/* -*- 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 mozilla_ClearOnShutdown_h
#define mozilla_ClearOnShutdown_h

#include "mozilla/LinkedList.h"
#include "mozilla/StaticPtr.h"
#include "mozilla/Array.h"
#include "MainThreadUtils.h"

/*
 * This header exports one public method in the mozilla namespace:
 *
 *   template<class SmartPtr>
 *   void ClearOnShutdown(SmartPtr *aPtr, aPhase=ShutdownPhase::ShutdownFinal)
 *
 * This function takes a pointer to a smart pointer and nulls the smart pointer
 * on shutdown (and a particular phase of shutdown as needed).  If a phase
 * is specified, the ptr will be cleared at the start of that phase.  Also,
 * if a phase has already occurred when ClearOnShutdown() is called it will
 * cause a MOZ_ASSERT.  In case a phase is not explicitly cleared we will
 * clear it on the next phase that occurs.
 *
 * This is useful if you have a global smart pointer object which you don't
 * want to "leak" on shutdown.
 *
 * Although ClearOnShutdown will work with any smart pointer (i.e., nsCOMPtr,
 * nsRefPtr, nsAutoPtr, StaticRefPtr, and StaticAutoPtr), you probably want to
 * use it only with StaticRefPtr and StaticAutoPtr.  There is no way to undo a
 * call to ClearOnShutdown, so you can call it only on smart pointers which you
 * know will live until the program shuts down.  In practice, these are likely
 * global variables, which should be Static{Ref,Auto}Ptr.
 *
 * ClearOnShutdown is currently main-thread only because we don't want to
 * accidentally free an object from a different thread than the one it was
 * created on.
 */

namespace mozilla {

// Must be contiguous starting at 0
enum class ShutdownPhase {
  NotInShutdown = 0,
  WillShutdown,
  Shutdown,
  ShutdownThreads,
  ShutdownLoaders,
  ShutdownFinal,
  ShutdownPhase_Length, // never pass this value
  First = WillShutdown, // for iteration
  Last = ShutdownFinal
};

namespace ClearOnShutdown_Internal {

class ShutdownObserver : public LinkedListElement<ShutdownObserver>
{
public:
  virtual void Shutdown() = 0;
  virtual ~ShutdownObserver()
  {
  }
};

template<class SmartPtr>
class PointerClearer : public ShutdownObserver
{
public:
  explicit PointerClearer(SmartPtr* aPtr)
    : mPtr(aPtr)
  {
  }

  virtual void Shutdown() override
  {
    if (mPtr) {
      *mPtr = nullptr;
    }
  }

private:
  SmartPtr* mPtr;
};

typedef LinkedList<ShutdownObserver> ShutdownList;
extern Array<StaticAutoPtr<ShutdownList>,
             static_cast<size_t>(ShutdownPhase::ShutdownPhase_Length)> sShutdownObservers;
extern ShutdownPhase sCurrentShutdownPhase;

} // namespace ClearOnShutdown_Internal

template<class SmartPtr>
inline void
ClearOnShutdown(SmartPtr* aPtr, ShutdownPhase aPhase = ShutdownPhase::ShutdownFinal)
{
  using namespace ClearOnShutdown_Internal;

  MOZ_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(aPhase != ShutdownPhase::ShutdownPhase_Length);

  // Adding a ClearOnShutdown for a "past" phase is an error.
  if (!(static_cast<size_t>(sCurrentShutdownPhase) < static_cast<size_t>(aPhase))) {
    MOZ_ASSERT(false, "ClearOnShutdown for phase that already was cleared");
    *aPtr = nullptr;
    return;
  }

  if (!(sShutdownObservers[static_cast<size_t>(aPhase)])) {
    sShutdownObservers[static_cast<size_t>(aPhase)] = new ShutdownList();
  }
  sShutdownObservers[static_cast<size_t>(aPhase)]->insertBack(new PointerClearer<SmartPtr>(aPtr));
}

// Called when XPCOM is shutting down, after all shutdown notifications have
// been sent and after all threads' event loops have been purged.
void KillClearOnShutdown(ShutdownPhase aPhase);

} // namespace mozilla

#endif