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diff --git a/mfbt/RefCounted.h b/mfbt/RefCounted.h
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+++ b/mfbt/RefCounted.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/. */
+
+/* CRTP refcounting templates.  Do not use unless you are an Expert. */
+
+#ifndef mozilla_RefCounted_h
+#define mozilla_RefCounted_h
+
+#include <utility>
+#include <type_traits>
+
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/RefCountType.h"
+
+#ifdef __wasi__
+#  include "mozilla/WasiAtomic.h"
+#else
+#  include <atomic>
+#endif  // __wasi__
+
+#if defined(MOZILLA_INTERNAL_API)
+#  include "nsXPCOM.h"
+#endif
+
+#if defined(MOZILLA_INTERNAL_API) && defined(NS_BUILD_REFCNT_LOGGING)
+#  define MOZ_REFCOUNTED_LEAK_CHECKING
+#endif
+
+namespace mozilla {
+
+/**
+ * RefCounted<T> is a sort of a "mixin" for a class T.  RefCounted
+ * manages, well, refcounting for T, and because RefCounted is
+ * parameterized on T, RefCounted<T> can call T's destructor directly.
+ * This means T doesn't need to have a virtual dtor and so doesn't
+ * need a vtable.
+ *
+ * RefCounted<T> is created with refcount == 0.  Newly-allocated
+ * RefCounted<T> must immediately be assigned to a RefPtr to make the
+ * refcount > 0.  It's an error to allocate and free a bare
+ * RefCounted<T>, i.e. outside of the RefPtr machinery.  Attempts to
+ * do so will abort DEBUG builds.
+ *
+ * Live RefCounted<T> have refcount > 0.  The lifetime (refcounts) of
+ * live RefCounted<T> are controlled by RefPtr<T> and
+ * RefPtr<super/subclass of T>.  Upon a transition from refcounted==1
+ * to 0, the RefCounted<T> "dies" and is destroyed.  The "destroyed"
+ * state is represented in DEBUG builds by refcount==0xffffdead.  This
+ * state distinguishes use-before-ref (refcount==0) from
+ * use-after-destroy (refcount==0xffffdead).
+ *
+ * Note that when deriving from RefCounted or AtomicRefCounted, you
+ * should add MOZ_DECLARE_REFCOUNTED_TYPENAME(ClassName) to the public
+ * section of your class, where ClassName is the name of your class.
+ *
+ * Note: SpiderMonkey should use js::RefCounted instead since that type
+ * will use appropriate js_delete and also not break ref-count logging.
+ */
+namespace detail {
+const MozRefCountType DEAD = 0xffffdead;
+
+// When building code that gets compiled into Gecko, try to use the
+// trace-refcount leak logging facilities.
+class RefCountLogger {
+ public:
+  // Called by `RefCounted`-like classes to log a successful AddRef call in the
+  // Gecko leak-logging system. This call is a no-op outside of Gecko. Should be
+  // called afer incrementing the reference count.
+  template <class T>
+  static void logAddRef(const T* aPointer, MozRefCountType aRefCount) {
+#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
+    const void* pointer = aPointer;
+    const char* typeName = aPointer->typeName();
+    uint32_t typeSize = aPointer->typeSize();
+    NS_LogAddRef(const_cast<void*>(pointer), aRefCount, typeName, typeSize);
+#endif
+  }
+
+  // Created by `RefCounted`-like classes to log a successful Release call in
+  // the Gecko leak-logging system. The constructor should be invoked before the
+  // refcount is decremented to avoid invoking `typeName()` with a zero
+  // reference count. This call is a no-op outside of Gecko.
+  class MOZ_STACK_CLASS ReleaseLogger final {
+   public:
+    template <class T>
+    explicit ReleaseLogger(const T* aPointer)
+#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
+        : mPointer(aPointer),
+          mTypeName(aPointer->typeName())
+#endif
+    {
+    }
+
+    void logRelease(MozRefCountType aRefCount) {
+#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
+      MOZ_ASSERT(aRefCount != DEAD);
+      NS_LogRelease(const_cast<void*>(mPointer), aRefCount, mTypeName);
+#endif
+    }
+
+#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
+    const void* mPointer;
+    const char* mTypeName;
+#endif
+  };
+};
+
+// This is used WeakPtr.h as well as this file.
+enum RefCountAtomicity { AtomicRefCount, NonAtomicRefCount };
+
+template <typename T, RefCountAtomicity Atomicity>
+class RC {
+ public:
+  explicit RC(T aCount) : mValue(aCount) {}
+
+  RC(const RC&) = delete;
+  RC& operator=(const RC&) = delete;
+  RC(RC&&) = delete;
+  RC& operator=(RC&&) = delete;
+
+  T operator++() { return ++mValue; }
+  T operator--() { return --mValue; }
+
+#ifdef DEBUG
+  void operator=(const T& aValue) { mValue = aValue; }
+#endif
+
+  operator T() const { return mValue; }
+
+ private:
+  T mValue;
+};
+
+template <typename T>
+class RC<T, AtomicRefCount> {
+ public:
+  explicit RC(T aCount) : mValue(aCount) {}
+
+  RC(const RC&) = delete;
+  RC& operator=(const RC&) = delete;
+  RC(RC&&) = delete;
+  RC& operator=(RC&&) = delete;
+
+  T operator++() {
+    // Memory synchronization is not required when incrementing a
+    // reference count.  The first increment of a reference count on a
+    // thread is not important, since the first use of the object on a
+    // thread can happen before it.  What is important is the transfer
+    // of the pointer to that thread, which may happen prior to the
+    // first increment on that thread.  The necessary memory
+    // synchronization is done by the mechanism that transfers the
+    // pointer between threads.
+    return mValue.fetch_add(1, std::memory_order_relaxed) + 1;
+  }
+
+  T operator--() {
+    // Since this may be the last release on this thread, we need
+    // release semantics so that prior writes on this thread are visible
+    // to the thread that destroys the object when it reads mValue with
+    // acquire semantics.
+    T result = mValue.fetch_sub(1, std::memory_order_release) - 1;
+    if (result == 0) {
+      // We're going to destroy the object on this thread, so we need
+      // acquire semantics to synchronize with the memory released by
+      // the last release on other threads, that is, to ensure that
+      // writes prior to that release are now visible on this thread.
+#if defined(MOZ_TSAN) || defined(__wasi__)
+      // TSan doesn't understand std::atomic_thread_fence, so in order
+      // to avoid a false positive for every time a refcounted object
+      // is deleted, we replace the fence with an atomic operation.
+      mValue.load(std::memory_order_acquire);
+#else
+      std::atomic_thread_fence(std::memory_order_acquire);
+#endif
+    }
+    return result;
+  }
+
+#ifdef DEBUG
+  // This method is only called in debug builds, so we're not too concerned
+  // about its performance.
+  void operator=(const T& aValue) {
+    mValue.store(aValue, std::memory_order_seq_cst);
+  }
+#endif
+
+  operator T() const {
+    // Use acquire semantics since we're not sure what the caller is
+    // doing.
+    return mValue.load(std::memory_order_acquire);
+  }
+
+  T IncrementIfNonzero() {
+    // This can be a relaxed load as any write of 0 that we observe will leave
+    // the field in a permanently zero (or `DEAD`) state (so a "stale" read of 0
+    // is fine), and any other value is confirmed by the CAS below.
+    //
+    // This roughly matches rust's Arc::upgrade implementation as of rust 1.49.0
+    T prev = mValue.load(std::memory_order_relaxed);
+    while (prev != 0) {
+      MOZ_ASSERT(prev != detail::DEAD,
+                 "Cannot IncrementIfNonzero if marked as dead!");
+      // TODO: It may be possible to use relaxed success ordering here?
+      if (mValue.compare_exchange_weak(prev, prev + 1,
+                                       std::memory_order_acquire,
+                                       std::memory_order_relaxed)) {
+        return prev + 1;
+      }
+    }
+    return 0;
+  }
+
+ private:
+  std::atomic<T> mValue;
+};
+
+template <typename T, RefCountAtomicity Atomicity>
+class RefCounted {
+ protected:
+  RefCounted() : mRefCnt(0) {}
+#ifdef DEBUG
+  ~RefCounted() { MOZ_ASSERT(mRefCnt == detail::DEAD); }
+#endif
+
+ public:
+  // Compatibility with RefPtr.
+  void AddRef() const {
+    // Note: this method must be thread safe for AtomicRefCounted.
+    MOZ_ASSERT(int32_t(mRefCnt) >= 0);
+    MozRefCountType cnt = ++mRefCnt;
+    detail::RefCountLogger::logAddRef(static_cast<const T*>(this), cnt);
+  }
+
+  void Release() const {
+    // Note: this method must be thread safe for AtomicRefCounted.
+    MOZ_ASSERT(int32_t(mRefCnt) > 0);
+    detail::RefCountLogger::ReleaseLogger logger(static_cast<const T*>(this));
+    MozRefCountType cnt = --mRefCnt;
+    // Note: it's not safe to touch |this| after decrementing the refcount,
+    // except for below.
+    logger.logRelease(cnt);
+    if (0 == cnt) {
+      // Because we have atomically decremented the refcount above, only
+      // one thread can get a 0 count here, so as long as we can assume that
+      // everything else in the system is accessing this object through
+      // RefPtrs, it's safe to access |this| here.
+#ifdef DEBUG
+      mRefCnt = detail::DEAD;
+#endif
+      delete static_cast<const T*>(this);
+    }
+  }
+
+  using HasThreadSafeRefCnt =
+      std::integral_constant<bool, Atomicity == AtomicRefCount>;
+
+  // Compatibility with wtf::RefPtr.
+  void ref() { AddRef(); }
+  void deref() { Release(); }
+  MozRefCountType refCount() const { return mRefCnt; }
+  bool hasOneRef() const {
+    MOZ_ASSERT(mRefCnt > 0);
+    return mRefCnt == 1;
+  }
+
+ private:
+  mutable RC<MozRefCountType, Atomicity> mRefCnt;
+};
+
+#ifdef MOZ_REFCOUNTED_LEAK_CHECKING
+// Passing override for the optional argument marks the typeName and
+// typeSize functions defined by this macro as overrides.
+#  define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...)           \
+    virtual const char* typeName() const __VA_ARGS__ { return #T; } \
+    virtual size_t typeSize() const __VA_ARGS__ { return sizeof(*this); }
+#else
+#  define MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(T, ...)
+#endif
+
+// Note that this macro is expanded unconditionally because it declares only
+// two small inline functions which will hopefully get eliminated by the linker
+// in non-leak-checking builds.
+#define MOZ_DECLARE_REFCOUNTED_TYPENAME(T)    \
+  const char* typeName() const { return #T; } \
+  size_t typeSize() const { return sizeof(*this); }
+
+}  // namespace detail
+
+template <typename T>
+class RefCounted : public detail::RefCounted<T, detail::NonAtomicRefCount> {
+ public:
+  ~RefCounted() {
+    static_assert(std::is_base_of<RefCounted, T>::value,
+                  "T must derive from RefCounted<T>");
+  }
+};
+
+namespace external {
+
+/**
+ * AtomicRefCounted<T> is like RefCounted<T>, with an atomically updated
+ * reference counter.
+ *
+ * NOTE: Please do not use this class, use NS_INLINE_DECL_THREADSAFE_REFCOUNTING
+ * instead.
+ */
+template <typename T>
+class AtomicRefCounted
+    : public mozilla::detail::RefCounted<T, mozilla::detail::AtomicRefCount> {
+ public:
+  ~AtomicRefCounted() {
+    static_assert(std::is_base_of<AtomicRefCounted, T>::value,
+                  "T must derive from AtomicRefCounted<T>");
+  }
+};
+
+}  // namespace external
+
+}  // namespace mozilla
+
+#endif  // mozilla_RefCounted_h