/* -*- 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/. */

/* CRTP refcounting templates.  Do not use unless you are an Expert. */

#ifndef mozilla_RefCounted_h

#define mozilla_RefCounted_h

#include "mozilla/AlreadyAddRefed.h"

#include "mozilla/Assertions.h"

#include "mozilla/Atomics.h"

#include "mozilla/Attributes.h"

#include "mozilla/Move.h"

#include "mozilla/RefCountType.h"

#include "mozilla/TypeTraits.h"

#include <atomic>

#if defined(MOZILLA_INTERNAL_API)

#include "nsXPCOM.h"

#endif

#if defined(MOZILLA_INTERNAL_API) && \

    (defined(DEBUG) || defined(FORCE_BUILD_REFCNT_LOGGING))

#define MOZ_REFCOUNTED_LEAK_CHECKING

#endif

namespace mozilla {

/**

 * RefCounted<T> is a sort of a "mixin" for a class T.  RefCounted

 * manages, well, refcounting for T, and because RefCounted is

 * parameterized on T, RefCounted<T> can call T's destructor directly.

 * This means T doesn't need to have a virtual dtor and so doesn't

 * need a vtable.

 *

 * RefCounted<T> is created with refcount == 0.  Newly-allocated

 * RefCounted<T> must immediately be assigned to a RefPtr to make the

 * refcount > 0.  It's an error to allocate and free a bare

 * RefCounted<T>, i.e. outside of the RefPtr machinery.  Attempts to

 * do so will abort DEBUG builds.

 *

 * Live RefCounted<T> have refcount > 0.  The lifetime (refcounts) of

 * live RefCounted<T> are controlled by RefPtr<T> and

 * RefPtr<super/subclass of T>.  Upon a transition from refcounted==1

 * to 0, the RefCounted<T> "dies" and is destroyed.  The "destroyed"

 * state is represented in DEBUG builds by refcount==0xffffdead.  This

 * state distinguishes use-before-ref (refcount==0) from

 * use-after-destroy (refcount==0xffffdead).

 *

 * Note that when deriving from RefCounted or AtomicRefCounted, you

 * should add MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public

 * section of your class, where ClassName is the name of your class.

 *

 * Note: SpiderMonkey should use js::RefCounted instead since that type

 * will use appropriate js_delete and also not break ref-count logging.

 */

namespace detail {

const MozRefCountType DEAD = 0xffffdead;

// When building code that gets compiled into Gecko, try to use the

// trace-refcount leak logging facilities.

#ifdef MOZ_REFCOUNTED_LEAK_CHECKING

class RefCountLogger {

 public:

  static void logAddRef(const void* aPointer, MozRefCountType aRefCount,

                        const char* aTypeName, uint32_t aInstanceSize) {

    MOZ_ASSERT(aRefCount != DEAD);

    NS_LogAddRef(const_cast<void*>(aPointer), aRefCount, aTypeName,

                 aInstanceSize);

  }

  static void logRelease(const void* aPointer, MozRefCountType aRefCount,

                         const char* aTypeName) {

    MOZ_ASSERT(aRefCount != DEAD);

    NS_LogRelease(const_cast<void*>(aPointer), aRefCount, aTypeName);

  }

};

#endif

// This is used WeakPtr.h as well as this file.

enum RefCountAtomicity { AtomicRefCount, NonAtomicRefCount };

template <typename T, RefCountAtomicity Atomicity>

class RC {

 public:

  explicit RC(T aCount) : mValue(aCount) {}

  T operator++() { return ++mValue; }

  T operator--() { return --mValue; }

  void operator=(const T& aValue) { mValue = aValue; }

  operator T() const { return mValue; }

 private:

  T mValue;

};

template <typename T>

class RC<T, AtomicRefCount> {

 public:

  explicit RC(T aCount) : mValue(aCount) {}

  T operator++() {

    // Memory synchronization is not required when incrementing a

    // reference count.  The first increment of a reference count on a

    // thread is not important, since the first use of the object on a

    // thread can happen before it.  What is important is the transfer

    // of the pointer to that thread, which may happen prior to the

    // first increment on that thread.  The necessary memory

    // synchronization is done by the mechanism that transfers the

    // pointer between threads.

    return mValue.fetch_add(1, std::memory_order_relaxed) + 1;

  }

  T operator--() {

    // Since this may be the last release on this thread, we need

    // release semantics so that prior writes on this thread are visible

    // to the thread that destroys the object when it reads mValue with

    // acquire semantics.

    T result = mValue.fetch_sub(1, std::memory_order_release) - 1;

    if (result == 0) {

      // We're going to destroy the object on this thread, so we need

      // acquire semantics to synchronize with the memory released by

      // the last release on other threads, that is, to ensure that

      // writes prior to that release are now visible on this thread.

      std::atomic_thread_fence(std::memory_order_acquire);

    }

    return result;

  }

  // This method is only called in debug builds, so we're not too concerned

  // about its performance.

  void operator=(const T& aValue) {

    mValue.store(aValue, std::memory_order_seq_cst);

  }

  operator T() const {

    // Use acquire semantics since we're not sure what the caller is

    // doing.

    return mValue.load(std::memory_order_acquire);

  }

 private:

  std::atomic<T> mValue;

};

template <typename T, RefCountAtomicity Atomicity>

class RefCounted {

 protected:

  RefCounted() : mRefCnt(0) {}

  ~RefCounted() { MOZ_ASSERT(mRefCnt == detail::DEAD); }

 public:

  // Compatibility with nsRefPtr.

  void AddRef() const {

    // Note: this method must be thread safe for AtomicRefCounted.

    MOZ_ASSERT(int32_t(mRefCnt) >= 0);

#ifndef MOZ_REFCOUNTED_LEAK_CHECKING

    ++mRefCnt;

#else

    const char* type = static_cast<const T*>(this)->typeName();

    uint32_t size = static_cast<const T*>(this)->typeSize();

    const void* ptr = static_cast<const T*>(this);

    MozRefCountType cnt = ++mRefCnt;

    detail::RefCountLogger::logAddRef(ptr, cnt, type, size);

#endif

  }

  void Release() const {

    // Note: this method must be thread safe for AtomicRefCounted.

    MOZ_ASSERT(int32_t(mRefCnt) > 0);

#ifndef MOZ_REFCOUNTED_LEAK_CHECKING

    MozRefCountType cnt = --mRefCnt;

#else

    const char* type = static_cast<const T*>(this)->typeName();

    const void* ptr = static_cast<const T*>(this);

    MozRefCountType cnt = --mRefCnt;

    // Note: it's not safe to touch |this| after decrementing the refcount,

    // except for below.

    detail::RefCountLogger::logRelease(ptr, cnt, type);

#endif

    if (0 == cnt) {

    // Because we have atomically decremented the refcount above, only

    // one thread can get a 0 count here, so as long as we can assume that

    // everything else in the system is accessing this object through

    // RefPtrs, it's safe to access |this| here.

#ifdef DEBUG

      mRefCnt = detail::DEAD;

#endif

      delete static_cast<const T*>(this);

    }

  }

  // Compatibility with wtf::RefPtr.

  void ref() { AddRef(); }

  void deref() { Release(); }

  MozRefCountType refCount() const { return mRefCnt; }

  bool hasOneRef() const {

    MOZ_ASSERT(mRefCnt > 0);

    return mRefCnt == 1;

  }

 private:

  mutable RC<MozRefCountType, Atomicity> mRefCnt;

};

#ifdef MOZ_REFCOUNTED_LEAK_CHECKING

// Passing override for the optional argument marks the typeName and

// typeSize functions defined by this macro as overrides.

#define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...)           \

  virtual const char* typeName() const __VA_ARGS__ { return #T; } \

  virtual size_t typeSize() const __VA_ARGS__ { return sizeof(*this); }

#else

#define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...)

#endif

// Note that this macro is expanded unconditionally because it declares only

// two small inline functions which will hopefully get eliminated by the linker

// in non-leak-checking builds.

#define MOZ_DECLARE_REFCOUNTED_TYPENAME(T)    \

  const char* typeName() const { return #T; } \

  size_t typeSize() const { return sizeof(*this); }

}  // namespace detail

template <typename T>

class RefCounted : public detail::RefCounted<T, detail::NonAtomicRefCount> {

 public:

  ~RefCounted() {

    static_assert(IsBaseOf<RefCounted, T>::value,

                  "T must derive from RefCounted<T>");

  }

};

namespace external {

/**

 * AtomicRefCounted<T> is like RefCounted<T>, with an atomically updated

 * reference counter.

 *

 * NOTE: Please do not use this class, use NS_INLINE_DECL_THREADSAFE_REFCOUNTING

 * instead.

 */

template <typename T>

class AtomicRefCounted

    : public mozilla::detail::RefCounted<T, mozilla::detail::AtomicRefCount> {

 public:

  ~AtomicRefCounted() {

    static_assert(IsBaseOf<AtomicRefCounted, T>::value,

                  "T must derive from AtomicRefCounted<T>");

  }

};

}  // namespace external

}  // namespace mozilla

#endif  // mozilla_RefCounted_h