Source/WTF/wtf/ThreadSafeRefCounted.h

/*
 * Copyright (C) 2007, 2008, 2010, 2013, 2014 Apple Inc. All rights reserved.
 * Copyright (C) 2007 Justin Haygood (jhaygood@reaktix.com)
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1.  Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 * 2.  Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
 * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#pragma once

#include <atomic>
#include <wtf/FastMalloc.h>
#include <wtf/MainThread.h>
#include <wtf/Noncopyable.h>

namespace WTF {

#if defined(NDEBUG) && !ENABLE(SECURITY_ASSERTIONS)
#define CHECK_THREAD_SAFE_REF_COUNTED_LIFECYCLE 0
#else
#define CHECK_THREAD_SAFE_REF_COUNTED_LIFECYCLE 1
#endif

class ThreadSafeRefCountedBase {
    WTF_MAKE_NONCOPYABLE(ThreadSafeRefCountedBase);
    WTF_MAKE_FAST_ALLOCATED;
public:
    ThreadSafeRefCountedBase() = default;

#if CHECK_THREAD_SAFE_REF_COUNTED_LIFECYCLE
    ~ThreadSafeRefCountedBase()
    {
        // When this ThreadSafeRefCounted object is a part of another object, derefBase() is never called on this object.
        m_deletionHasBegun = true;
    }
#endif

    void ref() const
    {
#if CHECK_THREAD_SAFE_REF_COUNTED_LIFECYCLE
        ASSERT_WITH_SECURITY_IMPLICATION(!m_deletionHasBegun);
#endif
        ++m_refCount;
    }

    bool hasOneRef() const
    {
#if CHECK_THREAD_SAFE_REF_COUNTED_LIFECYCLE
        ASSERT(!m_deletionHasBegun);
#endif
        return refCount() == 1;
    }

    unsigned refCount() const
    {
        return m_refCount;
    }

protected:
    // Returns whether the pointer should be freed or not.
    bool derefBase() const
    {
        ASSERT(m_refCount);

#if CHECK_THREAD_SAFE_REF_COUNTED_LIFECYCLE
        ASSERT_WITH_SECURITY_IMPLICATION(!m_deletionHasBegun);
#endif

        if (UNLIKELY(!--m_refCount)) {
            // Setting m_refCount to 1 here prevents double delete within the destructor but not from another thread
            // since such a thread could have ref'ed this object long after it had been deleted. See webkit.org/b/201576.
            m_refCount = 1;
#if CHECK_THREAD_SAFE_REF_COUNTED_LIFECYCLE
            m_deletionHasBegun = true;
#endif
            return true;
        }

        return false;
    }

private:
    mutable std::atomic<unsigned> m_refCount { 1 };

#if CHECK_THREAD_SAFE_REF_COUNTED_LIFECYCLE
    mutable std::atomic<bool> m_deletionHasBegun { false };
#endif
};

enum class DestructionThread { Any, Main, MainRunLoop };

template<class T, DestructionThread destructionThread = DestructionThread::Any> class ThreadSafeRefCounted : public ThreadSafeRefCountedBase {
public:
    void deref() const
    {
        if (!derefBase())
            return;

        auto deleteThis = [this] {
            delete static_cast<const T*>(this);
        };
        switch (destructionThread) {
        case DestructionThread::Any:
            break;
        case DestructionThread::Main:
            ensureOnMainThread(WTFMove(deleteThis));
            return;
        case DestructionThread::MainRunLoop:
            ensureOnMainRunLoop(WTFMove(deleteThis));
            return;
        }
        deleteThis();
    }

protected:
    ThreadSafeRefCounted() = default;
};

} // namespace WTF

using WTF::ThreadSafeRefCounted;