xpcom/base/ClearOnShutdown.h
author Simon Sapin <simon.sapin@exyr.org>
Mon, 31 Dec 2018 01:01:49 +0100
changeset 452667 91e7ea77c846ae753fb3a28ab81598d2dc7ac865
parent 448947 6f3709b3878117466168c40affa7bca0b60cf75b
child 457355 02f749ccdc4be100c7412d7698f250d65b296d12
permissions -rw-r--r--
Bug 1518045 - Publish selectors 0.21.0 on crates.io. r=emilio This cherry-picks https://github.com/servo/servo/pull/22577, with the relevant lockfile update.

/* -*- 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