task/sequence_manager/sequence_manager_impl.cc

// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/task/sequence_manager/sequence_manager_impl.h"

#include <array>
#include <atomic>
#include <optional>
#include <queue>
#include <string_view>
#include <vector>

#include "base/callback_list.h"
#include "base/compiler_specific.h"
#include "base/debug/crash_logging.h"
#include "base/debug/stack_trace.h"
#include "base/functional/bind.h"
#include "base/functional/callback.h"
#include "base/functional/callback_helpers.h"
#include "base/json/json_writer.h"
#include "base/logging.h"
#include "base/memory/ptr_util.h"
#include "base/notreached.h"
#include "base/observer_list.h"
#include "base/rand_util.h"
#include "base/ranges/algorithm.h"
#include "base/task/sequence_manager/enqueue_order.h"
#include "base/task/sequence_manager/task_queue_impl.h"
#include "base/task/sequence_manager/task_time_observer.h"
#include "base/task/sequence_manager/thread_controller_impl.h"
#include "base/task/sequence_manager/thread_controller_with_message_pump_impl.h"
#include "base/task/sequence_manager/time_domain.h"
#include "base/task/sequence_manager/wake_up_queue.h"
#include "base/task/sequence_manager/work_queue.h"
#include "base/task/sequence_manager/work_queue_sets.h"
#include "base/task/task_features.h"
#include "base/threading/thread_id_name_manager.h"
#include "base/time/default_tick_clock.h"
#include "base/time/tick_clock.h"
#include "base/trace_event/base_tracing.h"
#include "build/build_config.h"
#include "third_party/abseil-cpp/absl/base/attributes.h"

namespace base {
namespace sequence_manager {
namespace {

// Whether SequenceManagerImpl records crash keys. Enable via Finch when needed
// for an investigation. Disabled by default to avoid unnecessary overhead.
BASE_FEATURE(kRecordSequenceManagerCrashKeys,
             "RecordSequenceManagerCrashKeys",
             base::FEATURE_DISABLED_BY_DEFAULT);

ABSL_CONST_INIT thread_local internal::SequenceManagerImpl*
    thread_local_sequence_manager = nullptr;

class TracedBaseValue : public trace_event::ConvertableToTraceFormat {
 public:
  explicit TracedBaseValue(Value value) : value_(std::move(value)) {}
  ~TracedBaseValue() override = default;

  void AppendAsTraceFormat(std::string* out) const override {
    if (!value_.is_none()) {
      std::string tmp;
      JSONWriter::Write(value_, &tmp);
      *out += tmp;
    } else {
      *out += "{}";
    }
  }

 private:
  base::Value value_;
};

}  // namespace

std::unique_ptr<SequenceManager> CreateSequenceManagerOnCurrentThread(
    SequenceManager::Settings settings) {
  return internal::SequenceManagerImpl::CreateOnCurrentThread(
      std::move(settings));
}

std::unique_ptr<SequenceManager> CreateSequenceManagerOnCurrentThreadWithPump(
    std::unique_ptr<MessagePump> message_pump,
    SequenceManager::Settings settings) {
  std::unique_ptr<SequenceManager> manager =
      internal::SequenceManagerImpl::CreateUnbound(std::move(settings));
  manager->BindToMessagePump(std::move(message_pump));
  return manager;
}

std::unique_ptr<SequenceManager> CreateUnboundSequenceManager(
    SequenceManager::Settings settings) {
  return internal::SequenceManagerImpl::CreateUnbound(std::move(settings));
}

namespace internal {

std::unique_ptr<SequenceManagerImpl> CreateUnboundSequenceManagerImpl(
    PassKey<base::internal::SequenceManagerThreadDelegate>,
    SequenceManager::Settings settings) {
  return SequenceManagerImpl::CreateUnbound(std::move(settings));
}

using TimeRecordingPolicy =
    base::sequence_manager::TaskQueue::TaskTiming::TimeRecordingPolicy;

namespace {

constexpr TimeDelta kLongTaskTraceEventThreshold = Milliseconds(50);
// Proportion of tasks which will record thread time for metrics.
const double kTaskSamplingRateForRecordingCPUTime = 0.01;
// Proprortion of SequenceManagers which will record thread time for each task,
// enabling advanced metrics.
const double kThreadSamplingRateForRecordingCPUTime = 0.0001;

void ReclaimMemoryFromQueue(internal::TaskQueueImpl* queue, LazyNow* lazy_now) {
  queue->ReclaimMemory(lazy_now->Now());
  // If the queue was shut down as a side-effect of reclaiming memory, |queue|
  // will still be valid but the work queues will have been removed by
  // TaskQueueImpl::UnregisterTaskQueue.
  if (queue->delayed_work_queue()) {
    queue->delayed_work_queue()->RemoveAllCanceledTasksFromFront();
    queue->immediate_work_queue()->RemoveAllCanceledTasksFromFront();
  }
}

SequenceManager::MetricRecordingSettings InitializeMetricRecordingSettings(
    bool randomised_sampling_enabled) {
  if (!randomised_sampling_enabled)
    return SequenceManager::MetricRecordingSettings(0);
  bool records_cpu_time_for_each_task =
      base::RandDouble() < kThreadSamplingRateForRecordingCPUTime;
  return SequenceManager::MetricRecordingSettings(
      records_cpu_time_for_each_task ? 1
                                     : kTaskSamplingRateForRecordingCPUTime);
}

// Writes |address| in hexadecimal ("0x11223344") form starting from |output|
// and moving backwards in memory. Returns a pointer to the first digit of the
// result. Does *not* NUL-terminate the number.
#if !BUILDFLAG(IS_NACL) && !BUILDFLAG(IS_ANDROID)
char* PrependHexAddress(char* output, const void* address) {
  uintptr_t value = reinterpret_cast<uintptr_t>(address);
  static const char kHexChars[] = "0123456789ABCDEF";
  do {
    *output-- = kHexChars[value % 16];
    value /= 16;
  } while (value);
  *output-- = 'x';
  *output = '0';
  return output;
}
#endif  // !BUILDFLAG(IS_NACL) && !BUILDFLAG(IS_ANDROID)

// Atomic to avoid TSAN flags when a test  tries to access the value before the
// feature list is available.
std::atomic_bool g_record_crash_keys = false;

#if BUILDFLAG(IS_WIN)
bool g_explicit_high_resolution_timer_win = true;
#endif  // BUILDFLAG(IS_WIN)

}  // namespace

// static
SequenceManagerImpl* SequenceManagerImpl::GetCurrent() {
  // Workaround false-positive MSAN use-of-uninitialized-value on
  // thread_local storage for loaded libraries:
  // https://github.com/google/sanitizers/issues/1265
  MSAN_UNPOISON(&thread_local_sequence_manager, sizeof(SequenceManagerImpl*));

  return thread_local_sequence_manager;
}

SequenceManagerImpl::SequenceManagerImpl(
    std::unique_ptr<internal::ThreadController> controller,
    SequenceManager::Settings settings)
    : associated_thread_(controller->GetAssociatedThread()),
      controller_(std::move(controller)),
      settings_(std::move(settings)),
      metric_recording_settings_(InitializeMetricRecordingSettings(
          settings_.randomised_sampling_enabled)),
      add_queue_time_to_tasks_(settings_.add_queue_time_to_tasks),

      empty_queues_to_reload_(associated_thread_),
      main_thread_only_(this, associated_thread_, settings_, settings_.clock),
      clock_(settings_.clock) {
  TRACE_EVENT_OBJECT_CREATED_WITH_ID(
      TRACE_DISABLED_BY_DEFAULT("sequence_manager"), "SequenceManager", this);
  main_thread_only().selector.SetTaskQueueSelectorObserver(this);

  main_thread_only().next_time_to_reclaim_memory =
      main_thread_clock()->NowTicks() + kReclaimMemoryInterval;

  controller_->SetSequencedTaskSource(this);
}

SequenceManagerImpl::~SequenceManagerImpl() {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  TRACE_EVENT_OBJECT_DELETED_WITH_ID(
      TRACE_DISABLED_BY_DEFAULT("sequence_manager"), "SequenceManager", this);

#if BUILDFLAG(IS_IOS)
  if (settings_.message_loop_type == MessagePumpType::UI &&
      associated_thread_->IsBound()) {
    controller_->DetachFromMessagePump();
  }
#endif

  // Make sure no Task is running as given that RunLoop does not support the
  // Delegate being destroyed from a Task and
  // ThreadControllerWithMessagePumpImpl does not support being destroyed from a
  // Task. If we are using a ThreadControllerImpl (i.e. no pump) destruction is
  // fine
  DCHECK(!controller_->GetBoundMessagePump() ||
         main_thread_only().task_execution_stack.empty());

  for (internal::TaskQueueImpl* queue : main_thread_only().active_queues) {
    main_thread_only().selector.RemoveQueue(queue);
    queue->UnregisterTaskQueue();
  }

  // TODO(altimin): restore default task runner automatically when
  // ThreadController is destroyed.
  controller_->RestoreDefaultTaskRunner();

  main_thread_only().active_queues.clear();
  main_thread_only().selector.SetTaskQueueSelectorObserver(nullptr);

  // In the case of an early startup exits or in some tests a NestingObserver
  // may not have been registered.
  if (main_thread_only().nesting_observer_registered_)
    controller_->RemoveNestingObserver(this);

  // Let interested parties have one last shot at accessing this.
  for (auto& observer : main_thread_only().destruction_observers)
    observer.WillDestroyCurrentMessageLoop();

  // OK, now make it so that no one can find us.
  if (GetMessagePump()) {
    DCHECK_EQ(this, GetCurrent());
    thread_local_sequence_manager = nullptr;
  }
}

SequenceManagerImpl::MainThreadOnly::MainThreadOnly(
    SequenceManagerImpl* sequence_manager,
    const scoped_refptr<AssociatedThreadId>& associated_thread,
    const SequenceManager::Settings& settings,
    const base::TickClock* clock)
    : selector(associated_thread, settings),
      default_clock(clock),
      time_domain(nullptr),
      wake_up_queue(std::make_unique<DefaultWakeUpQueue>(associated_thread,
                                                         sequence_manager)),
      non_waking_wake_up_queue(
          std::make_unique<NonWakingWakeUpQueue>(associated_thread)) {
  if (settings.randomised_sampling_enabled) {
    metrics_subsampler = base::MetricsSubSampler();
  }
}

SequenceManagerImpl::MainThreadOnly::~MainThreadOnly() = default;

// static
std::unique_ptr<ThreadControllerImpl>
SequenceManagerImpl::CreateThreadControllerImplForCurrentThread(
    const TickClock* clock) {
  return ThreadControllerImpl::Create(GetCurrent(), clock);
}

// static
std::unique_ptr<SequenceManagerImpl> SequenceManagerImpl::CreateOnCurrentThread(
    SequenceManager::Settings settings) {
  auto thread_controller =
      CreateThreadControllerImplForCurrentThread(settings.clock);
  std::unique_ptr<SequenceManagerImpl> manager(new SequenceManagerImpl(
      std::move(thread_controller), std::move(settings)));
  manager->BindToCurrentThread();
  return manager;
}

// static
std::unique_ptr<SequenceManagerImpl> SequenceManagerImpl::CreateUnbound(
    SequenceManager::Settings settings) {
  auto thread_controller =
      ThreadControllerWithMessagePumpImpl::CreateUnbound(settings);
  return WrapUnique(new SequenceManagerImpl(std::move(thread_controller),
                                            std::move(settings)));
}

// static
void SequenceManagerImpl::InitializeFeatures() {
  TaskQueueImpl::InitializeFeatures();
  MessagePump::InitializeFeatures();
  ThreadControllerWithMessagePumpImpl::InitializeFeatures();
#if BUILDFLAG(IS_WIN)
  g_explicit_high_resolution_timer_win =
      FeatureList::IsEnabled(kExplicitHighResolutionTimerWin);
#endif  // BUILDFLAG(IS_WIN)

  g_record_crash_keys.store(
      FeatureList::IsEnabled(kRecordSequenceManagerCrashKeys),
      std::memory_order_relaxed);
  TaskQueueSelector::InitializeFeatures();
}

void SequenceManagerImpl::BindToMessagePump(std::unique_ptr<MessagePump> pump) {
  controller_->BindToCurrentThread(std::move(pump));
  CompleteInitializationOnBoundThread();

  // On Android attach to the native loop when there is one.
#if BUILDFLAG(IS_ANDROID)
  if (settings_.message_loop_type == MessagePumpType::UI ||
      settings_.message_loop_type == MessagePumpType::JAVA) {
    controller_->AttachToMessagePump();
  }
#endif

  // On iOS attach to the native loop when there is one.
#if BUILDFLAG(IS_IOS)
  if (settings_.message_loop_type == MessagePumpType::UI) {
    controller_->AttachToMessagePump();
  }
#endif
}

void SequenceManagerImpl::BindToCurrentThread() {
  associated_thread_->BindToCurrentThread();
  CompleteInitializationOnBoundThread();
}

scoped_refptr<SequencedTaskRunner>
SequenceManagerImpl::GetTaskRunnerForCurrentTask() {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  if (main_thread_only().task_execution_stack.empty())
    return nullptr;
  return main_thread_only()
      .task_execution_stack.back()
      .pending_task.task_runner;
}

void SequenceManagerImpl::CompleteInitializationOnBoundThread() {
  controller_->AddNestingObserver(this);
  main_thread_only().nesting_observer_registered_ = true;
  if (GetMessagePump()) {
    DCHECK(!GetCurrent())
        << "Can't register a second SequenceManagerImpl on the same thread.";
    thread_local_sequence_manager = this;
  }
  for (internal::TaskQueueImpl* queue : main_thread_only().active_queues) {
    queue->CompleteInitializationOnBoundThread();
  }
}

void SequenceManagerImpl::SetTimeDomain(TimeDomain* time_domain) {
  DCHECK(!main_thread_only().time_domain);
  DCHECK(time_domain);
  time_domain->OnAssignedToSequenceManager(this);
  controller_->SetTickClock(time_domain);
  main_thread_only().time_domain = time_domain;
  clock_.store(time_domain, std::memory_order_release);
}

void SequenceManagerImpl::ResetTimeDomain() {
  controller_->SetTickClock(main_thread_only().default_clock);
  clock_.store(main_thread_only().default_clock.get(),
               std::memory_order_release);
  main_thread_only().time_domain = nullptr;
}

std::unique_ptr<internal::TaskQueueImpl>
SequenceManagerImpl::CreateTaskQueueImpl(const TaskQueue::Spec& spec) {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  std::unique_ptr<internal::TaskQueueImpl> task_queue =
      std::make_unique<internal::TaskQueueImpl>(
          this,
          spec.non_waking ? main_thread_only().non_waking_wake_up_queue.get()
                          : main_thread_only().wake_up_queue.get(),
          spec);
  main_thread_only().active_queues.insert(task_queue.get());
  main_thread_only().selector.AddQueue(
      task_queue.get(), settings().priority_settings.default_priority());
  return task_queue;
}

void SequenceManagerImpl::SetAddQueueTimeToTasks(bool enable) {
  base::subtle::NoBarrier_Store(&add_queue_time_to_tasks_, enable ? 1 : 0);
}

bool SequenceManagerImpl::GetAddQueueTimeToTasks() {
  return base::subtle::NoBarrier_Load(&add_queue_time_to_tasks_);
}

void SequenceManagerImpl::SetObserver(Observer* observer) {
  main_thread_only().observer = observer;
}

void SequenceManagerImpl::UnregisterTaskQueueImpl(
    std::unique_ptr<internal::TaskQueueImpl> task_queue) {
  TRACE_EVENT1("sequence_manager", "SequenceManagerImpl::UnregisterTaskQueue",
               "queue_name", task_queue->GetName());
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);

  main_thread_only().selector.RemoveQueue(task_queue.get());

  // After UnregisterTaskQueue returns no new tasks can be posted.
  // It's important to call it first to avoid race condition between removing
  // the task queue from various lists here and adding it to the same lists
  // when posting a task.
  task_queue->UnregisterTaskQueue();

  // Add |task_queue| to |main_thread_only().queues_to_delete| so we can prevent
  // it from being freed while any of our structures hold hold a raw pointer to
  // it.
  main_thread_only().active_queues.erase(task_queue.get());
  main_thread_only().queues_to_delete[task_queue.get()] = std::move(task_queue);
}

AtomicFlagSet::AtomicFlag
SequenceManagerImpl::GetFlagToRequestReloadForEmptyQueue(
    TaskQueueImpl* task_queue) {
  return empty_queues_to_reload_.AddFlag(BindRepeating(
      &TaskQueueImpl::ReloadEmptyImmediateWorkQueue, Unretained(task_queue)));
}

void SequenceManagerImpl::ReloadEmptyWorkQueues() {
  work_tracker_.WillReloadImmediateWorkQueues();

  // There are two cases where a queue needs reloading.  First, it might be
  // completely empty and we've just posted a task (this method handles that
  // case). Secondly if the work queue becomes empty when calling
  // WorkQueue::TakeTaskFromWorkQueue (handled there).
  //
  // Invokes callbacks created by GetFlagToRequestReloadForEmptyQueue above.
  empty_queues_to_reload_.RunActiveCallbacks();
}

void SequenceManagerImpl::MoveReadyDelayedTasksToWorkQueues(LazyNow* lazy_now) {
  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
               "SequenceManagerImpl::MoveReadyDelayedTasksToWorkQueues");

  EnqueueOrder delayed_task_group_enqueue_order = GetNextSequenceNumber();
  main_thread_only().wake_up_queue->MoveReadyDelayedTasksToWorkQueues(
      lazy_now, delayed_task_group_enqueue_order);
  main_thread_only()
      .non_waking_wake_up_queue->MoveReadyDelayedTasksToWorkQueues(
          lazy_now, delayed_task_group_enqueue_order);
}

void SequenceManagerImpl::OnBeginNestedRunLoop() {
  main_thread_only().nesting_depth++;
  if (main_thread_only().observer)
    main_thread_only().observer->OnBeginNestedRunLoop();
}

void SequenceManagerImpl::OnExitNestedRunLoop() {
  main_thread_only().nesting_depth--;
  DCHECK_GE(main_thread_only().nesting_depth, 0);
  if (main_thread_only().nesting_depth == 0) {
    // While we were nested some non-nestable tasks may have been deferred. We
    // push them back onto the *front* of their original work queues, that's why
    // we iterate |non_nestable_task_queue| in LIFO order (we want
    // |non_nestable_task.front()| to be the last task pushed at the front of
    // |task_queue|).
    LazyNow exited_nested_now(main_thread_clock());
    while (!main_thread_only().non_nestable_task_queue.empty()) {
      internal::TaskQueueImpl::DeferredNonNestableTask& non_nestable_task =
          main_thread_only().non_nestable_task_queue.back();
      if (!non_nestable_task.task.queue_time.is_null()) {
        // Adjust the deferred tasks' queue time to now so that intentionally
        // deferred tasks are not unfairly considered as having been stuck in
        // the queue for a while. Note: this does not affect task ordering as
        // |enqueue_order| is untouched and deferred tasks will still be pushed
        // back to the front of the queue.
        non_nestable_task.task.queue_time = exited_nested_now.Now();
      }
      auto* const task_queue = non_nestable_task.task_queue;
      task_queue->RequeueDeferredNonNestableTask(std::move(non_nestable_task));
      main_thread_only().non_nestable_task_queue.pop_back();
    }
  }
  if (main_thread_only().observer)
    main_thread_only().observer->OnExitNestedRunLoop();
}

void SequenceManagerImpl::ScheduleWork() {
  controller_->ScheduleWork();
}

void SequenceManagerImpl::SetNextWakeUp(LazyNow* lazy_now,
                                        std::optional<WakeUp> wake_up) {
  auto next_wake_up = AdjustWakeUp(wake_up, lazy_now);
  if (next_wake_up && next_wake_up->is_immediate()) {
    ScheduleWork();
  } else {
    controller_->SetNextDelayedDoWork(lazy_now, next_wake_up);
  }
}

void SequenceManagerImpl::MaybeEmitTaskDetails(
    perfetto::EventContext& ctx,
    const SequencedTaskSource::SelectedTask& selected_task) const {
#if BUILDFLAG(ENABLE_BASE_TRACING)
  // Other parameters are included only when "scheduler" category is enabled.
  const uint8_t* scheduler_category_enabled =
      TRACE_EVENT_API_GET_CATEGORY_GROUP_ENABLED("scheduler");

  if (!*scheduler_category_enabled)
    return;
  auto* event = ctx.event<perfetto::protos::pbzero::ChromeTrackEvent>();
  auto* sequence_manager_task = event->set_sequence_manager_task();
  sequence_manager_task->set_priority(
      settings().priority_settings.TaskPriorityToProto(selected_task.priority));
  sequence_manager_task->set_queue_name(selected_task.task_queue_name);

#endif  //  BUILDFLAG(ENABLE_BASE_TRACING)
}

void SequenceManagerImpl::SetRunTaskSynchronouslyAllowed(
    bool can_run_tasks_synchronously) {
  work_tracker_.SetRunTaskSynchronouslyAllowed(can_run_tasks_synchronously);
}

std::optional<SequenceManagerImpl::SelectedTask>
SequenceManagerImpl::SelectNextTask(LazyNow& lazy_now,
                                    SelectTaskOption option) {
  std::optional<SelectedTask> selected_task =
      SelectNextTaskImpl(lazy_now, option);

  if (selected_task.has_value()) {
    work_tracker_.AssertHasWork();
  }

  return selected_task;
}

#if DCHECK_IS_ON() && !BUILDFLAG(IS_NACL)
void SequenceManagerImpl::LogTaskDebugInfo(
    const WorkQueue* selected_work_queue) const {
  const Task* task = selected_work_queue->GetFrontTask();
  switch (settings_.task_execution_logging) {
    case Settings::TaskLogging::kNone:
      break;

    case Settings::TaskLogging::kEnabled:
      LOG(INFO) << "#" << static_cast<uint64_t>(task->enqueue_order()) << " "
                << selected_work_queue->task_queue()->GetName()
                << (task->cross_thread_ ? " Run crossthread " : " Run ")
                << task->posted_from.ToString();
      break;

    case Settings::TaskLogging::kEnabledWithBacktrace: {
      std::array<const void*, PendingTask::kTaskBacktraceLength + 1> task_trace;
      task_trace[0] = task->posted_from.program_counter();
      ranges::copy(task->task_backtrace, task_trace.begin() + 1);
      size_t length = 0;
      while (length < task_trace.size() && task_trace[length])
        ++length;
      if (length == 0)
        break;
      LOG(INFO) << "#" << static_cast<uint64_t>(task->enqueue_order()) << " "
                << selected_work_queue->task_queue()->GetName()
                << (task->cross_thread_ ? " Run crossthread " : " Run ")
                << debug::StackTrace(task_trace.data(), length);
      break;
    }

    case Settings::TaskLogging::kReorderedOnly: {
      std::vector<const Task*> skipped_tasks;
      main_thread_only().selector.CollectSkippedOverLowerPriorityTasks(
          selected_work_queue, &skipped_tasks);

      if (skipped_tasks.empty())
        break;

      LOG(INFO) << "#" << static_cast<uint64_t>(task->enqueue_order()) << " "
                << selected_work_queue->task_queue()->GetName()
                << (task->cross_thread_ ? " Run crossthread " : " Run ")
                << task->posted_from.ToString();

      for (const Task* skipped_task : skipped_tasks) {
        LOG(INFO) << "# (skipped over) "
                  << static_cast<uint64_t>(skipped_task->enqueue_order()) << " "
                  << skipped_task->posted_from.ToString();
      }
    }
  }
}
#endif  // DCHECK_IS_ON() && !BUILDFLAG(IS_NACL)

std::optional<SequenceManagerImpl::SelectedTask>
SequenceManagerImpl::SelectNextTaskImpl(LazyNow& lazy_now,
                                        SelectTaskOption option) {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
               "SequenceManagerImpl::SelectNextTask");

  ReloadEmptyWorkQueues();
  MoveReadyDelayedTasksToWorkQueues(&lazy_now);

  // If we sampled now, check if it's time to reclaim memory next time we go
  // idle.
  if (lazy_now.has_value() &&
      lazy_now.Now() >= main_thread_only().next_time_to_reclaim_memory) {
    main_thread_only().memory_reclaim_scheduled = true;
  }

  while (true) {
    internal::WorkQueue* work_queue =
        main_thread_only().selector.SelectWorkQueueToService(option);
    TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(
        TRACE_DISABLED_BY_DEFAULT("sequence_manager.debug"), "SequenceManager",
        this,
        AsValueWithSelectorResultForTracing(work_queue,
                                            /* force_verbose */ false));

    if (!work_queue)
      return std::nullopt;

    // If the head task was canceled, remove it and run the selector again.
    if (UNLIKELY(work_queue->RemoveAllCanceledTasksFromFront()))
      continue;

    if (UNLIKELY(work_queue->GetFrontTask()->nestable ==
                     Nestable::kNonNestable &&
                 main_thread_only().nesting_depth > 0)) {
      // Defer non-nestable work. NOTE these tasks can be arbitrarily delayed so
      // the additional delay should not be a problem.
      // Note because we don't delete queues while nested, it's perfectly OK to
      // store the raw pointer for |queue| here.
      internal::TaskQueueImpl::DeferredNonNestableTask deferred_task{
          work_queue->TakeTaskFromWorkQueue(), work_queue->task_queue(),
          work_queue->queue_type()};
      main_thread_only().non_nestable_task_queue.push_back(
          std::move(deferred_task));
      continue;
    }

#if DCHECK_IS_ON() && !BUILDFLAG(IS_NACL)
    LogTaskDebugInfo(work_queue);
#endif  // DCHECK_IS_ON() && !BUILDFLAG(IS_NACL)

    main_thread_only().task_execution_stack.emplace_back(
        work_queue->TakeTaskFromWorkQueue(), work_queue->task_queue(),
        InitializeTaskTiming(work_queue->task_queue()));

    ExecutingTask& executing_task =
        *main_thread_only().task_execution_stack.rbegin();
    NotifyWillProcessTask(&executing_task, &lazy_now);

    // Maybe invalidate the delayed task handle. If already invalidated, then
    // don't run this task.
    if (!executing_task.pending_task.WillRunTask()) {
      executing_task.pending_task.task = DoNothing();
    }

    return SelectedTask(
        executing_task.pending_task,
        executing_task.task_queue->task_execution_trace_logger(),
        executing_task.priority, executing_task.task_queue_name);
  }
}

void SequenceManagerImpl::DidRunTask(LazyNow& lazy_now) {
  work_tracker_.AssertHasWork();

  ExecutingTask& executing_task =
      *main_thread_only().task_execution_stack.rbegin();

  NotifyDidProcessTask(&executing_task, &lazy_now);
  main_thread_only().task_execution_stack.pop_back();

  if (main_thread_only().nesting_depth == 0)
    CleanUpQueues();
}

void SequenceManagerImpl::RemoveAllCanceledDelayedTasksFromFront(
    LazyNow* lazy_now) {
  main_thread_only().wake_up_queue->RemoveAllCanceledDelayedTasksFromFront(
      lazy_now);
  main_thread_only()
      .non_waking_wake_up_queue->RemoveAllCanceledDelayedTasksFromFront(
          lazy_now);
}

std::optional<WakeUp> SequenceManagerImpl::GetPendingWakeUp(
    LazyNow* lazy_now,
    SelectTaskOption option) {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);

  RemoveAllCanceledDelayedTasksFromFront(lazy_now);

  if (main_thread_only().selector.GetHighestPendingPriority(option)) {
    // If the selector has non-empty queues we trivially know there is immediate
    // work to be done. However we may want to yield to native work if it is
    // more important.
    return WakeUp{};
  }

  // There may be some incoming immediate work which we haven't accounted for.
  // NB ReloadEmptyWorkQueues involves a memory barrier, so it's fastest to not
  // do this always.
  ReloadEmptyWorkQueues();

  if (main_thread_only().selector.GetHighestPendingPriority(option)) {
    return WakeUp{};
  }

  // Otherwise we need to find the shortest delay, if any.  NB we don't need to
  // call MoveReadyDelayedTasksToWorkQueues because it's assumed
  // DelayTillNextTask will return TimeDelta>() if the delayed task is due to
  // run now.
  return AdjustWakeUp(GetNextDelayedWakeUpWithOption(option), lazy_now);
}

std::optional<WakeUp> SequenceManagerImpl::GetNextDelayedWakeUp() const {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  return main_thread_only().wake_up_queue->GetNextDelayedWakeUp();
}

std::optional<WakeUp> SequenceManagerImpl::GetNextDelayedWakeUpWithOption(
    SelectTaskOption option) const {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);

  if (option == SelectTaskOption::kSkipDelayedTask)
    return std::nullopt;
  return GetNextDelayedWakeUp();
}

std::optional<WakeUp> SequenceManagerImpl::AdjustWakeUp(
    std::optional<WakeUp> wake_up,
    LazyNow* lazy_now) const {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  if (!wake_up)
    return std::nullopt;
  // Overdue work needs to be run immediately.
  if (lazy_now->Now() >= wake_up->earliest_time())
    return WakeUp{};
  // If |time_domain| is present, we don't want an actual OS level delayed wake
  // up scheduled, so pretend we have no more work. This will result in
  // appearing idle and |time_domain| will decide what to do in
  // MaybeFastForwardToWakeUp().
  if (main_thread_only().time_domain)
    return std::nullopt;
  return *wake_up;
}

void SequenceManagerImpl::MaybeAddLeewayToTask(Task& task) const {
  if (!main_thread_only().time_domain) {
    task.leeway = MessagePump::GetLeewayForCurrentThread();
  }
}

// TODO(crbug/1267874): Rename once ExplicitHighResolutionTimerWin experiment is
// shipped.
bool SequenceManagerImpl::HasPendingHighResolutionTasks() {
  // Only consider high-res tasks in the |wake_up_queue| (ignore the
  // |non_waking_wake_up_queue|).
#if BUILDFLAG(IS_WIN)
  if (g_explicit_high_resolution_timer_win) {
    std::optional<WakeUp> wake_up =
        main_thread_only().wake_up_queue->GetNextDelayedWakeUp();
    if (!wake_up)
      return false;
    // Under the kExplicitHighResolutionTimerWin experiment, rely on leeway
    // being larger than the minimum time of a low resolution timer (16ms). This
    // way, we don't need to activate the high resolution timer for precise
    // tasks that will run in more than 16ms if there are non precise tasks in
    // front of them.
    DCHECK_GE(MessagePump::GetLeewayIgnoringThreadOverride(),
              Milliseconds(Time::kMinLowResolutionThresholdMs));
    return wake_up->delay_policy == subtle::DelayPolicy::kPrecise;
  }
#endif  // BUILDFLAG(IS_WIN)
  return main_thread_only().wake_up_queue->has_pending_high_resolution_tasks();
}

void SequenceManagerImpl::OnBeginWork() {
  work_tracker_.OnBeginWork();
}

bool SequenceManagerImpl::OnIdle() {
  bool have_work_to_do = false;
  if (main_thread_only().time_domain) {
    auto wakeup = main_thread_only().wake_up_queue->GetNextDelayedWakeUp();
    have_work_to_do = main_thread_only().time_domain->MaybeFastForwardToWakeUp(
        wakeup, controller_->ShouldQuitRunLoopWhenIdle());
  }
  if (!have_work_to_do) {
    MaybeReclaimMemory();
    main_thread_only().on_next_idle_callbacks.Notify();
    if (main_thread_only().task_execution_stack.empty()) {
      work_tracker_.OnIdle();
    }
  }
  return have_work_to_do;
}

void SequenceManagerImpl::WillRequestReloadImmediateWorkQueue() {
  work_tracker_.WillRequestReloadImmediateWorkQueue();
}

SyncWorkAuthorization SequenceManagerImpl::TryAcquireSyncWorkAuthorization() {
  return work_tracker_.TryAcquireSyncWorkAuthorization();
}

void SequenceManagerImpl::WillQueueTask(Task* pending_task) {
  controller_->WillQueueTask(pending_task);
}

TaskQueue::TaskTiming SequenceManagerImpl::InitializeTaskTiming(
    internal::TaskQueueImpl* task_queue) {
  bool records_wall_time =
      ShouldRecordTaskTiming(task_queue) == TimeRecordingPolicy::DoRecord;
  bool records_thread_time = records_wall_time && ShouldRecordCPUTimeForTask();
  return TaskQueue::TaskTiming(records_wall_time, records_thread_time);
}

TimeRecordingPolicy SequenceManagerImpl::ShouldRecordTaskTiming(
    const internal::TaskQueueImpl* task_queue) {
  if (task_queue->RequiresTaskTiming())
    return TimeRecordingPolicy::DoRecord;
  if (main_thread_only().nesting_depth == 0 &&
      !main_thread_only().task_time_observers.empty()) {
    return TimeRecordingPolicy::DoRecord;
  }
  return TimeRecordingPolicy::DoNotRecord;
}

void SequenceManagerImpl::NotifyWillProcessTask(ExecutingTask* executing_task,
                                                LazyNow* time_before_task) {
  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
               "SequenceManagerImpl::NotifyWillProcessTaskObservers");

  if (g_record_crash_keys.load(std::memory_order_relaxed)) {
    RecordCrashKeys(executing_task->pending_task);
  }

  if (executing_task->task_queue->GetQuiescenceMonitored())
    main_thread_only().task_was_run_on_quiescence_monitored_queue = true;

  TimeRecordingPolicy recording_policy =
      ShouldRecordTaskTiming(executing_task->task_queue);
  if (recording_policy == TimeRecordingPolicy::DoRecord)
    executing_task->task_timing.RecordTaskStart(time_before_task);

  if (!executing_task->task_queue->GetShouldNotifyObservers())
    return;

  const bool was_blocked_or_low_priority =
      executing_task->task_queue->WasBlockedOrLowPriority(
          executing_task->pending_task.enqueue_order());

  {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
                 "SequenceManager.WillProcessTaskObservers");
    for (auto& observer : main_thread_only().task_observers) {
      observer.WillProcessTask(executing_task->pending_task,
                               was_blocked_or_low_priority);
    }
  }

  {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
                 "SequenceManager.QueueNotifyWillProcessTask");
    executing_task->task_queue->NotifyWillProcessTask(
        executing_task->pending_task, was_blocked_or_low_priority);
  }

  if (recording_policy != TimeRecordingPolicy::DoRecord)
    return;

  if (main_thread_only().nesting_depth == 0) {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
                 "SequenceManager.WillProcessTaskTimeObservers");
    for (auto& observer : main_thread_only().task_time_observers)
      observer.WillProcessTask(executing_task->task_timing.start_time());
  }

  {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
                 "SequenceManager.QueueOnTaskStarted");
    executing_task->task_queue->OnTaskStarted(executing_task->pending_task,
                                              executing_task->task_timing);
  }
}

void SequenceManagerImpl::NotifyDidProcessTask(ExecutingTask* executing_task,
                                               LazyNow* time_after_task) {
  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
               "SequenceManagerImpl::NotifyDidProcessTaskObservers");
  if (!executing_task->task_queue->GetShouldNotifyObservers())
    return;

  TaskQueue::TaskTiming& task_timing = executing_task->task_timing;

  {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
                 "SequenceManager.QueueOnTaskCompleted");
    if (task_timing.has_wall_time()) {
      executing_task->task_queue->OnTaskCompleted(
          executing_task->pending_task, &task_timing, time_after_task);
    }
  }

  bool has_valid_start =
      task_timing.state() != TaskQueue::TaskTiming::State::NotStarted;
  TimeRecordingPolicy recording_policy =
      ShouldRecordTaskTiming(executing_task->task_queue);
  // Record end time ASAP to avoid bias due to the overhead of observers.
  if (recording_policy == TimeRecordingPolicy::DoRecord && has_valid_start) {
    task_timing.RecordTaskEnd(time_after_task);
  }

  if (has_valid_start && task_timing.has_wall_time() &&
      main_thread_only().nesting_depth == 0) {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
                 "SequenceManager.DidProcessTaskTimeObservers");
    for (auto& observer : main_thread_only().task_time_observers) {
      observer.DidProcessTask(task_timing.start_time(), task_timing.end_time());
    }
  }

  {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
                 "SequenceManager.DidProcessTaskObservers");
    for (auto& observer : main_thread_only().task_observers)
      observer.DidProcessTask(executing_task->pending_task);
  }

  {
    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("sequence_manager"),
                 "SequenceManager.QueueNotifyDidProcessTask");
    executing_task->task_queue->NotifyDidProcessTask(
        executing_task->pending_task);
  }

  // TODO(altimin): Move this back to blink.
  if (task_timing.has_wall_time() &&
      recording_policy == TimeRecordingPolicy::DoRecord &&
      task_timing.wall_duration() > kLongTaskTraceEventThreshold &&
      main_thread_only().nesting_depth == 0) {
    TRACE_EVENT_INSTANT1("blink", "LongTask", TRACE_EVENT_SCOPE_THREAD,
                         "duration", task_timing.wall_duration().InSecondsF());
  }
}

void SequenceManagerImpl::SetWorkBatchSize(int work_batch_size) {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  DCHECK_GE(work_batch_size, 1);
  controller_->SetWorkBatchSize(work_batch_size);
}

void SequenceManagerImpl::AddTaskObserver(TaskObserver* task_observer) {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  main_thread_only().task_observers.AddObserver(task_observer);
}

void SequenceManagerImpl::RemoveTaskObserver(TaskObserver* task_observer) {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  main_thread_only().task_observers.RemoveObserver(task_observer);
}

void SequenceManagerImpl::AddTaskTimeObserver(
    TaskTimeObserver* task_time_observer) {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  main_thread_only().task_time_observers.AddObserver(task_time_observer);
}

void SequenceManagerImpl::RemoveTaskTimeObserver(
    TaskTimeObserver* task_time_observer) {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  main_thread_only().task_time_observers.RemoveObserver(task_time_observer);
}

bool SequenceManagerImpl::GetAndClearSystemIsQuiescentBit() {
  bool task_was_run =
      main_thread_only().task_was_run_on_quiescence_monitored_queue;
  main_thread_only().task_was_run_on_quiescence_monitored_queue = false;
  return !task_was_run;
}

EnqueueOrder SequenceManagerImpl::GetNextSequenceNumber() {
  return enqueue_order_generator_.GenerateNext();
}

std::unique_ptr<trace_event::ConvertableToTraceFormat>
SequenceManagerImpl::AsValueWithSelectorResultForTracing(
    internal::WorkQueue* selected_work_queue,
    bool force_verbose) const {
  return std::make_unique<TracedBaseValue>(
      Value(AsValueWithSelectorResult(selected_work_queue, force_verbose)));
}

Value::Dict SequenceManagerImpl::AsValueWithSelectorResult(
    internal::WorkQueue* selected_work_queue,
    bool force_verbose) const {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  TimeTicks now = NowTicks();
  Value::Dict state;
  Value::List active_queues;
  for (internal::TaskQueueImpl* const queue :
       main_thread_only().active_queues) {
    active_queues.Append(queue->AsValue(now, force_verbose));
  }
  state.Set("active_queues", std::move(active_queues));
  Value::List shutdown_queues;
  Value::List queues_to_delete;
  for (const auto& pair : main_thread_only().queues_to_delete)
    queues_to_delete.Append(pair.first->AsValue(now, force_verbose));
  state.Set("queues_to_delete", std::move(queues_to_delete));
  state.Set("selector", main_thread_only().selector.AsValue());
  if (selected_work_queue) {
    state.Set("selected_queue", selected_work_queue->task_queue()->GetName());
    state.Set("work_queue_name", selected_work_queue->name());
  }
  state.Set("time_domain", main_thread_only().time_domain
                               ? main_thread_only().time_domain->AsValue()
                               : Value::Dict());
  state.Set("wake_up_queue", main_thread_only().wake_up_queue->AsValue(now));
  state.Set("non_waking_wake_up_queue",
            main_thread_only().non_waking_wake_up_queue->AsValue(now));
  return state;
}

void SequenceManagerImpl::OnTaskQueueEnabled(internal::TaskQueueImpl* queue) {
  DCHECK_CALLED_ON_VALID_THREAD(associated_thread_->thread_checker);
  DCHECK(queue->IsQueueEnabled());
  // Only schedule DoWork if there's something to do.
  if (queue->HasTaskToRunImmediatelyOrReadyDelayedTask() &&
      !queue->BlockedByFence())
    ScheduleWork();
}

void SequenceManagerImpl::OnWorkAvailable() {
  work_tracker_.OnBeginWork();
}

void SequenceManagerImpl::MaybeReclaimMemory() {
  if (!main_thread_only().memory_reclaim_scheduled)
    return;

  TRACE_EVENT0("sequence_manager", "SequenceManagerImpl::MaybeReclaimMemory");
  ReclaimMemory();

  // To avoid performance regressions we only want to do this every so often.
  main_thread_only().next_time_to_reclaim_memory =
      NowTicks() + kReclaimMemoryInterval;
  main_thread_only().memory_reclaim_scheduled = false;
}

void SequenceManagerImpl::ReclaimMemory() {
  LazyNow lazy_now(main_thread_clock());
  for (auto it = main_thread_only().active_queues.begin();
       it != main_thread_only().active_queues.end();) {
    auto* const queue = (*it++).get();
    ReclaimMemoryFromQueue(queue, &lazy_now);
  }
}

void SequenceManagerImpl::CleanUpQueues() {
  main_thread_only().queues_to_delete.clear();
}

WeakPtr<SequenceManagerImpl> SequenceManagerImpl::GetWeakPtr() {
  return weak_factory_.GetWeakPtr();
}

void SequenceManagerImpl::SetDefaultTaskRunner(
    scoped_refptr<SingleThreadTaskRunner> task_runner) {
  controller_->SetDefaultTaskRunner(task_runner);
}

const TickClock* SequenceManagerImpl::GetTickClock() const {
  return any_thread_clock();
}

TimeTicks SequenceManagerImpl::NowTicks() const {
  return any_thread_clock()->NowTicks();
}

bool SequenceManagerImpl::ShouldRecordCPUTimeForTask() {
  DCHECK(ThreadTicks::IsSupported() ||
         !metric_recording_settings_.records_cpu_time_for_some_tasks());
  return metric_recording_settings_.records_cpu_time_for_some_tasks() &&
         main_thread_only().metrics_subsampler->ShouldSample(
             metric_recording_settings_
                 .task_sampling_rate_for_recording_cpu_time);
}

const SequenceManager::MetricRecordingSettings&
SequenceManagerImpl::GetMetricRecordingSettings() const {
  return metric_recording_settings_;
}

void SequenceManagerImpl::SetTaskExecutionAllowedInNativeNestedLoop(
    bool allowed) {
  controller_->SetTaskExecutionAllowedInNativeNestedLoop(allowed);
}

bool SequenceManagerImpl::IsTaskExecutionAllowedInNativeNestedLoop() const {
  return controller_->IsTaskExecutionAllowed();
}

#if BUILDFLAG(IS_IOS)
void SequenceManagerImpl::AttachToMessagePump() {
  return controller_->AttachToMessagePump();
}
#endif

bool SequenceManagerImpl::IsIdleForTesting() {
  ReloadEmptyWorkQueues();

  // Make sure that canceled tasks don't affect the return value.
  for (internal::TaskQueueImpl* queue : main_thread_only().active_queues) {
    queue->delayed_work_queue()->RemoveAllCanceledTasksFromFront();
    queue->immediate_work_queue()->RemoveAllCanceledTasksFromFront();
  }

  return !main_thread_only().selector.GetHighestPendingPriority().has_value();
}

void SequenceManagerImpl::EnableMessagePumpTimeKeeperMetrics(
    const char* thread_name) {
  controller_->EnableMessagePumpTimeKeeperMetrics(thread_name);
}

size_t SequenceManagerImpl::GetPendingTaskCountForTesting() const {
  size_t total = 0;
  for (internal::TaskQueueImpl* task_queue : main_thread_only().active_queues) {
    total += task_queue->GetNumberOfPendingTasks();
  }
  return total;
}

TaskQueue::Handle SequenceManagerImpl::CreateTaskQueue(
    const TaskQueue::Spec& spec) {
  return TaskQueue::Handle(CreateTaskQueueImpl(spec));
}

std::string SequenceManagerImpl::DescribeAllPendingTasks() const {
  Value::Dict value =
      AsValueWithSelectorResult(nullptr, /* force_verbose */ true);
  std::string result;
  JSONWriter::Write(value, &result);
  return result;
}

void SequenceManagerImpl::PrioritizeYieldingToNative(
    base::TimeTicks prioritize_until) {
  controller_->PrioritizeYieldingToNative(prioritize_until);
}

void SequenceManagerImpl::AddDestructionObserver(
    CurrentThread::DestructionObserver* destruction_observer) {
  main_thread_only().destruction_observers.AddObserver(destruction_observer);
}

void SequenceManagerImpl::RemoveDestructionObserver(
    CurrentThread::DestructionObserver* destruction_observer) {
  main_thread_only().destruction_observers.RemoveObserver(destruction_observer);
}

CallbackListSubscription SequenceManagerImpl::RegisterOnNextIdleCallback(
    OnceClosure on_next_idle_callback) {
  return main_thread_only().on_next_idle_callbacks.Add(
      std::move(on_next_idle_callback));
}

void SequenceManagerImpl::SetTaskRunner(
    scoped_refptr<SingleThreadTaskRunner> task_runner) {
  controller_->SetDefaultTaskRunner(task_runner);
}

scoped_refptr<SingleThreadTaskRunner> SequenceManagerImpl::GetTaskRunner() {
  return controller_->GetDefaultTaskRunner();
}

bool SequenceManagerImpl::IsBoundToCurrentThread() const {
  return associated_thread_->IsBoundToCurrentThread();
}

MessagePump* SequenceManagerImpl::GetMessagePump() const {
  return controller_->GetBoundMessagePump();
}

bool SequenceManagerImpl::IsType(MessagePumpType type) const {
  return settings_.message_loop_type == type;
}

void SequenceManagerImpl::EnableCrashKeys(const char* async_stack_crash_key) {
  DCHECK(!main_thread_only().async_stack_crash_key);
#if !BUILDFLAG(IS_NACL) && !BUILDFLAG(IS_ANDROID)
  main_thread_only().async_stack_crash_key = debug::AllocateCrashKeyString(
      async_stack_crash_key, debug::CrashKeySize::Size64);
  static_assert(sizeof(main_thread_only().async_stack_buffer) ==
                    static_cast<size_t>(debug::CrashKeySize::Size64),
                "Async stack buffer size must match crash key size.");
#endif  // !BUILDFLAG(IS_NACL) && !BUILDFLAG(IS_ANDROID)
}

void SequenceManagerImpl::RecordCrashKeys(const PendingTask& pending_task) {
#if !BUILDFLAG(IS_NACL) && !BUILDFLAG(IS_ANDROID)
  // SetCrashKeyString is a no-op even if the crash key is null, but we'd still
  // have construct the std::string_view that is passed in.
  if (!main_thread_only().async_stack_crash_key)
    return;

  // Write the async stack trace onto a crash key as whitespace-delimited hex
  // addresses. These will be symbolized by the crash reporting system. With
  // 63 characters we can fit the address of the task that posted the current
  // task and its predecessor. Avoid HexEncode since it incurs a memory
  // allocation and snprintf because it's about 3.5x slower on Android this
  // this.
  //
  // See
  // https://chromium.googlesource.com/chromium/src/+/main/docs/debugging_with_crash_keys.md
  // for instructions for symbolizing these crash keys.
  //
  // TODO(skyostil): Find a way to extract the destination function address
  // from the task.
  size_t max_size = main_thread_only().async_stack_buffer.size();
  char* const buffer = &main_thread_only().async_stack_buffer[0];
  char* const buffer_end = &buffer[max_size - 1];
  char* pos = buffer_end;
  // Leave space for the NUL terminator.
  pos = PrependHexAddress(pos - 1, pending_task.task_backtrace[0]);
  *(--pos) = ' ';
  pos = PrependHexAddress(pos - 1, pending_task.posted_from.program_counter());
  DCHECK_GE(pos, buffer);
  debug::SetCrashKeyString(
      main_thread_only().async_stack_crash_key,
      std::string_view(pos, static_cast<size_t>(buffer_end - pos)));
#endif  // !BUILDFLAG(IS_NACL) && !BUILDFLAG(IS_ANDROID)
}

internal::TaskQueueImpl* SequenceManagerImpl::currently_executing_task_queue()
    const {
  if (main_thread_only().task_execution_stack.empty())
    return nullptr;
  return main_thread_only().task_execution_stack.rbegin()->task_queue;
}

TaskQueue::QueuePriority SequenceManagerImpl::GetPriorityCount() const {
  return settings().priority_settings.priority_count();
}

constexpr TimeDelta SequenceManagerImpl::kReclaimMemoryInterval;

}  // namespace internal
}  // namespace sequence_manager
}  // namespace base