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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/Attributes.h"
+#include "mozilla/Move.h"
+#include "mozilla/ThreadLocal.h"
+#include "mozilla/Vector.h"
+
+#include <new>
+#include <string.h>
+
+namespace js {
+
+class ConditionVariable;
+
+namespace detail {
+
+class MutexImpl
+{
+public:
+  struct PlatformData;
+
+  MutexImpl();
+  ~MutexImpl();
+
+  MutexImpl(MutexImpl&& rhs)
+    : platformData_(rhs.platformData_)
+  {
+    MOZ_ASSERT(this != &rhs, "self move disallowed!");
+    rhs.platformData_ = nullptr;
+  }
+
+  MutexImpl& operator=(MutexImpl&& rhs) {
+    this->~MutexImpl();
+    new (this) MutexImpl(mozilla::Move(rhs));
+    return *this;
+  }
+
+  bool operator==(const MutexImpl& rhs) {
+    return platformData_ == rhs.platformData_;
+  }
+
+protected:
+  void lock();
+  void unlock();
+
+private:
+  MutexImpl(const MutexImpl&) = delete;
+  void operator=(const MutexImpl&) = delete;
+
+  friend class js::ConditionVariable;
+  PlatformData* platformData() {
+    MOZ_ASSERT(platformData_);
+    return platformData_;
+  };
+
+  PlatformData* platformData_;
+};
+
+} // namespace detail
+
+// 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;
+};
+
+#ifndef DEBUG
+
+class Mutex : public detail::MutexImpl
+{
+public:
+  static bool Init() { return true; }
+  static void ShutDown() {}
+
+  explicit Mutex(const MutexId& id) {}
+
+  using MutexImpl::lock;
+  using MutexImpl::unlock;
+};
+
+#else
+
+// 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 : public detail::MutexImpl
+{
+public:
+  static bool Init();
+  static void ShutDown();
+
+  explicit Mutex(const MutexId& id)
+   : id_(id)
+  {
+    MOZ_ASSERT(id_.order != 0);
+  }
+
+  void lock();
+  void unlock();
+
+private:
+  const MutexId id_;
+
+  using MutexVector = mozilla::Vector<const Mutex*>;
+  static MOZ_THREAD_LOCAL(MutexVector*) HeldMutexStack;
+  static MutexVector& heldMutexStack();
+};
+
+#endif
+
+} // namespace js
+
+#endif // threading_Mutex_h