xpcom/glue/nsProxyRelease.h

/* -*- 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 "nsThreadUtils.h"
#include "mozilla/Likely.h"
#include "mozilla/Move.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:
  explicit ProxyReleaseEvent(already_AddRefed<T> aDoomed)
  : mDoomed(aDoomed.take()) {}

  NS_IMETHOD Run() override
  {
    NS_IF_RELEASE(mDoomed);
    return NS_OK;
  }

private:
  T* MOZ_OWNING_REF mDoomed;
};

template<typename T>
void
ProxyRelease(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>(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(nsIEventTarget* aTarget,
                           already_AddRefed<T> aDoomed,
                           bool aAlwaysProxy)
  {
    ::detail::ProxyRelease(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(nsIEventTarget* aTarget,
                           already_AddRefed<T> aDoomed,
                           bool aAlwaysProxy)
  {
    ProxyReleaseISupports(aTarget, ToSupports(aDoomed.take()), aAlwaysProxy);
  }

  static void ProxyReleaseISupports(nsIEventTarget* aTarget,
                                    nsISupports* aDoomed,
                                    bool aAlwaysProxy);
};

} // namespace detail

/**
 * Ensures that the delete of a smart pointer occurs on the target thread.
 *
 * @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(nsIEventTarget* aTarget, already_AddRefed<T> aDoomed,
                bool aAlwaysProxy = false)
{
  ::detail::ProxyReleaseChooser<mozilla::IsBaseOf<nsISupports, T>::value>
    ::ProxyRelease(aTarget, mozilla::Move(aDoomed), aAlwaysProxy);
}

/**
 * Ensures that the delete of a smart pointer occurs on the main thread.
 *
 * @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(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<nsIThread> mainThread;
  if (!NS_IsMainThread() || aAlwaysProxy) {
    nsresult rv = NS_GetMainThread(getter_AddRefs(mainThread));

    if (NS_FAILED(rv)) {
      MOZ_ASSERT_UNREACHABLE("Could not get main thread; leaking an object!");
      mozilla::Unused << aDoomed.take();
      return;
    }
  }

  NS_ProxyRelease(mainThread, 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.
  explicit nsMainThreadPtrHolder(T* aPtr, bool aStrict = true)
    : mRawPtr(nullptr)
    , mStrict(aStrict)
  {
    // 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);
  }
  explicit nsMainThreadPtrHolder(already_AddRefed<T> aPtr, bool aString = true)
    : mRawPtr(aPtr.take())
    , mStrict(aString)
  {
    // 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) {
      NS_ReleaseOnMainThread(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;

  // 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