1 files changed, 147 insertions, 0 deletions
diff --git a/xpcom/base/ClearOnShutdown.h b/xpcom/base/ClearOnShutdown.h
new file mode 100644
index 0000000000..f927e3334a
--- /dev/null
+++ b/xpcom/base/ClearOnShutdown.h
@@ -0,0 +1,147 @@
+/* -*- 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 "ShutdownPhase.h"
+#include "MainThreadUtils.h"
+
+#include <functional>
+
+/*
+ * This header exports two public methods in the mozilla namespace:
+ *
+ *   template<class SmartPtr>
+ *   void ClearOnShutdown(SmartPtr *aPtr,
+ * aPhase=ShutdownPhase::XPCOMShutdownFinal)
+ *
+ * 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,
+ * RefPtr, 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.
+ *
+ *   template <typename CallableT>
+ *   void RunOnShutdown(CallableT&& aCallable,
+ *                      aPhase = ShutdownPhase::XPCOMShutdownFinal)
+ *
+ * This function takes a callable and executes it upon shutdown at the start of
+ * the specified phase. If the phase has already occurred when RunOnShutdown()
+ * 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.
+ *
+ * ClearOnShutdown and RunOnShutdown are both 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() = default;
+};
+
+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;
+};
+
+class FunctionInvoker : public ShutdownObserver {
+ public:
+  template <typename CallableT>
+  explicit FunctionInvoker(CallableT&& aCallable)
+      : mCallable(std::forward<CallableT>(aCallable)) {}
+
+  virtual void Shutdown() override {
+    if (!mCallable) {
+      return;
+    }
+
+    mCallable();
+  }
+
+ private:
+  std::function<void()> mCallable;
+};
+
+void InsertIntoShutdownList(ShutdownObserver* aShutdownObserver,
+                            ShutdownPhase aPhase);
+
+typedef LinkedList<ShutdownObserver> ShutdownList;
+extern Array<StaticAutoPtr<ShutdownList>,
+             static_cast<size_t>(ShutdownPhase::ShutdownPhase_Length)>
+    sShutdownObservers;
+extern ShutdownPhase sCurrentClearOnShutdownPhase;
+
+}  // namespace ClearOnShutdown_Internal
+
+template <class SmartPtr>
+inline void ClearOnShutdown(
+    SmartPtr* aPtr, ShutdownPhase aPhase = ShutdownPhase::XPCOMShutdownFinal) {
+  using namespace ClearOnShutdown_Internal;
+
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPhase != ShutdownPhase::ShutdownPhase_Length);
+
+  InsertIntoShutdownList(new PointerClearer<SmartPtr>(aPtr), aPhase);
+}
+
+template <typename CallableT>
+inline void RunOnShutdown(
+    CallableT&& aCallable,
+    ShutdownPhase aPhase = ShutdownPhase::XPCOMShutdownFinal) {
+  using namespace ClearOnShutdown_Internal;
+
+  MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(aPhase != ShutdownPhase::ShutdownPhase_Length);
+
+  InsertIntoShutdownList(
+      new FunctionInvoker(std::forward<CallableT>(aCallable)), aPhase);
+}
+
+inline bool PastShutdownPhase(ShutdownPhase aPhase) {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  return size_t(ClearOnShutdown_Internal::sCurrentClearOnShutdownPhase) >=
+         size_t(aPhase);
+}
+
+// Called by AdvanceShutdownPhase each time we switch a phase. Will null out
+// pointers added by ClearOnShutdown for all phases up to and including aPhase.
+void KillClearOnShutdown(ShutdownPhase aPhase);
+
+}  // namespace mozilla
+
+#endif