1 files changed, 124 insertions, 0 deletions
diff --git a/xpcom/base/ClearOnShutdown.h b/xpcom/base/ClearOnShutdown.h
new file mode 100644
index 0000000000..5c39c281cc
--- /dev/null
+++ b/xpcom/base/ClearOnShutdown.h
@@ -0,0 +1,124 @@
+/* -*- 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