mfbt/RefCounted.h
author Georg Fritzsche <georg.fritzsche@googlemail.com>
Mon, 22 May 2017 15:33:29 +0700
changeset 410493 af4327fd49cd93c717328e61caf1af692a0e52a0
parent 374526 47d317cfa2672e3e843da7a369072590460e96c6
child 442021 cde4f36e7c2372a61bab8ac0709febe754897bb1
permissions -rw-r--r--
Bug 1361661 - Part 1: Generate headers with process data from Processes.yaml. r=dexter Adding the Gecko enums to Processes.yaml allows us to generate mappings from ProcessID to GeckoProcessType. We generate string tables with the Telemetry process names, so we can use these names consistently throughout Telemetry.

/* -*- 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"

#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 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 typename Conditional<Atomicity == AtomicRefCount,
                               Atomic<MozRefCountType>,
                               MozRefCountType>::Type 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