diff -r 000000000000 -r 6474c204b198 xpcom/base/ClearOnShutdown.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/base/ClearOnShutdown.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,108 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set sw=2 ts=8 et ft=cpp : */
+/* 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 "MainThreadUtils.h"
+
+/*
+ * This header exports one public method in the mozilla namespace:
+ *
+ *   template<class SmartPtr>
+ *   void ClearOnShutdown(SmartPtr *aPtr)
+ *
+ * This function takes a pointer to a smart pointer and nulls the smart pointer
+ * on shutdown.
+ *
+ * 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 {
+namespace ClearOnShutdown_Internal {
+
+class ShutdownObserver : public LinkedListElement<ShutdownObserver>
+{
+public:
+  virtual void Shutdown() = 0;
+  virtual ~ShutdownObserver() {}
+};
+
+template<class SmartPtr>
+class PointerClearer : public ShutdownObserver
+{
+public:
+  PointerClearer(SmartPtr *aPtr)
+    : mPtr(aPtr)
+  {}
+
+  virtual void Shutdown()
+  {
+    if (mPtr) {
+      *mPtr = nullptr;
+    }
+  }
+
+private:
+  SmartPtr *mPtr;
+};
+
+extern bool sHasShutDown;
+extern StaticAutoPtr<LinkedList<ShutdownObserver> > sShutdownObservers;
+
+} // namespace ClearOnShutdown_Internal
+
+template<class SmartPtr>
+inline void ClearOnShutdown(SmartPtr *aPtr)
+{
+  using namespace ClearOnShutdown_Internal;
+
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(!sHasShutDown);
+
+  if (!sShutdownObservers) {
+    sShutdownObservers = new LinkedList<ShutdownObserver>();
+  }
+  sShutdownObservers->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.
+inline void KillClearOnShutdown()
+{
+  using namespace ClearOnShutdown_Internal;
+
+  MOZ_ASSERT(NS_IsMainThread());
+
+  if (sShutdownObservers) {
+    ShutdownObserver *observer;
+    while ((observer = sShutdownObservers->popFirst())) {
+      observer->Shutdown();
+      delete observer;
+    }
+  }
+
+  sShutdownObservers = nullptr;
+  sHasShutDown = true;
+}
+
+} // namespace mozilla
+
+#endif