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diff --git a/xpcom/threads/nsThread.h b/xpcom/threads/nsThread.h
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+++ b/xpcom/threads/nsThread.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 nsThread_h__
+#define nsThread_h__
+
+#include "MainThreadUtils.h"
+#include "mozilla/AlreadyAddRefed.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/EventQueue.h"
+#include "mozilla/LinkedList.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/NotNull.h"
+#include "mozilla/PerformanceCounter.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TaskDispatcher.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/UniquePtr.h"
+#include "nsIDirectTaskDispatcher.h"
+#include "nsIEventTarget.h"
+#include "nsISerialEventTarget.h"
+#include "nsISupportsPriority.h"
+#include "nsIThread.h"
+#include "nsIThreadInternal.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+class CycleCollectedJSContext;
+class SynchronizedEventQueue;
+class ThreadEventQueue;
+class ThreadEventTarget;
+
+template <typename T, size_t Length>
+class Array;
+}  // namespace mozilla
+
+using mozilla::NotNull;
+
+class nsIRunnable;
+class nsLocalExecutionRecord;
+class nsThreadEnumerator;
+
+// See https://www.w3.org/TR/longtasks
+#define LONGTASK_BUSY_WINDOW_MS 50
+
+// A class for managing performance counter state.
+namespace mozilla {
+class PerformanceCounterState {
+ public:
+  explicit PerformanceCounterState(const uint32_t& aNestedEventLoopDepthRef,
+                                   bool aIsMainThread)
+      : mNestedEventLoopDepth(aNestedEventLoopDepthRef),
+        mIsMainThread(aIsMainThread),
+        // Does it really make sense to initialize these to "now" when we
+        // haven't run any tasks?
+        mLastLongTaskEnd(TimeStamp::Now()),
+        mLastLongNonIdleTaskEnd(mLastLongTaskEnd) {}
+
+  class Snapshot {
+   public:
+    Snapshot(uint32_t aOldEventLoopDepth, PerformanceCounter* aCounter,
+             bool aOldIsIdleRunnable)
+        : mOldEventLoopDepth(aOldEventLoopDepth),
+          mOldPerformanceCounter(aCounter),
+          mOldIsIdleRunnable(aOldIsIdleRunnable) {}
+
+    Snapshot(const Snapshot&) = default;
+    Snapshot(Snapshot&&) = default;
+
+   private:
+    friend class PerformanceCounterState;
+
+    const uint32_t mOldEventLoopDepth;
+    // Non-const so we can move out of it and avoid the extra refcounting.
+    RefPtr<PerformanceCounter> mOldPerformanceCounter;
+    const bool mOldIsIdleRunnable;
+  };
+
+  // Notification that a runnable is about to run.  This captures a snapshot of
+  // our current state before we reset to prepare for the new runnable.  This
+  // muast be called after mNestedEventLoopDepth has been incremented for the
+  // runnable execution.  The performance counter passed in should be the one
+  // for the relevant runnable and may be null.  aIsIdleRunnable should be true
+  // if and only if the runnable has idle priority.
+  Snapshot RunnableWillRun(PerformanceCounter* Counter, TimeStamp aNow,
+                           bool aIsIdleRunnable);
+
+  // Notification that a runnable finished executing.  This must be passed the
+  // snapshot that RunnableWillRun returned for the same runnable.  This must be
+  // called before mNestedEventLoopDepth is decremented after the runnable's
+  // execution.
+  void RunnableDidRun(Snapshot&& aSnapshot);
+
+  const TimeStamp& LastLongTaskEnd() const { return mLastLongTaskEnd; }
+  const TimeStamp& LastLongNonIdleTaskEnd() const {
+    return mLastLongNonIdleTaskEnd;
+  }
+
+ private:
+  // Called to report accumulated time, as needed, when we're about to run a
+  // runnable or just finished running one.
+  void MaybeReportAccumulatedTime(TimeStamp aNow);
+
+  // Whether the runnable we are about to run, or just ran, is a nested
+  // runnable, in the sense that there is some other runnable up the stack
+  // spinning the event loop.  This must be called before we change our
+  // mCurrentEventLoopDepth (when about to run a new event) or after we restore
+  // it (after we ran one).
+  bool IsNestedRunnable() const {
+    return mNestedEventLoopDepth > mCurrentEventLoopDepth;
+  }
+
+  // The event loop depth of the currently running runnable.  Set to the max
+  // value of a uint32_t when there is no runnable running, so when starting to
+  // run a toplevel (not nested) runnable IsNestedRunnable() will test false.
+  uint32_t mCurrentEventLoopDepth = std::numeric_limits<uint32_t>::max();
+
+  // A reference to the nsThread's mNestedEventLoopDepth, so we can
+  // see what it is right now.
+  const uint32_t& mNestedEventLoopDepth;
+
+  // A boolean that indicates whether the currently running runnable is an idle
+  // runnable.  Only has a useful value between RunnableWillRun() being called
+  // and RunnableDidRun() returning.
+  bool mCurrentRunnableIsIdleRunnable = false;
+
+  // Whether we're attached to the mainthread nsThread.
+  const bool mIsMainThread;
+
+  // The timestamp from which time to be accounted for should be measured.  This
+  // can be the start of a runnable running or the end of a nested runnable
+  // running.
+  TimeStamp mCurrentTimeSliceStart;
+
+  // Information about when long tasks last ended.
+  TimeStamp mLastLongTaskEnd;
+  TimeStamp mLastLongNonIdleTaskEnd;
+
+  // The performance counter to use for accumulating the runtime of
+  // the currently running event.  May be null, in which case the
+  // event's running time should not be accounted to any performance
+  // counters.
+  RefPtr<PerformanceCounter> mCurrentPerformanceCounter;
+};
+}  // namespace mozilla
+
+// A native thread
+class nsThread : public nsIThreadInternal,
+                 public nsISupportsPriority,
+                 public nsIDirectTaskDispatcher,
+                 private mozilla::LinkedListElement<nsThread> {
+  friend mozilla::LinkedList<nsThread>;
+  friend mozilla::LinkedListElement<nsThread>;
+
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+  NS_DECL_NSIEVENTTARGET_FULL
+  NS_DECL_NSITHREAD
+  NS_DECL_NSITHREADINTERNAL
+  NS_DECL_NSISUPPORTSPRIORITY
+  NS_DECL_NSIDIRECTTASKDISPATCHER
+
+  enum MainThreadFlag { MAIN_THREAD, NOT_MAIN_THREAD };
+
+  nsThread(NotNull<mozilla::SynchronizedEventQueue*> aQueue,
+           MainThreadFlag aMainThread, uint32_t aStackSize);
+
+ private:
+  nsThread();
+
+ public:
+  // Initialize this as a named wrapper for a new PRThread.
+  nsresult Init(const nsACString& aName);
+
+  // Initialize this as a wrapper for the current PRThread.
+  nsresult InitCurrentThread();
+
+ private:
+  // Initializes the mThreadId and stack base/size members, and adds the thread
+  // to the ThreadList().
+  void InitCommon();
+
+ public:
+  // The PRThread corresponding to this thread.
+  PRThread* GetPRThread() const { return mThread; }
+
+  const void* StackBase() const { return mStackBase; }
+  size_t StackSize() const { return mStackSize; }
+
+  uint32_t ThreadId() const { return mThreadId; }
+
+  // If this flag is true, then the nsThread was created using
+  // nsIThreadManager::NewThread.
+  bool ShutdownRequired() { return mShutdownRequired; }
+
+  // Lets GetRunningEventDelay() determine if the pool this is part
+  // of has an unstarted thread
+  void SetPoolThreadFreePtr(mozilla::Atomic<bool, mozilla::Relaxed>* aPtr) {
+    mIsAPoolThreadFree = aPtr;
+  }
+
+  void SetScriptObserver(mozilla::CycleCollectedJSContext* aScriptObserver);
+
+  uint32_t RecursionDepth() const;
+
+  void ShutdownComplete(NotNull<struct nsThreadShutdownContext*> aContext);
+
+  void WaitForAllAsynchronousShutdowns();
+
+  static const uint32_t kRunnableNameBufSize = 1000;
+  static mozilla::Array<char, kRunnableNameBufSize> sMainThreadRunnableName;
+
+  mozilla::SynchronizedEventQueue* EventQueue() { return mEvents.get(); }
+
+  bool ShuttingDown() const { return mShutdownContext != nullptr; }
+
+  static bool GetLabeledRunnableName(nsIRunnable* aEvent, nsACString& aName,
+                                     mozilla::EventQueuePriority aPriority);
+
+  virtual mozilla::PerformanceCounter* GetPerformanceCounter(
+      nsIRunnable* aEvent) const;
+
+  static mozilla::PerformanceCounter* GetPerformanceCounterBase(
+      nsIRunnable* aEvent);
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  // Returns the size of this object, its PRThread, and its shutdown contexts,
+  // but excluding its event queues.
+  size_t ShallowSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  size_t SizeOfEventQueues(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  static nsThreadEnumerator Enumerate();
+
+  static uint32_t MaxActiveThreads();
+
+  // When entering local execution mode a new event queue is created and used as
+  // an event source. This queue is only accessible through an
+  // nsLocalExecutionGuard constructed from the nsLocalExecutionRecord returned
+  // by this function, effectively restricting the events that get run while in
+  // local execution mode to those dispatched by the owner of the guard object.
+  //
+  // Local execution is not nestable. When the nsLocalExecutionGuard is
+  // destructed, the thread exits the local execution mode.
+  //
+  // Note that code run in local execution mode is not considered a task in the
+  // spec sense. Events from the local queue are considered part of the
+  // enclosing task and as such do not trigger profiling hooks, observer
+  // notifications, etc.
+  nsLocalExecutionRecord EnterLocalExecution();
+
+  void SetUseHangMonitor(bool aValue) {
+    MOZ_ASSERT(IsOnCurrentThread());
+    mUseHangMonitor = aValue;
+  }
+
+ private:
+  void DoMainThreadSpecificProcessing() const;
+
+ protected:
+  friend class nsThreadShutdownEvent;
+
+  friend class nsThreadEnumerator;
+
+  virtual ~nsThread();
+
+  static void ThreadFunc(void* aArg);
+
+  // Helper
+  already_AddRefed<nsIThreadObserver> GetObserver() {
+    nsIThreadObserver* obs;
+    nsThread::GetObserver(&obs);
+    return already_AddRefed<nsIThreadObserver>(obs);
+  }
+
+  struct nsThreadShutdownContext* ShutdownInternal(bool aSync);
+
+  friend class nsThreadManager;
+  friend class nsThreadPool;
+
+  static mozilla::OffTheBooksMutex& ThreadListMutex();
+  static mozilla::LinkedList<nsThread>& ThreadList();
+  static void ClearThreadList();
+
+  // The current number of active threads.
+  static uint32_t sActiveThreads;
+  // The maximum current number of active threads we've had in this session.
+  static uint32_t sMaxActiveThreads;
+
+  void AddToThreadList();
+  void MaybeRemoveFromThreadList();
+
+  // Whether or not these members have a value determines whether the nsThread
+  // is treated as a full XPCOM thread or as a thin wrapper.
+  //
+  // For full nsThreads, they will always contain valid pointers. For thin
+  // wrappers around non-XPCOM threads, they will be null, and event dispatch
+  // methods which rely on them will fail (and assert) if called.
+  RefPtr<mozilla::SynchronizedEventQueue> mEvents;
+  RefPtr<mozilla::ThreadEventTarget> mEventTarget;
+
+  // The shutdown contexts for any other threads we've asked to shut down.
+  using ShutdownContexts =
+      nsTArray<mozilla::UniquePtr<struct nsThreadShutdownContext>>;
+
+  // Helper for finding a ShutdownContext in the contexts array.
+  struct ShutdownContextsComp {
+    bool Equals(const ShutdownContexts::elem_type& a,
+                const ShutdownContexts::elem_type::Pointer b) const;
+  };
+
+  ShutdownContexts mRequestedShutdownContexts;
+  // The shutdown context for ourselves.
+  struct nsThreadShutdownContext* mShutdownContext;
+
+  mozilla::CycleCollectedJSContext* mScriptObserver;
+
+  void* mStackBase = nullptr;
+  uint32_t mStackSize;
+  uint32_t mThreadId;
+
+  uint32_t mNestedEventLoopDepth;
+
+  mozilla::Atomic<bool> mShutdownRequired;
+
+  int8_t mPriority;
+
+  const bool mIsMainThread;
+  bool mUseHangMonitor;
+  mozilla::Atomic<bool, mozilla::Relaxed>* mIsAPoolThreadFree;
+
+  // Set to true if this thread creates a JSRuntime.
+  bool mCanInvokeJS;
+
+  bool mHasTLSEntry = false;
+
+  // The time the currently running event spent in event queues, and
+  // when it started running.  If no event is running, they are
+  // TimeDuration() & TimeStamp().
+  mozilla::TimeDuration mLastEventDelay;
+  mozilla::TimeStamp mLastEventStart;
+
+#ifdef EARLY_BETA_OR_EARLIER
+  nsCString mNameForWakeupTelemetry;
+  mozilla::TimeStamp mLastWakeupCheckTime;
+  uint32_t mWakeupCount = 0;
+#endif
+
+  mozilla::PerformanceCounterState mPerformanceCounterState;
+
+  bool mIsInLocalExecutionMode = false;
+
+  mozilla::SimpleTaskQueue mDirectTasks;
+};
+
+struct nsThreadShutdownContext {
+  nsThreadShutdownContext(NotNull<nsThread*> aTerminatingThread,
+                          NotNull<nsThread*> aJoiningThread,
+                          bool aAwaitingShutdownAck)
+      : mTerminatingThread(aTerminatingThread),
+        mTerminatingPRThread(aTerminatingThread->GetPRThread()),
+        mJoiningThread(aJoiningThread),
+        mAwaitingShutdownAck(aAwaitingShutdownAck),
+        mIsMainThreadJoining(NS_IsMainThread()) {
+    MOZ_COUNT_CTOR(nsThreadShutdownContext);
+  }
+  MOZ_COUNTED_DTOR(nsThreadShutdownContext)
+
+  // NB: This will be the last reference.
+  NotNull<RefPtr<nsThread>> mTerminatingThread;
+  PRThread* const mTerminatingPRThread;
+  NotNull<nsThread*> MOZ_UNSAFE_REF(
+      "Thread manager is holding reference to joining thread") mJoiningThread;
+  bool mAwaitingShutdownAck;
+  bool mIsMainThreadJoining;
+};
+
+// This RAII class controls the duration of the associated nsThread's local
+// execution mode and provides access to the local event target. (See
+// nsThread::EnterLocalExecution() for details.) It is constructed from an
+// nsLocalExecutionRecord, which can only be constructed by nsThread.
+class MOZ_RAII nsLocalExecutionGuard final {
+ public:
+  MOZ_IMPLICIT nsLocalExecutionGuard(
+      nsLocalExecutionRecord&& aLocalExecutionRecord);
+  nsLocalExecutionGuard(const nsLocalExecutionGuard&) = delete;
+  nsLocalExecutionGuard(nsLocalExecutionGuard&&) = delete;
+  ~nsLocalExecutionGuard();
+
+  nsCOMPtr<nsISerialEventTarget> GetEventTarget() const {
+    return mLocalEventTarget;
+  }
+
+ private:
+  mozilla::SynchronizedEventQueue& mEventQueueStack;
+  nsCOMPtr<nsISerialEventTarget> mLocalEventTarget;
+  bool& mLocalExecutionFlag;
+};
+
+class MOZ_TEMPORARY_CLASS nsLocalExecutionRecord final {
+ private:
+  friend class nsThread;
+  friend class nsLocalExecutionGuard;
+
+  nsLocalExecutionRecord(mozilla::SynchronizedEventQueue& aEventQueueStack,
+                         bool& aLocalExecutionFlag)
+      : mEventQueueStack(aEventQueueStack),
+        mLocalExecutionFlag(aLocalExecutionFlag) {}
+
+  nsLocalExecutionRecord(nsLocalExecutionRecord&&) = default;
+
+ public:
+  nsLocalExecutionRecord(const nsLocalExecutionRecord&) = delete;
+
+ private:
+  mozilla::SynchronizedEventQueue& mEventQueueStack;
+  bool& mLocalExecutionFlag;
+};
+
+class MOZ_STACK_CLASS nsThreadEnumerator final {
+ public:
+  nsThreadEnumerator() = default;
+
+  auto begin() { return nsThread::ThreadList().begin(); }
+  auto end() { return nsThread::ThreadList().end(); }
+
+ private:
+  mozilla::OffTheBooksMutexAutoLock mMal{nsThread::ThreadListMutex()};
+};
+
+#if defined(XP_UNIX) && !defined(ANDROID) && !defined(DEBUG) && HAVE_UALARM && \
+    defined(_GNU_SOURCE)
+#  define MOZ_CANARY
+
+extern int sCanaryOutputFD;
+#endif
+
+#endif  // nsThread_h__