path: root/base/threading/sequenced_worker_pool.cc

// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/threading/sequenced_worker_pool.h"

#include <list>
#include <map>
#include <set>
#include <utility>
#include <vector>

#include "base/atomicops.h"
#include "base/callback.h"
#include "base/compiler_specific.h"
#include "base/logging.h"
#include "base/memory/linked_ptr.h"
#include "base/message_loop_proxy.h"
#include "base/metrics/histogram.h"
#include "base/stl_util.h"
#include "base/stringprintf.h"
#include "base/synchronization/condition_variable.h"
#include "base/synchronization/lock.h"
#include "base/threading/platform_thread.h"
#include "base/threading/simple_thread.h"
#include "base/threading/thread_restrictions.h"
#include "base/time.h"
#include "base/tracked_objects.h"

#if defined(OS_MACOSX)
#include "base/mac/scoped_nsautorelease_pool.h"
#endif

namespace base {

namespace {

struct SequencedTask {
  SequencedTask()
      : sequence_token_id(0),
        shutdown_behavior(SequencedWorkerPool::BLOCK_SHUTDOWN) {}

  ~SequencedTask() {}

  int sequence_token_id;
  SequencedWorkerPool::WorkerShutdown shutdown_behavior;
  tracked_objects::Location location;
  Closure task;
};

// SequencedWorkerPoolTaskRunner ---------------------------------------------
// A TaskRunner which posts tasks to a SequencedWorkerPool with a
// fixed ShutdownBehavior.
//
// Note that this class is RefCountedThreadSafe (inherited from TaskRunner).
class SequencedWorkerPoolTaskRunner : public TaskRunner {
 public:
  SequencedWorkerPoolTaskRunner(
      const scoped_refptr<SequencedWorkerPool>& pool,
      SequencedWorkerPool::WorkerShutdown shutdown_behavior);

  // TaskRunner implementation
  virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
                               const Closure& task,
                               TimeDelta delay) OVERRIDE;
  virtual bool RunsTasksOnCurrentThread() const OVERRIDE;

 private:
  virtual ~SequencedWorkerPoolTaskRunner();

  // Helper function for posting a delayed task. Asserts that the delay is
  // zero because non-zero delays are not yet supported.
  bool PostDelayedTaskAssertZeroDelay(
      const tracked_objects::Location& from_here,
      const Closure& task,
      TimeDelta delay);

  const scoped_refptr<SequencedWorkerPool> pool_;

  const SequencedWorkerPool::WorkerShutdown shutdown_behavior_;

  DISALLOW_COPY_AND_ASSIGN(SequencedWorkerPoolTaskRunner);
};

SequencedWorkerPoolTaskRunner::SequencedWorkerPoolTaskRunner(
    const scoped_refptr<SequencedWorkerPool>& pool,
    SequencedWorkerPool::WorkerShutdown shutdown_behavior)
    : pool_(pool),
      shutdown_behavior_(shutdown_behavior) {
}

SequencedWorkerPoolTaskRunner::~SequencedWorkerPoolTaskRunner() {
}

bool SequencedWorkerPoolTaskRunner::PostDelayedTask(
    const tracked_objects::Location& from_here,
    const Closure& task,
    TimeDelta delay) {
  return PostDelayedTaskAssertZeroDelay(from_here, task, delay);
}

bool SequencedWorkerPoolTaskRunner::RunsTasksOnCurrentThread() const {
  return pool_->RunsTasksOnCurrentThread();
}

bool SequencedWorkerPoolTaskRunner::PostDelayedTaskAssertZeroDelay(
    const tracked_objects::Location& from_here,
    const Closure& task,
    TimeDelta delay) {
  // TODO(francoisk777@gmail.com): Change the following two statements once
  // SequencedWorkerPool supports non-zero delays.
  DCHECK_EQ(delay.InMillisecondsRoundedUp(), 0)
      << "SequencedWorkerPoolTaskRunner does not yet support non-zero delays";
  return pool_->PostWorkerTaskWithShutdownBehavior(
      from_here, task, shutdown_behavior_);
}

// SequencedWorkerPoolSequencedTaskRunner ------------------------------------
// A SequencedTaskRunner which posts tasks to a SequencedWorkerPool with a
// fixed sequence token.
//
// Note that this class is RefCountedThreadSafe (inherited from TaskRunner).
class SequencedWorkerPoolSequencedTaskRunner : public SequencedTaskRunner {
 public:
  SequencedWorkerPoolSequencedTaskRunner(
      const scoped_refptr<SequencedWorkerPool>& pool,
      SequencedWorkerPool::SequenceToken token,
      SequencedWorkerPool::WorkerShutdown shutdown_behavior);

  // TaskRunner implementation
  virtual bool PostDelayedTask(const tracked_objects::Location& from_here,
                               const Closure& task,
                               TimeDelta delay) OVERRIDE;
  virtual bool RunsTasksOnCurrentThread() const OVERRIDE;

  // SequencedTaskRunner implementation
  virtual bool PostNonNestableDelayedTask(
      const tracked_objects::Location& from_here,
      const Closure& task,
      TimeDelta delay) OVERRIDE;

 private:
  virtual ~SequencedWorkerPoolSequencedTaskRunner();

  // Helper function for posting a delayed task. Asserts that the delay is
  // zero because non-zero delays are not yet supported.
  bool PostDelayedTaskAssertZeroDelay(
      const tracked_objects::Location& from_here,
      const Closure& task,
      TimeDelta delay);

  const scoped_refptr<SequencedWorkerPool> pool_;

  const SequencedWorkerPool::SequenceToken token_;

  const SequencedWorkerPool::WorkerShutdown shutdown_behavior_;

  DISALLOW_COPY_AND_ASSIGN(SequencedWorkerPoolSequencedTaskRunner);
};

SequencedWorkerPoolSequencedTaskRunner::SequencedWorkerPoolSequencedTaskRunner(
    const scoped_refptr<SequencedWorkerPool>& pool,
    SequencedWorkerPool::SequenceToken token,
    SequencedWorkerPool::WorkerShutdown shutdown_behavior)
    : pool_(pool),
      token_(token),
      shutdown_behavior_(shutdown_behavior) {
}

SequencedWorkerPoolSequencedTaskRunner::
~SequencedWorkerPoolSequencedTaskRunner() {
}

bool SequencedWorkerPoolSequencedTaskRunner::PostDelayedTask(
    const tracked_objects::Location& from_here,
    const Closure& task,
    TimeDelta delay) {
  return PostDelayedTaskAssertZeroDelay(from_here, task, delay);
}

bool SequencedWorkerPoolSequencedTaskRunner::RunsTasksOnCurrentThread() const {
  return pool_->IsRunningSequenceOnCurrentThread(token_);
}

bool SequencedWorkerPoolSequencedTaskRunner::PostNonNestableDelayedTask(
    const tracked_objects::Location& from_here,
    const Closure& task,
    TimeDelta delay) {
  return PostDelayedTaskAssertZeroDelay(from_here, task, delay);
}

bool SequencedWorkerPoolSequencedTaskRunner::PostDelayedTaskAssertZeroDelay(
    const tracked_objects::Location& from_here,
    const Closure& task,
    TimeDelta delay) {
  // TODO(francoisk777@gmail.com): Change the following two statements once
  // SequencedWorkerPool supports non-zero delays.
  DCHECK_EQ(delay.InMillisecondsRoundedUp(), 0)
      << "SequencedWorkerPoolSequencedTaskRunner does not yet support non-zero"
         " delays";
  return pool_->PostSequencedWorkerTaskWithShutdownBehavior(
      token_, from_here, task, shutdown_behavior_);
}

}  // namespace

// Worker ---------------------------------------------------------------------

class SequencedWorkerPool::Worker : public SimpleThread {
 public:
  // Hold a (cyclic) ref to |worker_pool|, since we want to keep it
  // around as long as we are running.
  Worker(const scoped_refptr<SequencedWorkerPool>& worker_pool,
         int thread_number,
         const std::string& thread_name_prefix);
  virtual ~Worker();

  // SimpleThread implementation. This actually runs the background thread.
  virtual void Run() OVERRIDE;

  void set_running_sequence(SequenceToken token) {
    running_sequence_ = token;
  }

  SequenceToken running_sequence() const {
    return running_sequence_;
  }

 private:
  scoped_refptr<SequencedWorkerPool> worker_pool_;
  SequenceToken running_sequence_;

  DISALLOW_COPY_AND_ASSIGN(Worker);
};

// Inner ----------------------------------------------------------------------

class SequencedWorkerPool::Inner {
 public:
  // Take a raw pointer to |worker| to avoid cycles (since we're owned
  // by it).
  Inner(SequencedWorkerPool* worker_pool, size_t max_threads,
        const std::string& thread_name_prefix,
        TestingObserver* observer);

  ~Inner();

  SequenceToken GetSequenceToken();

  SequenceToken GetNamedSequenceToken(const std::string& name);

  // This function accepts a name and an ID. If the name is null, the
  // token ID is used. This allows us to implement the optional name lookup
  // from a single function without having to enter the lock a separate time.
  bool PostTask(const std::string* optional_token_name,
                SequenceToken sequence_token,
                WorkerShutdown shutdown_behavior,
                const tracked_objects::Location& from_here,
                const Closure& task);

  bool RunsTasksOnCurrentThread() const;

  bool IsRunningSequenceOnCurrentThread(SequenceToken sequence_token) const;

  void FlushForTesting();

  void SignalHasWorkForTesting();

  int GetWorkSignalCountForTesting() const;

  void Shutdown();

  // Runs the worker loop on the background thread.
  void ThreadLoop(Worker* this_worker);

 private:
  // Returns whether there are no more pending tasks and all threads
  // are idle.  Must be called under lock.
  bool IsIdle() const;

  // Called from within the lock, this converts the given token name into a
  // token ID, creating a new one if necessary.
  int LockedGetNamedTokenID(const std::string& name);

  // The calling code should clear the given delete_these_oustide_lock
  // vector the next time the lock is released. See the implementation for
  // a more detailed description.
  bool GetWork(SequencedTask* task,
               std::vector<Closure>* delete_these_outside_lock);

  // Peforms init and cleanup around running the given task. WillRun...
  // returns the value from PrepareToStartAdditionalThreadIfNecessary.
  // The calling code should call FinishStartingAdditionalThread once the
  // lock is released if the return values is nonzero.
  int WillRunWorkerTask(const SequencedTask& task);
  void DidRunWorkerTask(const SequencedTask& task);

  // Returns true if there are no threads currently running the given
  // sequence token.
  bool IsSequenceTokenRunnable(int sequence_token_id) const;

  // Checks if all threads are busy and the addition of one more could run an
  // additional task waiting in the queue. This must be called from within
  // the lock.
  //
  // If another thread is helpful, this will mark the thread as being in the
  // process of starting and returns the index of the new thread which will be
  // 0 or more. The caller should then call FinishStartingAdditionalThread to
  // complete initialization once the lock is released.
  //
  // If another thread is not necessary, returne 0;
  //
  // See the implementedion for more.
  int PrepareToStartAdditionalThreadIfHelpful();

  // The second part of thread creation after
  // PrepareToStartAdditionalThreadIfHelpful with the thread number it
  // generated. This actually creates the thread and should be called outside
  // the lock to avoid blocking important work starting a thread in the lock.
  void FinishStartingAdditionalThread(int thread_number);

  // Signal |has_work_| and increment |has_work_signal_count_|.
  void SignalHasWork();

  // Checks whether there is work left that's blocking shutdown. Must be
  // called inside the lock.
  bool CanShutdown() const;

  SequencedWorkerPool* const worker_pool_;

  // The last sequence number used. Managed by GetSequenceToken, since this
  // only does threadsafe increment operations, you do not need to hold the
  // lock.
  volatile subtle::Atomic32 last_sequence_number_;

  // This lock protects |everything in this class|. Do not read or modify
  // anything without holding this lock. Do not block while holding this
  // lock.
  mutable Lock lock_;

  // Condition variable that is waited on by worker threads until new
  // tasks are posted or shutdown starts.
  ConditionVariable has_work_cv_;

  // Condition variable that is waited on by non-worker threads (in
  // FlushForTesting()) until IsIdle() goes to true.
  ConditionVariable is_idle_cv_;

  // Condition variable that is waited on by non-worker threads (in
  // Shutdown()) until CanShutdown() goes to true.
  ConditionVariable can_shutdown_cv_;

  // The maximum number of worker threads we'll create.
  const size_t max_threads_;

  const std::string thread_name_prefix_;

  // Associates all known sequence token names with their IDs.
  std::map<std::string, int> named_sequence_tokens_;

  // Owning pointers to all threads we've created so far, indexed by
  // ID. Since we lazily create threads, this may be less than
  // max_threads_ and will be initially empty.
  typedef std::map<PlatformThreadId, linked_ptr<Worker> > ThreadMap;
  ThreadMap threads_;

  // Set to true when we're in the process of creating another thread.
  // See PrepareToStartAdditionalThreadIfHelpful for more.
  bool thread_being_created_;

  // Number of threads currently waiting for work.
  size_t waiting_thread_count_;

  // Number of threads currently running tasks that have the BLOCK_SHUTDOWN
  // flag set.
  size_t blocking_shutdown_thread_count_;

  // In-order list of all pending tasks. These are tasks waiting for a thread
  // to run on or that are blocked on a previous task in their sequence.
  //
  // We maintain the pending_task_count_ separately for metrics because
  // list.size() can be linear time.
  std::list<SequencedTask> pending_tasks_;
  size_t pending_task_count_;

  // Number of tasks in the pending_tasks_ list that are marked as blocking
  // shutdown.
  size_t blocking_shutdown_pending_task_count_;

  // Lists all sequence tokens currently executing.
  std::set<int> current_sequences_;

  // Set when Shutdown is called and no further tasks should be
  // allowed, though we may still be running existing tasks.
  bool shutdown_called_;

  TestingObserver* const testing_observer_;

  DISALLOW_COPY_AND_ASSIGN(Inner);
};

// Worker definitions ---------------------------------------------------------

SequencedWorkerPool::Worker::Worker(
    const scoped_refptr<SequencedWorkerPool>& worker_pool,
    int thread_number,
    const std::string& prefix)
    : SimpleThread(
          prefix + StringPrintf("Worker%d", thread_number).c_str()),
      worker_pool_(worker_pool) {
  Start();
}

SequencedWorkerPool::Worker::~Worker() {
}

void SequencedWorkerPool::Worker::Run() {
  // Just jump back to the Inner object to run the thread, since it has all the
  // tracking information and queues. It might be more natural to implement
  // using DelegateSimpleThread and have Inner implement the Delegate to avoid
  // having these worker objects at all, but that method lacks the ability to
  // send thread-specific information easily to the thread loop.
  worker_pool_->inner_->ThreadLoop(this);
  // Release our cyclic reference once we're done.
  worker_pool_ = NULL;
}

// Inner definitions ---------------------------------------------------------

SequencedWorkerPool::Inner::Inner(
    SequencedWorkerPool* worker_pool,
    size_t max_threads,
    const std::string& thread_name_prefix,
    TestingObserver* observer)
    : worker_pool_(worker_pool),
      last_sequence_number_(0),
      lock_(),
      has_work_cv_(&lock_),
      is_idle_cv_(&lock_),
      can_shutdown_cv_(&lock_),
      max_threads_(max_threads),
      thread_name_prefix_(thread_name_prefix),
      thread_being_created_(false),
      waiting_thread_count_(0),
      blocking_shutdown_thread_count_(0),
      pending_task_count_(0),
      blocking_shutdown_pending_task_count_(0),
      shutdown_called_(false),
      testing_observer_(observer) {}

SequencedWorkerPool::Inner::~Inner() {
  // You must call Shutdown() before destroying the pool.
  DCHECK(shutdown_called_);

  // Need to explicitly join with the threads before they're destroyed or else
  // they will be running when our object is half torn down.
  for (ThreadMap::iterator it = threads_.begin(); it != threads_.end(); ++it)
    it->second->Join();
  threads_.clear();

  if (testing_observer_)
    testing_observer_->OnDestruct();
}

SequencedWorkerPool::SequenceToken
SequencedWorkerPool::Inner::GetSequenceToken() {
  subtle::Atomic32 result =
      subtle::NoBarrier_AtomicIncrement(&last_sequence_number_, 1);
  return SequenceToken(static_cast<int>(result));
}

SequencedWorkerPool::SequenceToken
SequencedWorkerPool::Inner::GetNamedSequenceToken(const std::string& name) {
  AutoLock lock(lock_);
  return SequenceToken(LockedGetNamedTokenID(name));
}

bool SequencedWorkerPool::Inner::PostTask(
    const std::string* optional_token_name,
    SequenceToken sequence_token,
    WorkerShutdown shutdown_behavior,
    const tracked_objects::Location& from_here,
    const Closure& task) {
  SequencedTask sequenced;
  sequenced.sequence_token_id = sequence_token.id_;
  sequenced.shutdown_behavior = shutdown_behavior;
  sequenced.location = from_here;
  sequenced.task = task;

  int create_thread_id = 0;
  {
    AutoLock lock(lock_);
    if (shutdown_called_)
      return false;

    // Now that we have the lock, apply the named token rules.
    if (optional_token_name)
      sequenced.sequence_token_id = LockedGetNamedTokenID(*optional_token_name);

    pending_tasks_.push_back(sequenced);
    pending_task_count_++;
    if (shutdown_behavior == BLOCK_SHUTDOWN)
      blocking_shutdown_pending_task_count_++;

    create_thread_id = PrepareToStartAdditionalThreadIfHelpful();
  }

  // Actually start the additional thread or signal an existing one now that
  // we're outside the lock.
  if (create_thread_id)
    FinishStartingAdditionalThread(create_thread_id);
  else
    SignalHasWork();

  return true;
}

bool SequencedWorkerPool::Inner::RunsTasksOnCurrentThread() const {
  AutoLock lock(lock_);
  return ContainsKey(threads_, PlatformThread::CurrentId());
}

bool SequencedWorkerPool::Inner::IsRunningSequenceOnCurrentThread(
    SequenceToken sequence_token) const {
  AutoLock lock(lock_);
  ThreadMap::const_iterator found = threads_.find(PlatformThread::CurrentId());
  if (found == threads_.end())
    return false;
  return found->second->running_sequence().Equals(sequence_token);
}

void SequencedWorkerPool::Inner::FlushForTesting() {
  AutoLock lock(lock_);
  while (!IsIdle())
    is_idle_cv_.Wait();
}

void SequencedWorkerPool::Inner::SignalHasWorkForTesting() {
  SignalHasWork();
}

void SequencedWorkerPool::Inner::Shutdown() {
  // Mark us as terminated and go through and drop all tasks that aren't
  // required to run on shutdown. Since no new tasks will get posted once the
  // terminated flag is set, this ensures that all remaining tasks are required
  // for shutdown whenever the termianted_ flag is set.
  {
    AutoLock lock(lock_);

    if (shutdown_called_)
      return;
    shutdown_called_ = true;

    // Tickle the threads. This will wake up a waiting one so it will know that
    // it can exit, which in turn will wake up any other waiting ones.
    SignalHasWork();

    // There are no pending or running tasks blocking shutdown, we're done.
    if (CanShutdown())
      return;
  }

  // If we're here, then something is blocking shutdown.  So wait for
  // CanShutdown() to go to true.

  if (testing_observer_)
    testing_observer_->WillWaitForShutdown();

  TimeTicks shutdown_wait_begin = TimeTicks::Now();

  {
    base::ThreadRestrictions::ScopedAllowWait allow_wait;
    AutoLock lock(lock_);
    while (!CanShutdown())
      can_shutdown_cv_.Wait();
  }
  UMA_HISTOGRAM_TIMES("SequencedWorkerPool.ShutdownDelayTime",
                      TimeTicks::Now() - shutdown_wait_begin);
}

void SequencedWorkerPool::Inner::ThreadLoop(Worker* this_worker) {
  {
    AutoLock lock(lock_);
    DCHECK(thread_being_created_);
    thread_being_created_ = false;
    std::pair<ThreadMap::iterator, bool> result =
        threads_.insert(
            std::make_pair(this_worker->tid(), make_linked_ptr(this_worker)));
    DCHECK(result.second);

    while (true) {
#if defined(OS_MACOSX)
      base::mac::ScopedNSAutoreleasePool autorelease_pool;
#endif

      // See GetWork for what delete_these_outside_lock is doing.
      SequencedTask task;
      std::vector<Closure> delete_these_outside_lock;
      if (GetWork(&task, &delete_these_outside_lock)) {
        int new_thread_id = WillRunWorkerTask(task);
        {
          AutoUnlock unlock(lock_);
          // There may be more work available, so wake up another
          // worker thread. (Technically not required, since we
          // already get a signal for each new task, but it doesn't
          // hurt.)
          SignalHasWork();
          delete_these_outside_lock.clear();

          // Complete thread creation outside the lock if necessary.
          if (new_thread_id)
            FinishStartingAdditionalThread(new_thread_id);

          this_worker->set_running_sequence(
              SequenceToken(task.sequence_token_id));

          task.task.Run();

          this_worker->set_running_sequence(SequenceToken());

          // Make sure our task is erased outside the lock for the same reason
          // we do this with delete_these_oustide_lock.
          task.task = Closure();
        }
        DidRunWorkerTask(task);  // Must be done inside the lock.
      } else {
        // When we're terminating and there's no more work, we can
        // shut down.  You can't get more tasks posted once
        // shutdown_called_ is set. There may be some tasks stuck
        // behind running ones with the same sequence token, but
        // additional threads won't help this case.
        if (shutdown_called_)
          break;
        waiting_thread_count_++;
        // This is the only time that IsIdle() can go to true.
        if (IsIdle())
          is_idle_cv_.Signal();
        has_work_cv_.Wait();
        waiting_thread_count_--;
      }
    }
  }  // Release lock_.

  // We noticed we should exit. Wake up the next worker so it knows it should
  // exit as well (because the Shutdown() code only signals once).
  SignalHasWork();

  // Possibly unblock shutdown.
  can_shutdown_cv_.Signal();
}

bool SequencedWorkerPool::Inner::IsIdle() const {
  lock_.AssertAcquired();
  return pending_task_count_ == 0 && waiting_thread_count_ == threads_.size();
}

int SequencedWorkerPool::Inner::LockedGetNamedTokenID(
    const std::string& name) {
  lock_.AssertAcquired();
  DCHECK(!name.empty());

  std::map<std::string, int>::const_iterator found =
      named_sequence_tokens_.find(name);
  if (found != named_sequence_tokens_.end())
    return found->second;  // Got an existing one.

  // Create a new one for this name.
  SequenceToken result = GetSequenceToken();
  named_sequence_tokens_.insert(std::make_pair(name, result.id_));
  return result.id_;
}

bool SequencedWorkerPool::Inner::GetWork(
    SequencedTask* task,
    std::vector<Closure>* delete_these_outside_lock) {
  lock_.AssertAcquired();

  DCHECK_EQ(pending_tasks_.size(), pending_task_count_);
  UMA_HISTOGRAM_COUNTS_100("SequencedWorkerPool.TaskCount",
                           static_cast<int>(pending_task_count_));

  // Find the next task with a sequence token that's not currently in use.
  // If the token is in use, that means another thread is running something
  // in that sequence, and we can't run it without going out-of-order.
  //
  // This algorithm is simple and fair, but inefficient in some cases. For
  // example, say somebody schedules 1000 slow tasks with the same sequence
  // number. We'll have to go through all those tasks each time we feel like
  // there might be work to schedule. If this proves to be a problem, we
  // should make this more efficient.
  //
  // One possible enhancement would be to keep a map from sequence ID to a
  // list of pending but currently blocked SequencedTasks for that ID.
  // When a worker finishes a task of one sequence token, it can pick up the
  // next one from that token right away.
  //
  // This may lead to starvation if there are sufficient numbers of sequences
  // in use. To alleviate this, we could add an incrementing priority counter
  // to each SequencedTask. Then maintain a priority_queue of all runnable
  // tasks, sorted by priority counter. When a sequenced task is completed
  // we would pop the head element off of that tasks pending list and add it
  // to the priority queue. Then we would run the first item in the priority
  // queue.
  bool found_task = false;
  int unrunnable_tasks = 0;
  std::list<SequencedTask>::iterator i = pending_tasks_.begin();
  while (i != pending_tasks_.end()) {
    if (!IsSequenceTokenRunnable(i->sequence_token_id)) {
      unrunnable_tasks++;
      ++i;
      continue;
    }

    if (shutdown_called_ && i->shutdown_behavior != BLOCK_SHUTDOWN) {
      // We're shutting down and the task we just found isn't blocking
      // shutdown. Delete it and get more work.
      //
      // Note that we do not want to delete unrunnable tasks. Deleting a task
      // can have side effects (like freeing some objects) and deleting a
      // task that's supposed to run after one that's currently running could
      // cause an obscure crash.
      //
      // We really want to delete these tasks outside the lock in case the
      // closures are holding refs to objects that want to post work from
      // their destructorss (which would deadlock). The closures are
      // internally refcounted, so we just need to keep a copy of them alive
      // until the lock is exited. The calling code can just clear() the
      // vector they passed to us once the lock is exited to make this
      // happen.
      delete_these_outside_lock->push_back(i->task);
      i = pending_tasks_.erase(i);
      pending_task_count_--;
    } else {
      // Found a runnable task.
      *task = *i;
      i = pending_tasks_.erase(i);
      pending_task_count_--;
      if (task->shutdown_behavior == BLOCK_SHUTDOWN) {
        blocking_shutdown_pending_task_count_--;
      }

      found_task = true;
      break;
    }
  }

  // Track the number of tasks we had to skip over to see if we should be
  // making this more efficient. If this number ever becomes large or is
  // frequently "some", we should consider the optimization above.
  UMA_HISTOGRAM_COUNTS_100("SequencedWorkerPool.UnrunnableTaskCount",
                           unrunnable_tasks);
  return found_task;
}

int SequencedWorkerPool::Inner::WillRunWorkerTask(const SequencedTask& task) {
  lock_.AssertAcquired();

  // Mark the task's sequence number as in use.
  if (task.sequence_token_id)
    current_sequences_.insert(task.sequence_token_id);

  if (task.shutdown_behavior == BLOCK_SHUTDOWN)
    blocking_shutdown_thread_count_++;

  // We just picked up a task. Since StartAdditionalThreadIfHelpful only
  // creates a new thread if there is no free one, there is a race when posting
  // tasks that many tasks could have been posted before a thread started
  // running them, so only one thread would have been created. So we also check
  // whether we should create more threads after removing our task from the
  // queue, which also has the nice side effect of creating the workers from
  // background threads rather than the main thread of the app.
  //
  // If another thread wasn't created, we want to wake up an existing thread
  // if there is one waiting to pick up the next task.
  //
  // Note that we really need to do this *before* running the task, not
  // after. Otherwise, if more than one task is posted, the creation of the
  // second thread (since we only create one at a time) will be blocked by
  // the execution of the first task, which could be arbitrarily long.
  return PrepareToStartAdditionalThreadIfHelpful();
}

void SequencedWorkerPool::Inner::DidRunWorkerTask(const SequencedTask& task) {
  lock_.AssertAcquired();

  if (task.shutdown_behavior == BLOCK_SHUTDOWN) {
    DCHECK_GT(blocking_shutdown_thread_count_, 0u);
    blocking_shutdown_thread_count_--;
  }

  if (task.sequence_token_id)
    current_sequences_.erase(task.sequence_token_id);
}

bool SequencedWorkerPool::Inner::IsSequenceTokenRunnable(
    int sequence_token_id) const {
  lock_.AssertAcquired();
  return !sequence_token_id ||
      current_sequences_.find(sequence_token_id) ==
          current_sequences_.end();
}

int SequencedWorkerPool::Inner::PrepareToStartAdditionalThreadIfHelpful() {
  lock_.AssertAcquired();
  // How thread creation works:
  //
  // We'de like to avoid creating threads with the lock held. However, we
  // need to be sure that we have an accurate accounting of the threads for
  // proper Joining and deltion on shutdown.
  //
  // We need to figure out if we need another thread with the lock held, which
  // is what this function does. It then marks us as in the process of creating
  // a thread. When we do shutdown, we wait until the thread_being_created_
  // flag is cleared, which ensures that the new thread is properly added to
  // all the data structures and we can't leak it. Once shutdown starts, we'll
  // refuse to create more threads or they would be leaked.
  //
  // Note that this creates a mostly benign race condition on shutdown that
  // will cause fewer workers to be created than one would expect. It isn't
  // much of an issue in real life, but affects some tests. Since we only spawn
  // one worker at a time, the following sequence of events can happen:
  //
  //  1. Main thread posts a bunch of unrelated tasks that would normally be
  //     run on separate threads.
  //  2. The first task post causes us to start a worker. Other tasks do not
  //     cause a worker to start since one is pending.
  //  3. Main thread initiates shutdown.
  //  4. No more threads are created since the shutdown_called_ flag is set.
  //
  // The result is that one may expect that max_threads_ workers to be created
  // given the workload, but in reality fewer may be created because the
  // sequence of thread creation on the background threads is racing with the
  // shutdown call.
  if (!shutdown_called_ &&
      !thread_being_created_ &&
      threads_.size() < max_threads_ &&
      waiting_thread_count_ == 0) {
    // We could use an additional thread if there's work to be done.
    for (std::list<SequencedTask>::iterator i = pending_tasks_.begin();
         i != pending_tasks_.end(); ++i) {
      if (IsSequenceTokenRunnable(i->sequence_token_id)) {
        // Found a runnable task, mark the thread as being started.
        thread_being_created_ = true;
        return static_cast<int>(threads_.size() + 1);
      }
    }
  }
  return 0;
}

void SequencedWorkerPool::Inner::FinishStartingAdditionalThread(
    int thread_number) {
  // Called outside of the lock.
  DCHECK(thread_number > 0);

  // The worker is assigned to the list when the thread actually starts, which
  // will manage the memory of the pointer.
  new Worker(worker_pool_, thread_number, thread_name_prefix_);
}

void SequencedWorkerPool::Inner::SignalHasWork() {
  has_work_cv_.Signal();
  if (testing_observer_) {
    testing_observer_->OnHasWork();
  }
}

bool SequencedWorkerPool::Inner::CanShutdown() const {
  lock_.AssertAcquired();
  // See PrepareToStartAdditionalThreadIfHelpful for how thread creation works.
  return !thread_being_created_ &&
         blocking_shutdown_thread_count_ == 0 &&
         blocking_shutdown_pending_task_count_ == 0;
}

// SequencedWorkerPool --------------------------------------------------------

SequencedWorkerPool::SequencedWorkerPool(
    size_t max_threads,
    const std::string& thread_name_prefix)
    : constructor_message_loop_(MessageLoopProxy::current()),
      inner_(new Inner(ALLOW_THIS_IN_INITIALIZER_LIST(this),
                       max_threads, thread_name_prefix, NULL)) {
}

SequencedWorkerPool::SequencedWorkerPool(
    size_t max_threads,
    const std::string& thread_name_prefix,
    TestingObserver* observer)
    : constructor_message_loop_(MessageLoopProxy::current()),
      inner_(new Inner(ALLOW_THIS_IN_INITIALIZER_LIST(this),
                       max_threads, thread_name_prefix, observer)) {
}

SequencedWorkerPool::~SequencedWorkerPool() {}

void SequencedWorkerPool::OnDestruct() const {
  DCHECK(constructor_message_loop_.get());
  // Avoid deleting ourselves on a worker thread (which would
  // deadlock).
  if (RunsTasksOnCurrentThread()) {
    constructor_message_loop_->DeleteSoon(FROM_HERE, this);
  } else {
    delete this;
  }
}

SequencedWorkerPool::SequenceToken SequencedWorkerPool::GetSequenceToken() {
  return inner_->GetSequenceToken();
}

SequencedWorkerPool::SequenceToken SequencedWorkerPool::GetNamedSequenceToken(
    const std::string& name) {
  return inner_->GetNamedSequenceToken(name);
}

scoped_refptr<SequencedTaskRunner> SequencedWorkerPool::GetSequencedTaskRunner(
    SequenceToken token) {
  return GetSequencedTaskRunnerWithShutdownBehavior(token, BLOCK_SHUTDOWN);
}

scoped_refptr<SequencedTaskRunner>
SequencedWorkerPool::GetSequencedTaskRunnerWithShutdownBehavior(
    SequenceToken token, WorkerShutdown shutdown_behavior) {
  return new SequencedWorkerPoolSequencedTaskRunner(
      this, token, shutdown_behavior);
}

scoped_refptr<TaskRunner>
SequencedWorkerPool::GetTaskRunnerWithShutdownBehavior(
    WorkerShutdown shutdown_behavior) {
  return new SequencedWorkerPoolTaskRunner(this, shutdown_behavior);
}

bool SequencedWorkerPool::PostWorkerTask(
    const tracked_objects::Location& from_here,
    const Closure& task) {
  return inner_->PostTask(NULL, SequenceToken(), BLOCK_SHUTDOWN,
                          from_here, task);
}

bool SequencedWorkerPool::PostWorkerTaskWithShutdownBehavior(
    const tracked_objects::Location& from_here,
    const Closure& task,
    WorkerShutdown shutdown_behavior) {
  return inner_->PostTask(NULL, SequenceToken(), shutdown_behavior,
                          from_here, task);
}

bool SequencedWorkerPool::PostSequencedWorkerTask(
    SequenceToken sequence_token,
    const tracked_objects::Location& from_here,
    const Closure& task) {
  return inner_->PostTask(NULL, sequence_token, BLOCK_SHUTDOWN,
                          from_here, task);
}

bool SequencedWorkerPool::PostNamedSequencedWorkerTask(
    const std::string& token_name,
    const tracked_objects::Location& from_here,
    const Closure& task) {
  DCHECK(!token_name.empty());
  return inner_->PostTask(&token_name, SequenceToken(), BLOCK_SHUTDOWN,
                          from_here, task);
}

bool SequencedWorkerPool::PostSequencedWorkerTaskWithShutdownBehavior(
    SequenceToken sequence_token,
    const tracked_objects::Location& from_here,
    const Closure& task,
    WorkerShutdown shutdown_behavior) {
  return inner_->PostTask(NULL, sequence_token, shutdown_behavior,
                          from_here, task);
}

bool SequencedWorkerPool::PostDelayedTask(
    const tracked_objects::Location& from_here,
    const Closure& task,
    TimeDelta delay) {
  // TODO(akalin): Add support for non-zero delays.
  DCHECK_EQ(delay.InMillisecondsRoundedUp(), 0);
  return PostWorkerTask(from_here, task);
}

bool SequencedWorkerPool::RunsTasksOnCurrentThread() const {
  return inner_->RunsTasksOnCurrentThread();
}

bool SequencedWorkerPool::IsRunningSequenceOnCurrentThread(
    SequenceToken sequence_token) const {
  return inner_->IsRunningSequenceOnCurrentThread(sequence_token);
}

void SequencedWorkerPool::FlushForTesting() {
  inner_->FlushForTesting();
}

void SequencedWorkerPool::SignalHasWorkForTesting() {
  inner_->SignalHasWorkForTesting();
}

void SequencedWorkerPool::Shutdown() {
  DCHECK(constructor_message_loop_->BelongsToCurrentThread());
  inner_->Shutdown();
}

}  // namespace base