gecko-dev/xpcom/glue/nsProxyRelease.h

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/* -*- Mode: C++; tab-width: 3; indent-tabs-mode: nil; c-basic-offset: 3 -*- */
/* 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 "nsCOMPtr.h"
#include "nsAutoPtr.h"
#include "nsThreadUtils.h"
#ifdef XPCOM_GLUE_AVOID_NSPR
#error NS_ProxyRelease implementation depends on NSPR.
#endif
/**
* Ensure that a nsCOMPtr is released on the target thread.
*
* @see NS_ProxyRelease(nsIEventTarget*, nsISupports*, bool)
*/
template <class T>
inline NS_HIDDEN_(nsresult)
NS_ProxyRelease
(nsIEventTarget *target, nsCOMPtr<T> &doomed, bool alwaysProxy=false)
{
T* raw = nullptr;
doomed.swap(raw);
return NS_ProxyRelease(target, raw, alwaysProxy);
}
/**
* Ensure that a nsRefPtr is released on the target thread.
*
* @see NS_ProxyRelease(nsIEventTarget*, nsISupports*, bool)
*/
template <class T>
inline NS_HIDDEN_(nsresult)
NS_ProxyRelease
(nsIEventTarget *target, nsRefPtr<T> &doomed, bool alwaysProxy=false)
{
T* raw = nullptr;
doomed.swap(raw);
return NS_ProxyRelease(target, raw, alwaysProxy);
}
/**
* Ensures that the delete of a nsISupports object occurs on the target thread.
*
* @param target
* the target thread where the doomed object should be released.
* @param doomed
* the doomed object; the object to be released on the target thread.
* @param alwaysProxy
* normally, if NS_ProxyRelease is called on the target thread, then the
* doomed object will released directly. however, if this parameter is
* true, then an event will always be posted to the target thread for
* asynchronous release.
*/
NS_COM_GLUE nsresult
NS_ProxyRelease
(nsIEventTarget *target, nsISupports *doomed, bool alwaysProxy=false);
/**
* 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 nsRefPtr<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
{
public:
// We can only acquire a pointer on the main thread.
nsMainThreadPtrHolder(T* ptr) : mRawPtr(NULL) {
// We can only AddRef our pointer on the main thread, which means that the
// holder must be constructed on the main thread.
MOZ_ASSERT(NS_IsMainThread());
NS_IF_ADDREF(mRawPtr = ptr);
}
// We can be released on any thread.
~nsMainThreadPtrHolder() {
if (NS_IsMainThread()) {
NS_IF_RELEASE(mRawPtr);
} else {
nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
if (!mainThread) {
NS_WARNING("Couldn't get main thread! Leaking pointer.");
return;
}
NS_ProxyRelease(mainThread, mRawPtr);
}
}
T* get() {
// Nobody should be touching the raw pointer off-main-thread.
if (NS_UNLIKELY(!NS_IsMainThread()))
MOZ_CRASH();
return mRawPtr;
}
NS_IMETHOD_(nsrefcnt) Release();
NS_IMETHOD_(nsrefcnt) AddRef();
private:
// This class is threadsafe and reference-counted.
nsAutoRefCnt mRefCnt;
// Our wrapped pointer.
T* mRawPtr;
// Copy constructor and operator= not implemented. Once constructed, the
// holder is immutable.
T& operator=(nsMainThreadPtrHolder& other);
nsMainThreadPtrHolder(const nsMainThreadPtrHolder& other);
};
template<class T>
NS_IMPL_THREADSAFE_ADDREF(nsMainThreadPtrHolder<T>)
template<class T>
NS_IMPL_THREADSAFE_RELEASE(nsMainThreadPtrHolder<T>)
template<class T>
class nsMainThreadPtrHandle
{
nsRefPtr<nsMainThreadPtrHolder<T> > mPtr;
public:
nsMainThreadPtrHandle(nsMainThreadPtrHolder<T> *aHolder) : mPtr(aHolder) {}
nsMainThreadPtrHandle(const nsMainThreadPtrHandle& aOther) : mPtr(aOther.mPtr) {}
nsMainThreadPtrHandle& operator=(const nsMainThreadPtrHandle& aOther) {
mPtr = aOther.mPtr;
}
operator nsMainThreadPtrHolder<T>*() { return mPtr.get(); }
// 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() { return mPtr.get()->get(); }
operator T*() { return get(); }
T* operator->() { return get(); }
};
#endif