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diff --git a/mfbt/WeakPtr.h b/mfbt/WeakPtr.h
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+/* -*- 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/. */
+
+/* Weak pointer functionality, implemented as a mixin for use with any class. */
+
+/**
+ * SupportsWeakPtr lets you have a pointer to an object 'Foo' without affecting
+ * its lifetime. It works by creating a single shared reference counted object
+ * (WeakReference) that each WeakPtr will access 'Foo' through. This lets 'Foo'
+ * clear the pointer in the WeakReference without having to know about all of
+ * the WeakPtrs to it and allows the WeakReference to live beyond the lifetime
+ * of 'Foo'.
+ *
+ * PLEASE NOTE: This weak pointer implementation is not thread-safe.
+ *
+ * Note that when deriving from SupportsWeakPtr you should add
+ * MOZ_DECLARE_WEAKREFERENCE_TYPENAME(ClassName) to the public section of your
+ * class, where ClassName is the name of your class.
+ *
+ * The overhead of WeakPtr is that accesses to 'Foo' becomes an additional
+ * dereference, and an additional heap allocated pointer sized object shared
+ * between all of the WeakPtrs.
+ *
+ * Example of usage:
+ *
+ *   // To have a class C support weak pointers, inherit from
+ *   // SupportsWeakPtr<C>.
+ *   class C : public SupportsWeakPtr<C>
+ *   {
+ *   public:
+ *     MOZ_DECLARE_WEAKREFERENCE_TYPENAME(C)
+ *     int mNum;
+ *     void act();
+ *   };
+ *
+ *   C* ptr = new C();
+ *
+ *   // Get weak pointers to ptr. The first time a weak pointer
+ *   // is obtained, a reference counted WeakReference object is created that
+ *   // can live beyond the lifetime of 'ptr'. The WeakReference
+ *   // object will be notified of 'ptr's destruction.
+ *   WeakPtr<C> weak = ptr;
+ *   WeakPtr<C> other = ptr;
+ *
+ *   // Test a weak pointer for validity before using it.
+ *   if (weak) {
+ *     weak->mNum = 17;
+ *     weak->act();
+ *   }
+ *
+ *   // Destroying the underlying object clears weak pointers to it.
+ *   delete ptr;
+ *
+ *   MOZ_ASSERT(!weak, "Deleting |ptr| clears weak pointers to it.");
+ *   MOZ_ASSERT(!other, "Deleting |ptr| clears all weak pointers to it.");
+ *
+ * WeakPtr is typesafe and may be used with any class. It is not required that
+ * the class be reference-counted or allocated in any particular way.
+ *
+ * The API was loosely inspired by Chromium's weak_ptr.h:
+ * http://src.chromium.org/svn/trunk/src/base/memory/weak_ptr.h
+ */
+
+#ifndef mozilla_WeakPtr_h
+#define mozilla_WeakPtr_h
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/RefCounted.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TypeTraits.h"
+
+#include <string.h>
+
+// Weak referencing is not implemeted as thread safe.  When a WeakPtr
+// is created or dereferenced on thread A but the real object is just
+// being Released() on thread B, there is a possibility of a race
+// when the proxy object (detail::WeakReference) is notified about
+// the real object destruction just between when thread A is storing
+// the object pointer locally and is about to add a reference to it.
+//
+// Hence, a non-null weak proxy object is considered to have a single
+// "owning thread".  It means that each query for a weak reference,
+// its dereference, and destruction of the real object must all happen
+// on a single thread.  The following macros implement assertions for
+// checking these conditions.
+//
+// We disable this on MinGW. MinGW has two threading models: win32
+// API based, which disables std::thread; and POSIX based which
+// enables it but requires an emulation library (winpthreads).
+// Rather than attempting to switch to pthread emulation at this point,
+// we are disabling the std::thread based assertion checking.
+//
+// In the future, to enable it we could
+// a. have libgcc/stdc++ support win32 threads natively
+// b. switch to POSIX-based threading in MinGW with pthread emulation
+// c. refactor it to not use std::thread
+
+#if !defined(__MINGW32__) && (defined(DEBUG) || (defined(NIGHTLY_BUILD) && !defined(MOZ_PROFILING)))
+
+#include <thread>
+#define MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK \
+  std::thread::id _owningThread; \
+  bool _empty; // If it was initialized as a placeholder with mPtr = nullptr.
+#define MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK() \
+  do { \
+    _owningThread = std::this_thread::get_id(); \
+    _empty = !p; \
+  } while (false)
+#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY() \
+  MOZ_DIAGNOSTIC_ASSERT(_empty || _owningThread == std::this_thread::get_id(), \
+                        "WeakPtr used on multiple threads")
+#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(that) \
+  (that)->AssertThreadSafety();
+
+#define MOZ_WEAKPTR_THREAD_SAFETY_CHECKING 1
+
+#else
+
+#define MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK
+#define MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK() do { } while (false)
+#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY() do { } while (false)
+#define MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(that) do { } while (false)
+
+#endif
+
+namespace mozilla {
+
+template <typename T> class WeakPtr;
+template <typename T> class SupportsWeakPtr;
+
+#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
+#define MOZ_DECLARE_WEAKREFERENCE_TYPENAME(T) \
+  static const char* weakReferenceTypeName() { return "WeakReference<" #T ">"; }
+#else
+#define MOZ_DECLARE_WEAKREFERENCE_TYPENAME(T)
+#endif
+
+namespace detail {
+
+// This can live beyond the lifetime of the class derived from
+// SupportsWeakPtr.
+template<class T>
+class WeakReference : public ::mozilla::RefCounted<WeakReference<T> >
+{
+public:
+  explicit WeakReference(T* p) : mPtr(p)
+  {
+    MOZ_WEAKPTR_INIT_THREAD_SAFETY_CHECK();
+  }
+
+  T* get() const {
+    MOZ_WEAKPTR_ASSERT_THREAD_SAFETY();
+    return mPtr;
+  }
+
+#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
+  const char* typeName() const
+  {
+    // The first time this is called mPtr is null, so don't
+    // invoke any methods on mPtr.
+    return T::weakReferenceTypeName();
+  }
+  size_t typeSize() const { return sizeof(*this); }
+#endif
+
+#ifdef MOZ_WEAKPTR_THREAD_SAFETY_CHECKING
+  void AssertThreadSafety() { MOZ_WEAKPTR_ASSERT_THREAD_SAFETY(); }
+#endif
+
+private:
+  friend class mozilla::SupportsWeakPtr<T>;
+
+  void detach() {
+    MOZ_WEAKPTR_ASSERT_THREAD_SAFETY();
+    mPtr = nullptr;
+  }
+
+  T* MOZ_NON_OWNING_REF mPtr;
+  MOZ_WEAKPTR_DECLARE_THREAD_SAFETY_CHECK
+};
+
+} // namespace detail
+
+template <typename T>
+class SupportsWeakPtr
+{
+protected:
+  ~SupportsWeakPtr()
+  {
+    static_assert(IsBaseOf<SupportsWeakPtr<T>, T>::value,
+                  "T must derive from SupportsWeakPtr<T>");
+    if (mSelfReferencingWeakPtr) {
+      mSelfReferencingWeakPtr.mRef->detach();
+    }
+  }
+
+private:
+  const WeakPtr<T>& SelfReferencingWeakPtr()
+  {
+    if (!mSelfReferencingWeakPtr) {
+      mSelfReferencingWeakPtr.mRef = new detail::WeakReference<T>(static_cast<T*>(this));
+    } else {
+      MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mSelfReferencingWeakPtr.mRef);
+    }
+    return mSelfReferencingWeakPtr;
+  }
+
+  const WeakPtr<const T>& SelfReferencingWeakPtr() const
+  {
+    const WeakPtr<T>& p = const_cast<SupportsWeakPtr*>(this)->SelfReferencingWeakPtr();
+    return reinterpret_cast<const WeakPtr<const T>&>(p);
+  }
+
+  friend class WeakPtr<T>;
+  friend class WeakPtr<const T>;
+
+  WeakPtr<T> mSelfReferencingWeakPtr;
+};
+
+template <typename T>
+class WeakPtr
+{
+  typedef detail::WeakReference<T> WeakReference;
+
+public:
+  WeakPtr& operator=(const WeakPtr& aOther)
+  {
+    mRef = aOther.mRef;
+    MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mRef);
+    return *this;
+  }
+
+  WeakPtr(const WeakPtr& aOther)
+  {
+    // The thread safety check is performed inside of the operator= method.
+    *this = aOther;
+  }
+
+  WeakPtr& operator=(T* aOther)
+  {
+    if (aOther) {
+      *this = aOther->SelfReferencingWeakPtr();
+    } else if (!mRef || mRef->get()) {
+      // Ensure that mRef is dereferenceable in the uninitialized state.
+      mRef = new WeakReference(nullptr);
+    }
+    // The thread safety check happens inside SelfReferencingWeakPtr
+    // or is initialized in the WeakReference constructor.
+    return *this;
+  }
+
+  MOZ_IMPLICIT WeakPtr(T* aOther)
+  {
+    *this = aOther;
+    MOZ_WEAKPTR_ASSERT_THREAD_SAFETY_DELEGATED(mRef);
+  }
+
+  // Ensure that mRef is dereferenceable in the uninitialized state.
+  WeakPtr() : mRef(new WeakReference(nullptr)) {}
+
+  operator T*() const { return mRef->get(); }
+  T& operator*() const { return *mRef->get(); }
+
+  T* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN { return mRef->get(); }
+
+  T* get() const { return mRef->get(); }
+
+private:
+  friend class SupportsWeakPtr<T>;
+
+  explicit WeakPtr(const RefPtr<WeakReference>& aOther) : mRef(aOther) {}
+
+  RefPtr<WeakReference> mRef;
+};
+
+} // namespace mozilla
+
+#endif /* mozilla_WeakPtr_h */