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diff --git a/js/src/threading/Mutex.h b/js/src/threading/Mutex.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/. */
+
+#ifndef threading_Mutex_h
+#define threading_Mutex_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/PlatformMutex.h"
+#include "mozilla/ThreadLocal.h"
+#include "mozilla/Vector.h"
+
+#include <utility>
+
+#include "threading/ThreadId.h"
+
+namespace js {
+
+// A MutexId secifies the name and mutex order for a mutex.
+//
+// The mutex order defines the allowed order of mutex acqusition on a single
+// thread. Mutexes must be acquired in strictly increasing order. Mutexes with
+// the same order may not be held at the same time by that thread.
+struct MutexId {
+  const char* name;
+  uint32_t order;
+};
+
+// The Mutex class below wraps mozilla::detail::MutexImpl, but we don't want to
+// use public inheritance, and private inheritance is problematic because
+// Mutex's friends can access the private parent class as if it was public
+// inheritance.  So use a data member, but for Mutex to access the data member
+// we must override it and make Mutex a friend.
+class MutexImpl : public mozilla::detail::MutexImpl {
+ protected:
+  MutexImpl() : mozilla::detail::MutexImpl() {}
+
+  friend class Mutex;
+};
+
+// In debug builds, js::Mutex is a wrapper over MutexImpl that checks correct
+// locking order is observed.
+//
+// The class maintains a per-thread stack of currently-held mutexes to enable it
+// to check this.
+class Mutex {
+ private:
+  MutexImpl impl_;
+
+#ifdef DEBUG
+  const MutexId id_;
+  Mutex* prev_ = nullptr;
+  ThreadId owningThread_;
+
+  static MOZ_THREAD_LOCAL(Mutex*) HeldMutexStack;
+#endif
+
+ public:
+#ifdef DEBUG
+  static bool Init();
+
+  explicit Mutex(const MutexId& id) : id_(id) { MOZ_ASSERT(id_.order != 0); }
+
+  void lock();
+  void unlock();
+  void assertOwnedByCurrentThread() const;
+#else
+  static bool Init() { return true; }
+
+  explicit Mutex(const MutexId& id) {}
+
+  void lock() { impl_.lock(); }
+  void unlock() { impl_.unlock(); }
+  void assertOwnedByCurrentThread() const {};
+#endif
+
+ private:
+#ifdef DEBUG
+  void preLockChecks() const;
+  void postLockChecks();
+  void preUnlockChecks();
+#endif
+
+  friend class ConditionVariable;
+};
+
+}  // namespace js
+
+#endif  // threading_Mutex_h