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diff --git a/js/src/threading/Thread.h b/js/src/threading/Thread.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_Thread_h
+#define threading_Thread_h
+
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/Tuple.h"
+
+#include <stdint.h>
+#include <type_traits>
+#include <utility>
+
+#include "js/Initialization.h"
+#include "js/Utility.h"
+#include "threading/LockGuard.h"
+#include "threading/Mutex.h"
+#include "threading/ThreadId.h"
+#include "vm/MutexIDs.h"
+
+#ifdef XP_WIN
+#  define THREAD_RETURN_TYPE unsigned int
+#  define THREAD_CALL_API __stdcall
+#else
+#  define THREAD_RETURN_TYPE void*
+#  define THREAD_CALL_API
+#endif
+
+namespace js {
+namespace detail {
+template <typename F, typename... Args>
+class ThreadTrampoline;
+}  // namespace detail
+
+// Execute the given functor concurrent with the currently executing instruction
+// stream and within the current address space. Use with care.
+class Thread {
+ public:
+  // Provides optional parameters to a Thread.
+  class Options {
+    size_t stackSize_;
+
+   public:
+    Options() : stackSize_(0) {}
+
+    Options& setStackSize(size_t sz) {
+      stackSize_ = sz;
+      return *this;
+    }
+    size_t stackSize() const { return stackSize_; }
+  };
+
+  // Create a Thread in an initially unjoinable state. A thread of execution can
+  // be created for this Thread by calling |init|. Some of the thread's
+  // properties may be controlled by passing options to this constructor.
+  template <typename O = Options,
+            // SFINAE to make sure we don't try and treat functors for the other
+            // constructor as an Options and vice versa.
+            typename NonConstO = std::remove_const_t<O>,
+            typename DerefO = std::remove_reference_t<NonConstO>,
+            typename = std::enable_if_t<std::is_same_v<DerefO, Options>>>
+  explicit Thread(O&& options = Options())
+      : id_(ThreadId()), options_(std::forward<O>(options)) {
+    MOZ_ASSERT(isInitialized());
+  }
+
+  // Start a thread of execution at functor |f| with parameters |args|. This
+  // method will return false if thread creation fails. This Thread must not
+  // already have been created. Note that the arguments must be either POD or
+  // rvalue references (std::move). Attempting to pass a reference will
+  // result in the value being copied, which may not be the intended behavior.
+  // See the comment below on ThreadTrampoline::args for an explanation.
+  template <typename F, typename... Args>
+  MOZ_MUST_USE bool init(F&& f, Args&&... args) {
+    MOZ_RELEASE_ASSERT(id_ == ThreadId());
+    using Trampoline = detail::ThreadTrampoline<F, Args...>;
+    auto trampoline =
+        js_new<Trampoline>(std::forward<F>(f), std::forward<Args>(args)...);
+    if (!trampoline) {
+      return false;
+    }
+
+    // We hold this lock while create() sets the thread id.
+    LockGuard<Mutex> lock(trampoline->createMutex);
+    return create(Trampoline::Start, trampoline);
+  }
+
+  // The thread must be joined or detached before destruction.
+  ~Thread();
+
+  // Move the thread into the detached state without blocking. In the detatched
+  // state, the thread continues to run until it exits, but cannot be joined.
+  // After this method returns, this Thread no longer represents a thread of
+  // execution. When the thread exits, its resources will be cleaned up by the
+  // system. At process exit, if the thread is still running, the thread's TLS
+  // storage will be destructed, but the thread stack will *not* be unrolled.
+  void detach();
+
+  // Block the current thread until this Thread returns from the functor it was
+  // created with. The thread's resources will be cleaned up before this
+  // function returns. After this method returns, this Thread no longer
+  // represents a thread of execution.
+  void join();
+
+  // Return true if this thread has not yet been joined or detached. If this
+  // method returns false, this Thread does not have an associated thread of
+  // execution, for example, if it has been previously moved or joined.
+  bool joinable();
+
+  // Returns the id of this thread if this represents a thread of execution or
+  // the default constructed Id() if not. The thread ID is guaranteed to
+  // uniquely identify a thread and can be compared with the == operator.
+  ThreadId get_id();
+
+  // Allow threads to be moved so that they can be stored in containers.
+  Thread(Thread&& aOther);
+  Thread& operator=(Thread&& aOther);
+
+ private:
+  // Disallow copy as that's not sensible for unique resources.
+  Thread(const Thread&) = delete;
+  void operator=(const Thread&) = delete;
+
+  // Provide a process global ID to each thread.
+  ThreadId id_;
+
+  // Overridable thread creation options.
+  Options options_;
+
+  // Dispatch to per-platform implementation of thread creation.
+  MOZ_MUST_USE bool create(THREAD_RETURN_TYPE(THREAD_CALL_API* aMain)(void*),
+                           void* aArg);
+
+  // An internal version of JS_IsInitialized() that returns whether SpiderMonkey
+  // is currently initialized or is in the process of being initialized.
+  static inline bool isInitialized() {
+    using namespace JS::detail;
+    return libraryInitState == InitState::Initializing ||
+           libraryInitState == InitState::Running;
+  }
+};
+
+namespace ThisThread {
+
+// Set the current thread name. Note that setting the thread name may not be
+// available on all platforms; on these platforms setName() will simply do
+// nothing.
+void SetName(const char* name);
+
+// Get the current thread name. As with SetName, not available on all
+// platforms. On these platforms getName() will give back an empty string (by
+// storing NUL in nameBuffer[0]). 'len' is the bytes available to be written in
+// 'nameBuffer', including the terminating NUL.
+void GetName(char* nameBuffer, size_t len);
+
+}  // namespace ThisThread
+
+namespace detail {
+
+// Platform thread APIs allow passing a single void* argument to the target
+// thread. This class is responsible for safely ferrying the arg pack and
+// functor across that void* membrane and running it in the other thread.
+template <typename F, typename... Args>
+class ThreadTrampoline {
+  // The functor to call.
+  F f;
+
+  // A std::decay copy of the arguments, as specified by std::thread. Using an
+  // rvalue reference for the arguments to Thread and ThreadTrampoline gives us
+  // move semantics for large structures, allowing us to quickly and easily pass
+  // enormous amounts of data to a new thread. Unfortunately, there is a
+  // downside: rvalue references becomes lvalue references when used with POD
+  // types. This becomes dangerous when attempting to pass POD stored on the
+  // stack to the new thread; the rvalue reference will implicitly become an
+  // lvalue reference to the stack location. Thus, the value may not exist if
+  // the parent thread leaves the frame before the read happens in the new
+  // thread. To avoid this dangerous and highly non-obvious footgun, the
+  // standard requires a "decay" copy of the arguments at the cost of making it
+  // impossible to pass references between threads.
+  mozilla::Tuple<std::decay_t<Args>...> args;
+
+  // Protect the thread id during creation.
+  Mutex createMutex;
+
+  // Thread can access createMutex.
+  friend class js::Thread;
+
+ public:
+  // Note that this template instatiation duplicates and is identical to the
+  // class template instantiation. It is required for perfect forwarding of
+  // rvalue references, which is only enabled for calls to a function template,
+  // even if the class template arguments are correct.
+  template <typename G, typename... ArgsT>
+  explicit ThreadTrampoline(G&& aG, ArgsT&&... aArgsT)
+      : f(std::forward<F>(aG)),
+        args(std::forward<Args>(aArgsT)...),
+        createMutex(mutexid::ThreadId) {}
+
+  static THREAD_RETURN_TYPE THREAD_CALL_API Start(void* aPack) {
+    auto* pack = static_cast<ThreadTrampoline<F, Args...>*>(aPack);
+    pack->callMain(std::index_sequence_for<Args...>{});
+    js_delete(pack);
+    return 0;
+  }
+
+  template <size_t... Indices>
+  void callMain(std::index_sequence<Indices...>) {
+    // Pretend createMutex is a semaphore and wait for a notification that the
+    // thread that spawned us is ready.
+    createMutex.lock();
+    createMutex.unlock();
+    f(mozilla::Get<Indices>(args)...);
+  }
+};
+
+}  // namespace detail
+}  // namespace js
+
+#undef THREAD_RETURN_TYPE
+
+#endif  // threading_Thread_h