naiveproxy/src/base/test/task_environment.cc

// Copyright 2017 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/test/task_environment.h"

#include <algorithm>
#include <memory>
#include <ostream>

#include "base/callback_helpers.h"
#include "base/check.h"
#include "base/debug/stack_trace.h"
#include "base/lazy_instance.h"
#include "base/location.h"
#include "base/logging.h"
#include "base/memory/ptr_util.h"
#include "base/memory/raw_ptr.h"
#include "base/memory/raw_ref.h"
#include "base/message_loop/message_pump.h"
#include "base/message_loop/message_pump_type.h"
#include "base/no_destructor.h"
#include "base/process/process.h"
#include "base/run_loop.h"
#include "base/synchronization/condition_variable.h"
#include "base/synchronization/lock.h"
#include "base/task/common/lazy_now.h"
#include "base/task/sequence_manager/sequence_manager_impl.h"
#include "base/task/sequence_manager/time_domain.h"
#include "base/task/simple_task_executor.h"
#include "base/task/thread_pool/thread_pool_impl.h"
#include "base/task/thread_pool/thread_pool_instance.h"
#include "base/test/bind.h"
#include "base/test/test_mock_time_task_runner.h"
#include "base/test/test_timeouts.h"
#include "base/thread_annotations.h"
#include "base/threading/platform_thread.h"
#include "base/threading/sequence_local_storage_map.h"
#include "base/threading/thread_checker_impl.h"
#include "base/threading/thread_local.h"
#include "base/threading/thread_restrictions.h"
#include "base/threading/thread_task_runner_handle.h"
#include "base/time/clock.h"
#include "base/time/tick_clock.h"
#include "base/time/time.h"
#include "base/time/time_override.h"
#include "build/build_config.h"
#include "testing/gtest/include/gtest/gtest.h"

#if BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
#include "base/files/file_descriptor_watcher_posix.h"
#include "third_party/abseil-cpp/absl/types/optional.h"
#endif

#if BUILDFLAG(ENABLE_BASE_TRACING)
#include "base/trace_event/trace_log.h"  // nogncheck
#endif                                   // BUILDFLAG(ENABLE_BASE_TRACING)

namespace base {
namespace test {

namespace {

ObserverList<TaskEnvironment::DestructionObserver>& GetDestructionObservers() {
  static NoDestructor<ObserverList<TaskEnvironment::DestructionObserver>>
      instance;
  return *instance;
}

// A pointer to the current TestTaskTracker, if any, constant throughout the
// lifetime of a ThreadPoolInstance managed by a TaskEnvironment.
TaskEnvironment::TestTaskTracker* g_task_tracker = nullptr;

base::MessagePumpType GetMessagePumpTypeForMainThreadType(
    TaskEnvironment::MainThreadType main_thread_type) {
  switch (main_thread_type) {
    case TaskEnvironment::MainThreadType::DEFAULT:
      return MessagePumpType::DEFAULT;
    case TaskEnvironment::MainThreadType::UI:
      return MessagePumpType::UI;
    case TaskEnvironment::MainThreadType::IO:
      return MessagePumpType::IO;
  }
  NOTREACHED();
  return MessagePumpType::DEFAULT;
}

std::unique_ptr<sequence_manager::SequenceManager>
CreateSequenceManagerForMainThreadType(
    TaskEnvironment::MainThreadType main_thread_type) {
  auto type = GetMessagePumpTypeForMainThreadType(main_thread_type);
  return sequence_manager::CreateSequenceManagerOnCurrentThreadWithPump(
      MessagePump::Create(type),
      base::sequence_manager::SequenceManager::Settings::Builder()
          .SetMessagePumpType(type)
          .Build());
}

class TickClockBasedClock : public Clock {
 public:
  explicit TickClockBasedClock(const TickClock* tick_clock)
      : tick_clock_(*tick_clock),
        start_ticks_(tick_clock_->NowTicks()),
        start_time_(Time::UnixEpoch()) {}

  Time Now() const override {
    return start_time_ + (tick_clock_->NowTicks() - start_ticks_);
  }

 private:
  const raw_ref<const TickClock> tick_clock_;
  const TimeTicks start_ticks_;
  const Time start_time_;
};

}  // namespace

class TaskEnvironment::TestTaskTracker
    : public internal::ThreadPoolImpl::TaskTrackerImpl {
 public:
  TestTaskTracker();

  TestTaskTracker(const TestTaskTracker&) = delete;
  TestTaskTracker& operator=(const TestTaskTracker&) = delete;

  // Allow running tasks. Returns whether tasks were previously allowed to run.
  bool AllowRunTasks();

  // Disallow running tasks. Returns true on success; success requires there to
  // be no tasks currently running. Returns false if >0 tasks are currently
  // running. Prior to returning false, it will attempt to block until at least
  // one task has completed (in an attempt to avoid callers busy-looping
  // DisallowRunTasks() calls with the same set of slowly ongoing tasks).
  // Returns false if none of the ongoing tasks complete within |timeout| in an
  // attempt to prevent a deadlock in the event that the only task remaining is
  // blocked on the main thread.
  bool DisallowRunTasks(TimeDelta timeout = Milliseconds(1));

  // Returns true if tasks are currently allowed to run.
  bool TasksAllowedToRun() const;

  // For debugging purposes. Returns a string with information about all the
  // currently running tasks on the thread pool.
  std::string DescribeRunningTasks() const;

  // Returns true if this is invoked on this TaskTracker's owning thread
  // (i.e. test main thread).
  bool OnControllerThread() const {
    return controller_thread_checker_.CalledOnValidThread();
  }

 private:
  friend class TaskEnvironment;

  // internal::ThreadPoolImpl::TaskTrackerImpl:
  void RunTask(internal::Task task,
               internal::TaskSource* sequence,
               const TaskTraits& traits) override;
  void BeginCompleteShutdown(base::WaitableEvent& shutdown_event) override;
  void AssertFlushForTestingAllowed() override;

  // Synchronizes accesses to members below.
  mutable Lock lock_;

  // True if running tasks is allowed.
  bool can_run_tasks_ GUARDED_BY(lock_) = true;

  // Signaled when |can_run_tasks_| becomes true.
  ConditionVariable can_run_tasks_cv_ GUARDED_BY(lock_);

  // Signaled when a task is completed.
  ConditionVariable task_completed_cv_ GUARDED_BY(lock_);

  // Next task number so that each task has some unique-ish id.
  int64_t next_task_number_ GUARDED_BY(lock_) = 1;
  // The set of tasks currently running, keyed by the id from
  // |next_task_number_|.
  base::flat_map<int64_t, Location> running_tasks_ GUARDED_BY(lock_);

  // Used to implement OnControllerThread().
  ThreadCheckerImpl controller_thread_checker_;
};

class TaskEnvironment::MockTimeDomain : public sequence_manager::TimeDomain {
 public:
  explicit MockTimeDomain(
      sequence_manager::internal::SequenceManagerImpl* sequence_manager)
      : sequence_manager_(sequence_manager) {
    DCHECK_EQ(nullptr, current_mock_time_domain_);
    current_mock_time_domain_ = this;
  }

  ~MockTimeDomain() override {
    DCHECK_EQ(this, current_mock_time_domain_);
    current_mock_time_domain_ = nullptr;
  }

  static MockTimeDomain* current_mock_time_domain_;

  static Time GetTime() {
    return Time::UnixEpoch() +
           (current_mock_time_domain_->NowTicks() - TimeTicks());
  }

  static TimeTicks GetTimeTicks() {
    return current_mock_time_domain_->NowTicks();
  }

  void AdvanceClock(TimeDelta delta) {
    DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
    {
      AutoLock lock(now_ticks_lock_);
      now_ticks_ += delta;
    }
    if (thread_pool_)
      thread_pool_->ProcessRipeDelayedTasksForTesting();
  }

  void SetThreadPool(internal::ThreadPoolImpl* thread_pool,
                     const TestTaskTracker* thread_pool_task_tracker) {
    DCHECK(!thread_pool_);
    DCHECK(!thread_pool_task_tracker_);
    thread_pool_ = thread_pool;
    thread_pool_task_tracker_ = thread_pool_task_tracker;
  }

  // sequence_manager::TimeDomain:

  // This method is called when the underlying message pump has run out of
  // non-delayed work. Advances time to the next task unless
  // |quit_when_idle_requested| or TaskEnvironment controls mock time.
  bool MaybeFastForwardToWakeUp(
      absl::optional<sequence_manager::WakeUp> next_wake_up,
      bool quit_when_idle_requested) override {
    if (quit_when_idle_requested)
      return false;

    return FastForwardToNextTaskOrCap(next_wake_up, TimeTicks::Max()) ==
           NextTaskSource::kMainThreadHasWork;
  }

  const char* GetName() const override { return "MockTimeDomain"; }

  // TickClock implementation:
  TimeTicks NowTicks() const override {
    // This can be called from any thread.
    AutoLock lock(now_ticks_lock_);
    return now_ticks_;
  }

  // Used by FastForwardToNextTaskOrCap() to return which task source time was
  // advanced to.
  enum class NextTaskSource {
    // Out of tasks under |fast_forward_cap|.
    kNone,
    // There's now >=1 immediate task on the main thread (ThreadPool might have
    // some too).
    kMainThreadHasWork,
    // There's now >=1 immediate task in the thread pool.
    kThreadPoolOnly,
  };

  // Advances time to the first of : next main thread delayed task, next thread
  // pool task, or |fast_forward_cap| (if it's not Max()). Ignores immediate
  // tasks, expected to be called after being just idle, racily scheduling
  // immediate tasks doesn't affect the outcome of this call.
  NextTaskSource FastForwardToNextTaskOrCap(
      absl::optional<sequence_manager::WakeUp> next_main_thread_wake_up,
      TimeTicks fast_forward_cap) {
    DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);

    // Consider the next thread pool tasks iff they're running.
    absl::optional<TimeTicks> next_thread_pool_task_time;
    if (thread_pool_ && thread_pool_task_tracker_->TasksAllowedToRun()) {
      next_thread_pool_task_time =
          thread_pool_->NextScheduledRunTimeForTesting();
    }

    // Custom comparison logic to consider nullopt the largest rather than
    // smallest value. Could consider using TimeTicks::Max() instead of nullopt
    // to represent out-of-tasks?
    absl::optional<TimeTicks> next_task_time;
    if (!next_main_thread_wake_up) {
      next_task_time = next_thread_pool_task_time;
    } else if (!next_thread_pool_task_time) {
      next_task_time = next_main_thread_wake_up->time;
    } else {
      next_task_time =
          std::min(next_main_thread_wake_up->time, *next_thread_pool_task_time);
    }

    if (next_task_time && *next_task_time <= fast_forward_cap) {
      {
        AutoLock lock(now_ticks_lock_);
        // It's possible for |next_task_time| to be in the past in the following
        // scenario:
        // Start with Now() == 100ms
        // Thread A : Post 200ms delayed task T (construct and enqueue)
        // Thread B : Construct 20ms delayed task U
        //              => |delayed_run_time| == 120ms.
        // Thread A : FastForwardToNextTaskOrCap() => fast-forwards to T @
        //            300ms (task U is not yet in queue).
        // Thread B : Complete enqueue of task U.
        // Thread A : FastForwardToNextTaskOrCap() => must stay at 300ms and run
        //            U, not go back to 120ms.
        // Hence we need std::max() to protect against this because construction
        // and enqueuing isn't atomic in time (LazyNow support in
        // base/task/thread_pool could help).
        now_ticks_ = std::max(now_ticks_, *next_task_time);
      }

      if (next_task_time == next_thread_pool_task_time)
        thread_pool_->ProcessRipeDelayedTasksForTesting();

      if (next_main_thread_wake_up &&
          next_task_time == next_main_thread_wake_up->time) {
        return NextTaskSource::kMainThreadHasWork;
      }

      // The main thread doesn't have immediate work so it'll go to sleep after
      // returning from this call. We must make sure it wakes up when the
      // ThreadPool is done or the test may stall : crbug.com/1263149.
      //
      // Note: It is necessary to reach in SequenceManagerImpl to ScheduleWork
      // instead of alternatives to waking the main thread, like posting a
      // no-op task, as alternatives would prevent the main thread from
      // achieving quiescence (which some task monitoring tests verify).
      thread_pool_->FlushAsyncForTesting(BindOnce(
          &sequence_manager::internal::SequenceManagerImpl::ScheduleWork,
          Unretained(sequence_manager_)));
      return NextTaskSource::kThreadPoolOnly;
    }

    if (!fast_forward_cap.is_max()) {
      AutoLock lock(now_ticks_lock_);
      // It's possible that Now() is already beyond |fast_forward_cap| when the
      // caller nests multiple FastForwardBy() calls.
      now_ticks_ = std::max(now_ticks_, fast_forward_cap);
    }

    return NextTaskSource::kNone;
  }

 private:
  SEQUENCE_CHECKER(sequence_checker_);

  raw_ptr<internal::ThreadPoolImpl> thread_pool_ = nullptr;
  raw_ptr<const TestTaskTracker> thread_pool_task_tracker_ = nullptr;

  const raw_ptr<sequence_manager::internal::SequenceManagerImpl>
      sequence_manager_;

  // Protects |now_ticks_|
  mutable Lock now_ticks_lock_;

  // Only ever written to from the main sequence. Start from real Now() instead
  // of zero to give a more realistic view to tests.
  TimeTicks now_ticks_ GUARDED_BY(now_ticks_lock_){
      base::subtle::TimeTicksNowIgnoringOverride()
          .SnappedToNextTick(TimeTicks(), Milliseconds(1))};
};

TaskEnvironment::MockTimeDomain*
    TaskEnvironment::MockTimeDomain::current_mock_time_domain_ = nullptr;

TaskEnvironment::TaskEnvironment(
    TimeSource time_source,
    MainThreadType main_thread_type,
    ThreadPoolExecutionMode thread_pool_execution_mode,
    ThreadingMode threading_mode,
    ThreadPoolCOMEnvironment thread_pool_com_environment,
    bool subclass_creates_default_taskrunner,
    trait_helpers::NotATraitTag)
    : main_thread_type_(main_thread_type),
      thread_pool_execution_mode_(thread_pool_execution_mode),
      threading_mode_(threading_mode),
      thread_pool_com_environment_(thread_pool_com_environment),
      subclass_creates_default_taskrunner_(subclass_creates_default_taskrunner),
      sequence_manager_(
          CreateSequenceManagerForMainThreadType(main_thread_type)),
      mock_time_domain_(
          time_source != TimeSource::SYSTEM_TIME
              ? std::make_unique<TaskEnvironment::MockTimeDomain>(
                    static_cast<
                        sequence_manager::internal::SequenceManagerImpl*>(
                        sequence_manager_.get()))
              : nullptr),
      time_overrides_(time_source == TimeSource::MOCK_TIME
                          ? std::make_unique<subtle::ScopedTimeClockOverrides>(
                                &MockTimeDomain::GetTime,
                                &MockTimeDomain::GetTimeTicks,
                                nullptr)
                          : nullptr),
      mock_clock_(mock_time_domain_ ? std::make_unique<TickClockBasedClock>(
                                          mock_time_domain_.get())
                                    : nullptr),
      scoped_lazy_task_runner_list_for_testing_(
          std::make_unique<internal::ScopedLazyTaskRunnerListForTesting>()),
      // TODO(https://crbug.com/922098): Enable Run() timeouts even for
      // instances created with TimeSource::MOCK_TIME.
      run_loop_timeout_(
          mock_time_domain_
              ? nullptr
              : std::make_unique<ScopedRunLoopTimeout>(
                    FROM_HERE,
                    TestTimeouts::action_timeout(),
                    BindRepeating(&sequence_manager::SequenceManager::
                                      DescribeAllPendingTasks,
                                  Unretained(sequence_manager_.get())))) {
  CHECK(!base::ThreadTaskRunnerHandle::IsSet());
  // If |subclass_creates_default_taskrunner| is true then initialization is
  // deferred until DeferredInitFromSubclass().
  if (!subclass_creates_default_taskrunner) {
    task_queue_ =
        sequence_manager_->CreateTaskQueue(sequence_manager::TaskQueue::Spec(
            sequence_manager::QueueName::TASK_ENVIRONMENT_DEFAULT_TQ));
    task_runner_ = task_queue_->task_runner();
    sequence_manager_->SetDefaultTaskRunner(task_runner_);
    if (mock_time_domain_)
      sequence_manager_->SetTimeDomain(mock_time_domain_.get());
    simple_task_executor_ = std::make_unique<SimpleTaskExecutor>(task_runner_);
    CHECK(base::ThreadTaskRunnerHandle::IsSet())
        << "ThreadTaskRunnerHandle should've been set now.";
    CompleteInitialization();
  }

  if (threading_mode_ != ThreadingMode::MAIN_THREAD_ONLY)
    InitializeThreadPool();

  if (thread_pool_execution_mode_ == ThreadPoolExecutionMode::QUEUED &&
      task_tracker_) {
    CHECK(task_tracker_->DisallowRunTasks());
  }
}

// static
TaskEnvironment::TestTaskTracker* TaskEnvironment::CreateThreadPool() {
  CHECK(!ThreadPoolInstance::Get())
      << "Someone has already installed a ThreadPoolInstance. If nothing in "
         "your test does so, then a test that ran earlier may have installed "
         "one and leaked it. base::TestSuite will trap leaked globals, unless "
         "someone has explicitly disabled it with "
         "DisableCheckForLeakedGlobals().";

  auto task_tracker = std::make_unique<TestTaskTracker>();
  TestTaskTracker* raw_task_tracker = task_tracker.get();
  auto thread_pool = std::make_unique<internal::ThreadPoolImpl>(
      std::string(), std::move(task_tracker));
  ThreadPoolInstance::Set(std::move(thread_pool));
  DCHECK(!g_task_tracker);
  g_task_tracker = raw_task_tracker;
  return raw_task_tracker;
}

void TaskEnvironment::InitializeThreadPool() {
#if BUILDFLAG(ENABLE_BASE_TRACING)
  // Force the creation of TraceLog instance before starting ThreadPool and
  // creating additional threads to avoid race conditions.
  trace_event::TraceLog::GetInstance();
#endif  // BUILDFLAG(ENABLE_BASE_TRACING)

  task_tracker_ = CreateThreadPool();
  if (mock_time_domain_) {
    mock_time_domain_->SetThreadPool(
        static_cast<internal::ThreadPoolImpl*>(ThreadPoolInstance::Get()),
        task_tracker_);
  }

  ThreadPoolInstance::InitParams init_params(kNumForegroundThreadPoolThreads);
  init_params.suggested_reclaim_time = TimeDelta::Max();
#if BUILDFLAG(IS_WIN)
  if (thread_pool_com_environment_ == ThreadPoolCOMEnvironment::COM_MTA) {
    init_params.common_thread_pool_environment =
        ThreadPoolInstance::InitParams::CommonThreadPoolEnvironment::COM_MTA;
  }
#endif
  ThreadPoolInstance::Get()->Start(init_params);
}

void TaskEnvironment::CompleteInitialization() {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);

#if BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
  if (main_thread_type() == MainThreadType::IO) {
    file_descriptor_watcher_ =
        std::make_unique<FileDescriptorWatcher>(GetMainThreadTaskRunner());
  }
#endif  // BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
}

TaskEnvironment::TaskEnvironment(TaskEnvironment&& other) = default;

TaskEnvironment::~TaskEnvironment() {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);
  DestroyTaskEnvironment();
}

void TaskEnvironment::DestroyTaskEnvironment() {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);

  // If we've been moved or already destroyed (i.e. subclass invoked
  // DestroyTaskEnvironment() before ~TaskEnvironment()) then bail out.
  if (!owns_instance_)
    return;
  owns_instance_.reset();

  for (auto& observer : GetDestructionObservers())
    observer.WillDestroyCurrentTaskEnvironment();

  DestroyThreadPool();
  task_queue_ = nullptr;
  // SequenceManagerImpl must outlive ThreadPoolInstance() (DestroyThreadPool()
  // above) as TaskEnvironment::MockTimeDomain can invoke its
  // SequenceManagerImpl* from worker threads.
  sequence_manager_.reset();
}

void TaskEnvironment::DestroyThreadPool() {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);

  if (threading_mode_ == ThreadingMode::MAIN_THREAD_ONLY)
    return;
  DCHECK(ThreadPoolInstance::Get());

  // Ideally this would RunLoop().RunUntilIdle() here to catch any errors or
  // infinite post loop in the remaining work but this isn't possible right now
  // because base::~MessageLoop() didn't use to do this and adding it here would
  // make the migration away from MessageLoop that much harder.

  // Without FlushForTesting(), DeleteSoon() and ReleaseSoon() tasks could be
  // skipped, resulting in memory leaks.
  task_tracker_->AllowRunTasks();
  ThreadPoolInstance::Get()->FlushForTesting();
  ThreadPoolInstance::Get()->Shutdown();
  ThreadPoolInstance::Get()->JoinForTesting();
  DCHECK_EQ(g_task_tracker, task_tracker_);
  g_task_tracker = nullptr;
  // Destroying ThreadPoolInstance state can result in waiting on worker
  // threads. Make sure this is allowed to avoid flaking tests that have
  // disallowed waits on their main thread.
  ScopedAllowBaseSyncPrimitivesForTesting allow_waits_to_destroy_task_tracker;
  ThreadPoolInstance::Set(nullptr);
}

sequence_manager::TimeDomain* TaskEnvironment::GetMockTimeDomain() const {
  return mock_time_domain_.get();
}

sequence_manager::SequenceManager* TaskEnvironment::sequence_manager() const {
  DCHECK(subclass_creates_default_taskrunner_);
  return sequence_manager_.get();
}

void TaskEnvironment::DeferredInitFromSubclass(
    scoped_refptr<base::SingleThreadTaskRunner> task_runner) {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);

  task_runner_ = std::move(task_runner);
  sequence_manager_->SetDefaultTaskRunner(task_runner_);
  CompleteInitialization();
}

scoped_refptr<base::SingleThreadTaskRunner>
TaskEnvironment::GetMainThreadTaskRunner() {
  DCHECK(task_runner_);
  return task_runner_;
}

bool TaskEnvironment::MainThreadIsIdle() const {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);

  sequence_manager::internal::SequenceManagerImpl* sequence_manager_impl =
      static_cast<sequence_manager::internal::SequenceManagerImpl*>(
          sequence_manager_.get());
  // ReclaimMemory sweeps canceled delayed tasks.
  sequence_manager_impl->ReclaimMemory();
  return sequence_manager_impl->IsIdleForTesting();
}

void TaskEnvironment::RunUntilIdle() {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);

  if (threading_mode_ == ThreadingMode::MAIN_THREAD_ONLY) {
    RunLoop(RunLoop::Type::kNestableTasksAllowed).RunUntilIdle();
    return;
  }

  // TODO(gab): This can be heavily simplified to essentially:
  //     bool HasMainThreadTasks() {
  //      if (message_loop_)
  //        return !message_loop_->IsIdleForTesting();
  //      return mock_time_task_runner_->NextPendingTaskDelay().is_zero();
  //     }
  //     while (task_tracker_->HasIncompleteTasks() || HasMainThreadTasks()) {
  //       base::RunLoop().RunUntilIdle();
  //       // Avoid busy-looping.
  //       if (task_tracker_->HasIncompleteTasks())
  //         PlatformThread::Sleep(Milliseconds(1));
  //     }
  // Update: This can likely be done now that MessageLoop::IsIdleForTesting()
  // checks all queues.
  //
  // Other than that it works because once |task_tracker_->HasIncompleteTasks()|
  // is false we know for sure that the only thing that can make it true is a
  // main thread task (TaskEnvironment owns all the threads). As such we can't
  // racily see it as false on the main thread and be wrong as if it the main
  // thread sees the atomic count at zero, it's the only one that can make it go
  // up. And the only thing that can make it go up on the main thread are main
  // thread tasks and therefore we're done if there aren't any left.
  //
  // This simplification further allows simplification of DisallowRunTasks().
  //
  // This can also be simplified even further once TaskTracker becomes directly
  // aware of main thread tasks. https://crbug.com/660078.

  const bool could_run_tasks = task_tracker_->AllowRunTasks();

  for (;;) {
    task_tracker_->AllowRunTasks();

    // First run as many tasks as possible on the main thread in parallel with
    // tasks in ThreadPool. This increases likelihood of TSAN catching
    // threading errors and eliminates possibility of hangs should a
    // ThreadPool task synchronously block on a main thread task
    // (ThreadPoolInstance::FlushForTesting() can't be used here for that
    // reason).
    RunLoop(RunLoop::Type::kNestableTasksAllowed).RunUntilIdle();

    // Then halt ThreadPool. DisallowRunTasks() failing indicates that there
    // were ThreadPool tasks currently running. In that case, try again from
    // top when DisallowRunTasks() yields control back to this thread as they
    // may have posted main thread tasks.
    if (!task_tracker_->DisallowRunTasks())
      continue;

    // Once ThreadPool is halted. Run any remaining main thread tasks (which
    // may have been posted by ThreadPool tasks that completed between the
    // above main thread RunUntilIdle() and ThreadPool DisallowRunTasks()).
    // Note: this assumes that no main thread task synchronously blocks on a
    // ThreadPool tasks (it certainly shouldn't); this call could otherwise
    // hang.
    RunLoop(RunLoop::Type::kNestableTasksAllowed).RunUntilIdle();

    // The above RunUntilIdle() guarantees there are no remaining main thread
    // tasks (the ThreadPool being halted during the last RunUntilIdle() is
    // key as it prevents a task being posted to it racily with it determining
    // it had no work remaining). Therefore, we're done if there is no more work
    // on ThreadPool either (there can be ThreadPool work remaining if
    // DisallowRunTasks() preempted work and/or the last RunUntilIdle() posted
    // more ThreadPool tasks).
    // Note: this last |if| couldn't be turned into a |do {} while();|. A
    // conditional loop makes it such that |continue;| results in checking the
    // condition (not unconditionally loop again) which would be incorrect for
    // the above logic as it'd then be possible for a ThreadPool task to be
    // running during the DisallowRunTasks() test, causing it to fail, but then
    // post to the main thread and complete before the loop's condition is
    // verified which could result in HasIncompleteUndelayedTasksForTesting()
    // returning false and the loop erroneously exiting with a pending task on
    // the main thread.
    if (!task_tracker_->HasIncompleteTaskSourcesForTesting())
      break;
  }

  // The above loop always ends with running tasks being disallowed. Re-enable
  // parallel execution before returning if it was allowed at the beginning of
  // this call.
  if (could_run_tasks)
    task_tracker_->AllowRunTasks();
}

void TaskEnvironment::FastForwardBy(TimeDelta delta) {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);
  DCHECK(mock_time_domain_);
  DCHECK_GE(delta, TimeDelta());

  const bool could_run_tasks = task_tracker_ && task_tracker_->AllowRunTasks();

  const TimeTicks fast_forward_until = mock_time_domain_->NowTicks() + delta;
  do {
    RunUntilIdle();
    // ReclaimMemory sweeps canceled delayed tasks, making sure
    // FastForwardToNextTaskOrCap isn't affected by canceled tasks.
    sequence_manager_->ReclaimMemory();
  } while (mock_time_domain_->FastForwardToNextTaskOrCap(
               sequence_manager_->GetNextDelayedWakeUp(), fast_forward_until) !=
           MockTimeDomain::NextTaskSource::kNone);

  if (task_tracker_ && !could_run_tasks)
    task_tracker_->DisallowRunTasks();
}

void TaskEnvironment::FastForwardUntilNoTasksRemain() {
  // TimeTicks::operator+(TimeDelta) uses saturated arithmetic so it's safe to
  // pass in TimeDelta::Max().
  FastForwardBy(TimeDelta::Max());
}

void TaskEnvironment::AdvanceClock(TimeDelta delta) {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);
  DCHECK(mock_time_domain_);
  DCHECK_GE(delta, TimeDelta());
  mock_time_domain_->AdvanceClock(delta);
}

const TickClock* TaskEnvironment::GetMockTickClock() const {
  DCHECK(mock_time_domain_);
  return mock_time_domain_.get();
}

base::TimeTicks TaskEnvironment::NowTicks() const {
  DCHECK(mock_time_domain_);
  return mock_time_domain_->NowTicks();
}

const Clock* TaskEnvironment::GetMockClock() const {
  DCHECK(mock_clock_);
  return mock_clock_.get();
}

size_t TaskEnvironment::GetPendingMainThreadTaskCount() const {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);

  // ReclaimMemory sweeps canceled delayed tasks.
  sequence_manager_->ReclaimMemory();
  return sequence_manager_->GetPendingTaskCountForTesting();
}

TimeDelta TaskEnvironment::NextMainThreadPendingTaskDelay() const {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);

  // ReclaimMemory sweeps canceled delayed tasks.
  sequence_manager_->ReclaimMemory();
  DCHECK(mock_time_domain_);
  LazyNow lazy_now(mock_time_domain_->NowTicks());
  if (!sequence_manager_->IsIdleForTesting())
    return TimeDelta();
  absl::optional<sequence_manager::WakeUp> wake_up =
      sequence_manager_->GetNextDelayedWakeUp();
  return wake_up ? wake_up->time - lazy_now.Now() : TimeDelta::Max();
}

bool TaskEnvironment::NextTaskIsDelayed() const {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);

  TimeDelta delay = NextMainThreadPendingTaskDelay();
  return !delay.is_zero() && !delay.is_max();
}

void TaskEnvironment::DescribeCurrentTasks() const {
  DCHECK_CALLED_ON_VALID_THREAD(main_thread_checker_);
  LOG(INFO) << task_tracker_->DescribeRunningTasks();
  LOG(INFO) << sequence_manager_->DescribeAllPendingTasks();
}

void TaskEnvironment::DetachFromThread() {
  DETACH_FROM_THREAD(main_thread_checker_);
  if (task_tracker_)
    task_tracker_->controller_thread_checker_.DetachFromThread();
}

// static
void TaskEnvironment::AddDestructionObserver(DestructionObserver* observer) {
  GetDestructionObservers().AddObserver(observer);
}

// static
void TaskEnvironment::RemoveDestructionObserver(DestructionObserver* observer) {
  GetDestructionObservers().RemoveObserver(observer);
}

TaskEnvironment::ParallelExecutionFence::ParallelExecutionFence(
    const char* error_message) {
  CHECK(!g_task_tracker || g_task_tracker->OnControllerThread())
      << error_message;
  if (g_task_tracker) {
    // Do not attempt to install a fence post shutdown, the only remaining tasks
    // at that point are CONTINUE_ON_SHUTDOWN and attempting to wait for them
    // causes more issues (test timeouts) than the fence solves (data races on
    // global state). CONTINUE_ON_SHUTDOWN tasks should generally not be
    // touching global state and while not all users of ParallelExecutionFence
    // (FeatureList) guard against access from CONTINUE_ON_SHUTDOWN tasks, any
    // such tasks abusing this would be flagged by TSAN and have to be fixed
    // manually. Note: this is only relevant in browser tests as unit tests
    // already go through a full join in TaskEnvironment::DestroyThreadPool().
    previously_allowed_to_run_ = g_task_tracker->TasksAllowedToRun() &&
                                 !g_task_tracker->IsShutdownComplete();

    // DisallowRunTasks typically yields back if it fails to reach quiescence
    // within 1ms. This is typically done to let the main thread run tasks that
    // could potentially be blocking main thread tasks. In this case however,
    // main thread making progress while installing the fence would be more
    // surprising. So allow more time but report errors after a while.
    while (previously_allowed_to_run_ &&
           !g_task_tracker->DisallowRunTasks(Seconds(5))) {
      LOG(WARNING) << "Installing ParallelExecutionFence is slow because of "
                      "these running tasks:\n"
                   << g_task_tracker->DescribeRunningTasks()
                   << "\nParallelExecutionFence requested by:\n"
                   << debug::StackTrace();
    }
  } else if (ThreadPoolInstance::Get()) {
    LOG(WARNING)
        << "ParallelExecutionFence is ineffective when ThreadPoolInstance is "
           "not managed by a TaskEnvironment.\n"
           "Test fixtures should use a TaskEnvironment member or statically "
           "invoke TaskEnvironment::CreateThreadPool() + "
           "ThreadPoolInstance::Get()->StartWithDefaultParams() when the "
           "former is not possible.";
  }
}

TaskEnvironment::ParallelExecutionFence::~ParallelExecutionFence() {
  if (previously_allowed_to_run_)
    g_task_tracker->AllowRunTasks();
}

TaskEnvironment::TestTaskTracker::TestTaskTracker()
    : can_run_tasks_cv_(&lock_), task_completed_cv_(&lock_) {
  // Consider threads blocked on these as idle (avoids instantiating
  // ScopedBlockingCalls and confusing some //base internals tests).
  can_run_tasks_cv_.declare_only_used_while_idle();
  task_completed_cv_.declare_only_used_while_idle();
}

bool TaskEnvironment::TestTaskTracker::AllowRunTasks() {
  AutoLock auto_lock(lock_);
  const bool could_run_tasks = can_run_tasks_;
  can_run_tasks_ = true;
  can_run_tasks_cv_.Broadcast();
  return could_run_tasks;
}

bool TaskEnvironment::TestTaskTracker::TasksAllowedToRun() const {
  AutoLock auto_lock(lock_);
  return can_run_tasks_;
}

bool TaskEnvironment::TestTaskTracker::DisallowRunTasks(TimeDelta timeout) {
  // Disallowing task running should only be done from the main thread to avoid
  // racing with shutdown.
  DCHECK(OnControllerThread());

  AutoLock auto_lock(lock_);

  // Can't disallow run task if there are tasks running.
  for (TimeTicks now = subtle::TimeTicksNowIgnoringOverride(),
                 end = now + timeout;
       !running_tasks_.empty() && now < end;
       now = subtle::TimeTicksNowIgnoringOverride()) {
    task_completed_cv_.TimedWait(end - now);
  }
  // Timed out waiting for running tasks, yield to caller.
  if (!running_tasks_.empty()) {
    // This condition should never be sought after shutdown and this call
    // shouldn't be racing shutdown either per the above `OnControllerThread()`
    // contract.
    DCHECK(!IsShutdownComplete());
    return false;
  }

  can_run_tasks_ = false;
  return true;
}

void TaskEnvironment::TestTaskTracker::RunTask(internal::Task task,
                                               internal::TaskSource* sequence,
                                               const TaskTraits& traits) {
  int task_number;
  {
    AutoLock auto_lock(lock_);

    while (!can_run_tasks_)
      can_run_tasks_cv_.Wait();

    task_number = next_task_number_++;
    auto pair = running_tasks_.emplace(task_number, task.posted_from);
    CHECK(pair.second);  // If false, the |task_number| was already present.
  }

  {
    // Using TimeTicksNowIgnoringOverride() because in tests that mock time,
    // Now() can advance very far very fast, and that's not a problem. This is
    // watching for tests that have actually long running tasks which cause our
    // test suites to run slowly.
    base::TimeTicks before = base::subtle::TimeTicksNowIgnoringOverride();
    const Location posted_from = task.posted_from;
    internal::ThreadPoolImpl::TaskTrackerImpl::RunTask(std::move(task),
                                                       sequence, traits);
    base::TimeTicks after = base::subtle::TimeTicksNowIgnoringOverride();

    const TimeDelta kTimeout = TestTimeouts::action_max_timeout();
    if ((after - before) > kTimeout) {
      ADD_FAILURE() << "TaskEnvironment: RunTask took more than "
                    << kTimeout.InSeconds() << " seconds. Posted from "
                    << posted_from.ToString();
    }
  }

  {
    AutoLock auto_lock(lock_);
    CHECK(can_run_tasks_);
    size_t found = running_tasks_.erase(task_number);
    CHECK_EQ(1u, found);

    task_completed_cv_.Broadcast();
  }
}

std::string TaskEnvironment::TestTaskTracker::DescribeRunningTasks() const {
  base::flat_map<int64_t, Location> running_tasks_copy;
  {
    AutoLock auto_lock(lock_);
    running_tasks_copy = running_tasks_;
  }
  std::string running_tasks_str = "ThreadPool currently running tasks:";
  if (running_tasks_copy.empty()) {
    running_tasks_str += " none.";
  } else {
    for (auto& pair : running_tasks_copy)
      running_tasks_str += "\n  Task posted from: " + pair.second.ToString();
  }
  return running_tasks_str;
}

void TaskEnvironment::TestTaskTracker::BeginCompleteShutdown(
    base::WaitableEvent& shutdown_event) {
  const TimeDelta kTimeout = TestTimeouts::action_max_timeout();
  if (shutdown_event.TimedWait(kTimeout))
    return;  // All tasks completed in time, yay! Yield back to shutdown.

  // If we had to wait too long for the shutdown tasks to complete, then we
  // should fail the test and report which tasks are currently running.
  std::string failure_tasks = DescribeRunningTasks();

  ADD_FAILURE() << "TaskEnvironment: CompleteShutdown took more than "
                << kTimeout.InSeconds() << " seconds.\n"
                << failure_tasks;
  base::Process::TerminateCurrentProcessImmediately(-1);
}

void TaskEnvironment::TestTaskTracker::AssertFlushForTestingAllowed() {
  AutoLock auto_lock(lock_);
  ASSERT_TRUE(can_run_tasks_)
      << "FlushForTesting() requires ThreadPool tasks to be allowed to run or "
         "it will hang. Note: DisallowRunTasks happens implicitly on-and-off "
         "during TaskEnvironment::RunUntilIdle and main thread tasks running "
         "under it should thus never FlushForTesting().";
}

}  // namespace test
}  // namespace base