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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 "nsIThread.h"

 #include "nsCOMPtr.h"

 #include "nsAutoPtr.h"

 #include "MainThreadUtils.h"

 #include "mozilla/Likely.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 MOZ_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(T* ptr, bool strict = true) : mRawPtr(nullptr), mStrict(strict) {

     // 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 = ptr);

   }

   // We can be released on any thread.

   ~nsMainThreadPtrHolder() {

     if (NS_IsMainThread()) {

       NS_IF_RELEASE(mRawPtr);

     } else if (mRawPtr) {

       nsCOMPtr<nsIThread> mainThread;

       NS_GetMainThread(getter_AddRefs(mainThread));

       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 (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; }

   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(nsMainThreadPtrHolder<T>)

 private:

   // Our wrapped pointer.

   T* mRawPtr;

   // Whether to strictly enforce thread invariants in this class.

   bool mStrict;

   // Copy constructor and operator= not implemented. Once constructed, the

   // holder is immutable.

   T& operator=(nsMainThreadPtrHolder& other);

   nsMainThreadPtrHolder(const nsMainThreadPtrHolder& other);

 };

 template<class T>

 class nsMainThreadPtrHandle

 {

   nsRefPtr<nsMainThreadPtrHolder<T> > mPtr;

   public:

   nsMainThreadPtrHandle() : mPtr(nullptr) {}

   nsMainThreadPtrHandle(nsMainThreadPtrHolder<T> *aHolder) : mPtr(aHolder) {}

   nsMainThreadPtrHandle(const nsMainThreadPtrHandle& aOther) : mPtr(aOther.mPtr) {}

   nsMainThreadPtrHandle& operator=(const nsMainThreadPtrHandle& aOther) {

     mPtr = aOther.mPtr;

     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->() { 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!() { return !mPtr; }

 };

 #endif