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diff --git a/xpcom/threads/TaskController.h b/xpcom/threads/TaskController.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 mozilla_TaskController_h
+#define mozilla_TaskController_h
+
+#include "MainThreadUtils.h"
+#include "mozilla/CondVar.h"
+#include "mozilla/IdlePeriodState.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/StaticMutex.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/EventQueue.h"
+#include "nsISupportsImpl.h"
+#include "nsIEventTarget.h"
+
+#include <atomic>
+#include <memory>
+#include <vector>
+#include <set>
+#include <list>
+#include <stack>
+
+class nsIRunnable;
+class nsIThreadObserver;
+
+namespace mozilla {
+
+class Task;
+class TaskController;
+class PerformanceCounter;
+class PerformanceCounterState;
+
+const EventQueuePriority kDefaultPriorityValue = EventQueuePriority::Normal;
+
+// This file contains the core classes to access the Gecko scheduler. The
+// scheduler forms a graph of prioritize tasks, and is responsible for ensuring
+// the execution of tasks or their dependencies in order of inherited priority.
+//
+// The core class is the 'Task' class. The task class describes a single unit of
+// work. Users scheduling work implement this class and are required to
+// reimplement the 'Run' function in order to do work.
+//
+// The TaskManager class is reimplemented by users that require
+// the ability to reprioritize or suspend tasks.
+//
+// The TaskController is responsible for scheduling the work itself. The AddTask
+// function is used to schedule work. The ReprioritizeTask function may be used
+// to change the priority of a task already in the task graph, without
+// unscheduling it.
+
+// The TaskManager is the baseclass used to atomically manage a large set of
+// tasks. API users reimplementing TaskManager may reimplement a number of
+// functions that they may use to indicate to the scheduler changes in the state
+// for any tasks they manage. They may be used to reprioritize or suspend tasks
+// under their control, and will also be notified before and after tasks under
+// their control are executed. Their methods will only be called once per event
+// loop turn, however they may still incur some performance overhead. In
+// addition to this frequent reprioritizations may incur a significant
+// performance overhead and are discouraged. A TaskManager may currently only be
+// used to manage tasks that are bound to the Gecko Main Thread.
+class TaskManager {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(TaskManager)
+
+  TaskManager() : mTaskCount(0) {}
+
+  // Subclasses implementing task manager will have this function called to
+  // determine whether their associated tasks are currently suspended. This
+  // will only be called once per iteration of the task queue, this means that
+  // suspension of tasks managed by a single TaskManager may be assumed to
+  // occur atomically.
+  virtual bool IsSuspended(const MutexAutoLock& aProofOfLock) { return false; }
+
+  // Subclasses may implement this in order to supply a priority adjustment
+  // to their managed tasks. This is called once per iteration of the task
+  // queue, and may be assumed to occur atomically for all managed tasks.
+  virtual int32_t GetPriorityModifierForEventLoopTurn(
+      const MutexAutoLock& aProofOfLock) {
+    return 0;
+  }
+
+  void DidQueueTask() { ++mTaskCount; }
+  // This is called when a managed task is about to be executed by the
+  // scheduler. Anyone reimplementing this should ensure to call the parent or
+  // decrement mTaskCount.
+  virtual void WillRunTask() { --mTaskCount; }
+  // This is called when a managed task has finished being executed by the
+  // scheduler.
+  virtual void DidRunTask() {}
+  uint32_t PendingTaskCount() { return mTaskCount; }
+
+ protected:
+  virtual ~TaskManager() {}
+
+ private:
+  friend class TaskController;
+
+  enum class IterationType { NOT_EVENT_LOOP_TURN, EVENT_LOOP_TURN };
+  bool UpdateCachesForCurrentIterationAndReportPriorityModifierChanged(
+      const MutexAutoLock& aProofOfLock, IterationType aIterationType);
+
+  bool mCurrentSuspended = false;
+  int32_t mCurrentPriorityModifier = 0;
+
+  std::atomic<uint32_t> mTaskCount;
+};
+
+// A Task is the the base class for any unit of work that may be scheduled.
+// Subclasses may specify their priority and whether they should be bound to
+// the Gecko Main thread. When not bound to the main thread tasks may be
+// executed on any available thread (including the main thread), but they may
+// also be executed in parallel to any other task they do not have a dependency
+// relationship with. Tasks will be run in order of object creation.
+class Task {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(Task)
+
+  bool IsMainThreadOnly() { return mMainThreadOnly; }
+
+  // This returns the current task priority with its modifier applied.
+  uint32_t GetPriority() { return mPriority + mPriorityModifier; }
+  uint64_t GetSeqNo() { return mSeqNo; }
+
+  // Callee needs to assume this may be called on any thread.
+  // aInterruptPriority passes the priority of the higher priority task that
+  // is ready to be executed. The task may safely ignore this function, or
+  // interrupt any work being done. It may return 'false' from its run function
+  // in order to be run automatically in the future, or true if it will
+  // reschedule incomplete work manually.
+  virtual void RequestInterrupt(uint32_t aInterruptPriority) {}
+
+  // At the moment this -must- be called before the task is added to the
+  // controller. Calling this after tasks have been added to the controller
+  // results in undefined behavior!
+  // At submission, tasks must depend only on tasks managed by the same, or
+  // no idle manager.
+  void AddDependency(Task* aTask) {
+    MOZ_ASSERT(aTask);
+    MOZ_ASSERT(!mIsInGraph);
+    mDependencies.insert(aTask);
+  }
+
+  // This sets the TaskManager for the current task. Calling this after the
+  // task has been added to the TaskController results in undefined behavior.
+  void SetManager(TaskManager* aManager) {
+    MOZ_ASSERT(mMainThreadOnly);
+    MOZ_ASSERT(!mIsInGraph);
+    mTaskManager = aManager;
+  }
+  TaskManager* GetManager() { return mTaskManager; }
+
+  struct PriorityCompare {
+    bool operator()(const RefPtr<Task>& aTaskA,
+                    const RefPtr<Task>& aTaskB) const {
+      uint32_t prioA = aTaskA->GetPriority();
+      uint32_t prioB = aTaskB->GetPriority();
+      return (prioA > prioB) ||
+             (prioA == prioB && (aTaskA->GetSeqNo() < aTaskB->GetSeqNo()));
+    }
+  };
+
+  // Tell the task about its idle deadline.  Will only be called for
+  // tasks managed by an IdleTaskManager, right before the task runs.
+  virtual void SetIdleDeadline(TimeStamp aDeadline) {}
+
+  virtual PerformanceCounter* GetPerformanceCounter() const { return nullptr; }
+
+  // Get a name for this task. This returns false if the task has no name.
+#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
+  virtual bool GetName(nsACString& aName) = 0;
+#else
+  virtual bool GetName(nsACString& aName) { return false; }
+#endif
+
+ protected:
+  Task(bool aMainThreadOnly,
+       uint32_t aPriority = static_cast<uint32_t>(kDefaultPriorityValue))
+      : mMainThreadOnly(aMainThreadOnly),
+        mSeqNo(sCurrentTaskSeqNo++),
+        mPriority(aPriority) {}
+
+  Task(bool aMainThreadOnly,
+       EventQueuePriority aPriority = kDefaultPriorityValue)
+      : mMainThreadOnly(aMainThreadOnly),
+        mSeqNo(sCurrentTaskSeqNo++),
+        mPriority(static_cast<uint32_t>(aPriority)) {}
+
+  virtual ~Task() {}
+
+  friend class TaskController;
+
+  // When this returns false, the task is considered incomplete and will be
+  // rescheduled at the current 'mPriority' level.
+  virtual bool Run() = 0;
+
+ private:
+  Task* GetHighestPriorityDependency();
+
+  // Iterator pointing to this task's position in
+  // mThreadableTasks/mMainThreadTasks if, and only if this task is currently
+  // scheduled to be executed. This allows fast access to the task's position
+  // in the set, allowing for fast removal.
+  // This is safe, and remains valid unless the task is removed from the set.
+  // See also iterator invalidation in:
+  // https://en.cppreference.com/w/cpp/container
+  //
+  // Or the spec:
+  // "All Associative Containers: The insert and emplace members shall not
+  // affect the validity of iterators and references to the container
+  // [26.2.6/9]" "All Associative Containers: The erase members shall invalidate
+  // only iterators and references to the erased elements [26.2.6/9]"
+  std::set<RefPtr<Task>, PriorityCompare>::iterator mIterator;
+  std::set<RefPtr<Task>, PriorityCompare> mDependencies;
+
+  RefPtr<TaskManager> mTaskManager;
+
+  // Access to these variables is protected by the GraphMutex.
+  bool mMainThreadOnly;
+  bool mCompleted = false;
+  bool mInProgress = false;
+#ifdef DEBUG
+  bool mIsInGraph = false;
+#endif
+
+  static std::atomic<uint64_t> sCurrentTaskSeqNo;
+  int64_t mSeqNo;
+  uint32_t mPriority;
+  // Modifier currently being applied to this task by its taskmanager.
+  int32_t mPriorityModifier = 0;
+  // Time this task was inserted into the task graph, this is used by the
+  // profiler.
+  mozilla::TimeStamp mInsertionTime;
+};
+
+struct PoolThread {
+  PRThread* mThread;
+  RefPtr<Task> mCurrentTask;
+  // This may be higher than mCurrentTask's priority due to priority
+  // propagation. This is -only- valid when mCurrentTask != nullptr.
+  uint32_t mEffectiveTaskPriority;
+};
+
+// A task manager implementation for priority levels that should only
+// run during idle periods.
+class IdleTaskManager : public TaskManager {
+ public:
+  explicit IdleTaskManager(already_AddRefed<nsIIdlePeriod>&& aIdlePeriod)
+      : mIdlePeriodState(std::move(aIdlePeriod)), mProcessedTaskCount(0) {}
+
+  IdlePeriodState& State() { return mIdlePeriodState; }
+
+  bool IsSuspended(const MutexAutoLock& aProofOfLock) override {
+    TimeStamp idleDeadline = State().GetCachedIdleDeadline();
+    return !idleDeadline;
+  }
+
+  void DidRunTask() override {
+    TaskManager::DidRunTask();
+    ++mProcessedTaskCount;
+  }
+
+  uint64_t ProcessedTaskCount() { return mProcessedTaskCount; }
+
+ private:
+  // Tracking of our idle state of various sorts.
+  IdlePeriodState mIdlePeriodState;
+
+  std::atomic<uint64_t> mProcessedTaskCount;
+};
+
+// The TaskController is the core class of the scheduler. It is used to
+// schedule tasks to be executed, as well as to reprioritize tasks that have
+// already been scheduled. The core functions to do this are AddTask and
+// ReprioritizeTask.
+class TaskController {
+ public:
+  TaskController();
+
+  static TaskController* Get();
+
+  static void Initialize();
+
+  void SetThreadObserver(nsIThreadObserver* aObserver) {
+    MutexAutoLock lock(mGraphMutex);
+    mObserver = aObserver;
+  }
+  void SetConditionVariable(CondVar* aExternalCondVar) {
+    MutexAutoLock lock(mGraphMutex);
+    mExternalCondVar = aExternalCondVar;
+  }
+
+  void SetIdleTaskManager(IdleTaskManager* aIdleTaskManager) {
+    mIdleTaskManager = aIdleTaskManager;
+  }
+  IdleTaskManager* GetIdleTaskManager() { return mIdleTaskManager.get(); }
+
+  uint64_t RunOutOfMTTasksCount() { return mRunOutOfMTTasksCounter; }
+
+  // Initialization and shutdown code.
+  void SetPerformanceCounterState(
+      PerformanceCounterState* aPerformanceCounterState);
+
+  static void Shutdown();
+
+  // This adds a task to the TaskController graph.
+  // This may be called on any thread.
+  void AddTask(already_AddRefed<Task>&& aTask);
+
+  // This wait function is the theoretical function you would need if our main
+  // thread needs to also process OS messages or something along those lines.
+  void WaitForTaskOrMessage();
+
+  // This gets the next (highest priority) task that is only allowed to execute
+  // on the main thread.
+  void ExecuteNextTaskOnlyMainThread();
+
+  // Process all pending main thread tasks.
+  void ProcessPendingMTTask(bool aMayWait = false);
+
+  // This allows reprioritization of a task already in the task graph.
+  // This may be called on any thread.
+  void ReprioritizeTask(Task* aTask, uint32_t aPriority);
+
+  void DispatchRunnable(already_AddRefed<nsIRunnable>&& aRunnable,
+                        uint32_t aPriority, TaskManager* aManager = nullptr);
+
+  nsIRunnable* GetRunnableForMTTask(bool aReallyWait);
+
+  bool HasMainThreadPendingTasks();
+
+  uint64_t PendingMainthreadTaskCountIncludingSuspended();
+
+  // Let users know whether the last main thread task runnable did work.
+  bool MTTaskRunnableProcessedTask() {
+    MOZ_ASSERT(NS_IsMainThread());
+    return mMTTaskRunnableProcessedTask;
+  }
+
+  static int32_t GetPoolThreadCount();
+  static size_t GetThreadStackSize();
+
+ private:
+  friend void ThreadFuncPoolThread(void* aIndex);
+
+  void InitializeThreadPool();
+
+  // This gets the next (highest priority) task that is only allowed to execute
+  // on the main thread, if any, and executes it.
+  // Returns true if it succeeded.
+  bool ExecuteNextTaskOnlyMainThreadInternal(const MutexAutoLock& aProofOfLock);
+
+  // The guts of ExecuteNextTaskOnlyMainThreadInternal, which get idle handling
+  // wrapped around them.  Returns whether a task actually ran.
+  bool DoExecuteNextTaskOnlyMainThreadInternal(
+      const MutexAutoLock& aProofOfLock);
+
+  Task* GetFinalDependency(Task* aTask);
+  void MaybeInterruptTask(Task* aTask);
+  Task* GetHighestPriorityMTTask();
+
+  void EnsureMainThreadTasksScheduled();
+
+  void ProcessUpdatedPriorityModifier(TaskManager* aManager);
+
+  void ShutdownThreadPoolInternal();
+  void ShutdownInternal();
+
+  void RunPoolThread();
+
+  static std::unique_ptr<TaskController> sSingleton;
+  static StaticMutex sSingletonMutex MOZ_UNANNOTATED;
+
+  // This protects access to the task graph.
+  Mutex mGraphMutex MOZ_UNANNOTATED;
+
+  // This protects thread pool initialization. We cannot do this from within
+  // the GraphMutex, since thread creation on Windows can generate events on
+  // the main thread that need to be handled.
+  Mutex mPoolInitializationMutex =
+      Mutex("TaskController::mPoolInitializationMutex");
+  // Created under the PoolInitialization mutex, then never extended, and
+  // only freed when the object is freed.  mThread is set at creation time;
+  // mCurrentTask and mEffectiveTaskPriority are only accessed from the
+  // thread, so no locking is needed to access this.
+  std::vector<PoolThread> mPoolThreads;
+
+  CondVar mThreadPoolCV;
+  CondVar mMainThreadCV;
+
+  // Variables below are protected by mGraphMutex.
+
+  std::stack<RefPtr<Task>> mCurrentTasksMT;
+
+  // A list of all tasks ordered by priority.
+  std::set<RefPtr<Task>, Task::PriorityCompare> mThreadableTasks;
+  std::set<RefPtr<Task>, Task::PriorityCompare> mMainThreadTasks;
+
+  // TaskManagers currently active.
+  // We can use a raw pointer since tasks always hold on to their TaskManager.
+  std::set<TaskManager*> mTaskManagers;
+
+  // This ensures we keep running the main thread if we processed a task there.
+  bool mMayHaveMainThreadTask = true;
+  bool mShuttingDown = false;
+
+  // This stores whether the last main thread task runnable did work.
+  // Accessed only on MainThread
+  bool mMTTaskRunnableProcessedTask = false;
+
+  // Whether our thread pool is initialized. We use this currently to avoid
+  // starting the threads in processes where it's never used. This is protected
+  // by mPoolInitializationMutex.
+  bool mThreadPoolInitialized = false;
+
+  // Whether we have scheduled a runnable on the main thread event loop.
+  // This is used for nsIRunnable compatibility.
+  RefPtr<nsIRunnable> mMTProcessingRunnable;
+  RefPtr<nsIRunnable> mMTBlockingProcessingRunnable;
+
+  // XXX - Thread observer to notify when a new event has been dispatched
+  // Set immediately, then simply accessed from any thread
+  nsIThreadObserver* mObserver = nullptr;
+  // XXX - External condvar to notify when we have received an event
+  CondVar* mExternalCondVar = nullptr;
+  // Idle task manager so we can properly do idle state stuff.
+  RefPtr<IdleTaskManager> mIdleTaskManager;
+
+  // How many times the main thread was empty.
+  std::atomic<uint64_t> mRunOutOfMTTasksCounter;
+
+  // Our tracking of our performance counter and long task state,
+  // shared with nsThread.
+  // Set once when MainThread is created, never changed, only accessed from
+  // DoExecuteNextTaskOnlyMainThreadInternal()
+  PerformanceCounterState* mPerformanceCounterState = nullptr;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_TaskController_h