gecko-dev/xpcom/threads/nsProxyRelease.h

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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 http://mozilla.org/MPL/2.0/. */
#ifndef nsProxyRelease_h__
#define nsProxyRelease_h__
#include "nsIEventTarget.h"
#include "nsIThread.h"
#include "nsCOMPtr.h"
#include "nsAutoPtr.h"
#include "MainThreadUtils.h"
#include "nsPrintfCString.h"
#include "nsThreadUtils.h"
#include "mozilla/Likely.h"
#include "mozilla/Move.h"
#include "mozilla/SystemGroup.h"
#include "mozilla/TypeTraits.h"
#include "mozilla/Unused.h"
#ifdef XPCOM_GLUE_AVOID_NSPR
#error NS_ProxyRelease implementation depends on NSPR.
#endif
namespace detail {
template<typename T>
class ProxyReleaseEvent : public mozilla::Runnable
{
public:
ProxyReleaseEvent(const char* aName, already_AddRefed<T> aDoomed)
: Runnable(aName), mDoomed(aDoomed.take()) {}
NS_IMETHOD Run() override
{
NS_IF_RELEASE(mDoomed);
return NS_OK;
}
NS_IMETHOD GetName(nsACString& aName) override
{
#ifdef RELEASE_OR_BETA
aName.Truncate();
#else
if (mName) {
aName.Append(nsPrintfCString("ProxyReleaseEvent for %s", mName));
} else {
aName.AssignLiteral("ProxyReleaseEvent");
}
#endif
return NS_OK;
}
private:
T* MOZ_OWNING_REF mDoomed;
};
template<typename T>
void
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;
}
if (!aAlwaysProxy) {
bool onCurrentThread = false;
rv = aTarget->IsOnCurrentThread(&onCurrentThread);
if (NS_SUCCEEDED(rv) && onCurrentThread) {
return;
}
}
nsCOMPtr<nsIRunnable> ev = new ProxyReleaseEvent<T>(aName, doomed.forget());
rv = aTarget->Dispatch(ev, NS_DISPATCH_NORMAL);
if (NS_FAILED(rv)) {
NS_WARNING("failed to post proxy release event, leaking!");
// It is better to leak the aDoomed object than risk crashing as
// a result of deleting it on the wrong thread.
}
}
template<bool nsISupportsBased>
struct ProxyReleaseChooser
{
template<typename T>
static void ProxyRelease(const char* aName,
nsIEventTarget* aTarget,
already_AddRefed<T> aDoomed,
bool aAlwaysProxy)
{
::detail::ProxyRelease(aName, aTarget, mozilla::Move(aDoomed), aAlwaysProxy);
}
};
template<>
struct ProxyReleaseChooser<true>
{
// We need an intermediate step for handling classes with ambiguous
// inheritance to nsISupports.
template<typename T>
static void ProxyRelease(const char* aName,
nsIEventTarget* aTarget,
already_AddRefed<T> aDoomed,
bool aAlwaysProxy)
{
ProxyReleaseISupports(aName, aTarget, ToSupports(aDoomed.take()), aAlwaysProxy);
}
static void 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.
*
* @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.
*/
template<class T>
inline NS_HIDDEN_(void)
NS_ProxyRelease(const char* aName, nsIEventTarget* aTarget,
already_AddRefed<T> aDoomed, bool aAlwaysProxy = false)
{
::detail::ProxyReleaseChooser<mozilla::IsBaseOf<nsISupports, T>::value>
::ProxyRelease(aName, aTarget, mozilla::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_ReleaseOnMainThreadSystemGroup 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_ReleaseOnMainThreadSystemGroup(const char* aName,
already_AddRefed<T> aDoomed,
bool aAlwaysProxy = false)
{
// 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> systemGroupEventTarget;
if (!NS_IsMainThread() || aAlwaysProxy) {
systemGroupEventTarget = mozilla::SystemGroup::EventTargetFor(mozilla::TaskCategory::Other);
if (!systemGroupEventTarget) {
MOZ_ASSERT_UNREACHABLE("Could not get main thread; leaking an object!");
mozilla::Unused << aDoomed.take();
return;
}
}
NS_ProxyRelease(aName, systemGroupEventTarget, mozilla::Move(aDoomed),
aAlwaysProxy);
}
template<class T>
inline NS_HIDDEN_(void)
NS_ReleaseOnMainThreadSystemGroup(already_AddRefed<T> aDoomed,
bool aAlwaysProxy = false)
{
NS_ReleaseOnMainThreadSystemGroup("NS_ReleaseOnMainThreadSystemGroup",
mozilla::Move(aDoomed), 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 recently began to
* apply 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 (now) 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 nsMainThreadPtrHolder final
{
public:
// We can only acquire a pointer on the main thread. We 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(nullptr)
, 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 = aPtr);
}
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.
}
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()
{
// 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;
// Whether to strictly enforce thread invariants in this class.
bool mStrict;
nsCOMPtr<nsIEventTarget> mMainThreadEventTarget;
#ifndef RELEASE_OR_BETA
const char* mName = nullptr;
#endif
// Copy constructor and operator= not implemented. Once constructed, the
// holder is immutable.
T& operator=(nsMainThreadPtrHolder& aOther);
nsMainThreadPtrHolder(const nsMainThreadPtrHolder& aOther);
};
template<class T>
class nsMainThreadPtrHandle
{
RefPtr<nsMainThreadPtrHolder<T>> mPtr;
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)
: mPtr(aOther.mPtr)
{
}
nsMainThreadPtrHandle& operator=(const nsMainThreadPtrHandle& aOther)
{
mPtr = aOther.mPtr;
return *this;
}
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. Off-main-thread consumers must treat
// these handles as opaque.
T* get()
{
if (mPtr) {
return mPtr.get()->get();
}
return nullptr;
}
const T* get() const
{
if (mPtr) {
return mPtr.get()->get();
}
return nullptr;
}
operator T*() { return get(); }
T* operator->() 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;
}
};
namespace mozilla {
template<typename T>
using PtrHolder = nsMainThreadPtrHolder<T>;
template<typename T>
using PtrHandle = nsMainThreadPtrHandle<T>;
} // namespace mozilla
#endif