCount() const { return mRefCnt; }
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bool hasOneRef() const {
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MOZ_ASSERT(mRefCnt > 0);
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return mRefCnt == 1;
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}
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private:
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mutable RC<MozRefCountType, Atomicity> mRefCnt;
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};
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#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
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// Passing override for the optional argument marks the typeName and
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// typeSize functions defined by this macro as overrides.
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# define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...) \
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virtual const char* typeName() const __VA_ARGS__ { return #T; } \
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virtual size_t typeSize() const __VA_ARGS__ { return sizeof(*this); }
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#else
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# define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...)
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#endif
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// Note that this macro is expanded unconditionally because it declares only
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// two small inline functions which will hopefully get eliminated by the linker
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// in non-leak-checking builds.
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#define MOZ_DECLARE_REFCOUNTED_TYPENAME(T) \
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const char* typeName() const { return #T; } \
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size_t typeSize() const { return sizeof(*this); }
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} // namespace detail
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template <typename T>
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class RefCounted : public detail::RefCounted<T, detail::NonAtomicRefCount> {
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public:
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~RefCounted() {
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static_assert(std::is_base_of<RefCounted, T>::value,
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"T must derive from RefCounted<T>");
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}
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};
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namespace external {
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/**
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* AtomicRefCounted<T> is like RefCounted<T>, with an atomically updated
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* reference counter.
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*
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* NOTE: Please do not use this class, use NS_INLINE_DECL_THREADSAFE_REFCOUNTING
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* instead.
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*/
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template <typename T>
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class AtomicRefCounted
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: public mozilla::detail::RefCounted<T, mozilla::detail::AtomicRefCount> {
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public:
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~AtomicRefCounted() {
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static_assert(std::is_base_of<AtomicRefCounted, T>::value,
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"T must derive from AtomicRefCounted<T>");
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}
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};
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} // namespace external
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} // namespace mozilla
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#endif // mozilla_RefCounted_h
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