path: root/base/threading/sequenced_worker_pool.h

// Copyright (c) 2011 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.

#ifndef BASE_THREADING_SEQUENCED_WORKER_POOL_H_
#define BASE_THREADING_SEQUENCED_WORKER_POOL_H_
#pragma once

#include <string>

#include "base/callback.h"
#include "base/memory/linked_ptr.h"
#include "base/memory/ref_counted.h"
#include "base/tracked_objects.h"
#include "base/base_export.h"

namespace base {

// A worker thread pool that enforces ordering between sets of tasks. It also
// allows you to specify what should happen to your tasks on shutdown.
//
// To enforce ordering, get a unique sequence token from the pool and post all
// tasks you want to order with the token. All tasks with the same token are
// guaranteed to execute serially, though not necessarily on the same thread.
//
// Example:
//   SequencedWorkerPool::SequenceToken token = pool.GetSequenceToken();
//   pool.PostSequencedWorkerTask(token, SequencedWorkerPool::SKIP_ON_SHUTDOWN,
//                                FROM_HERE, base::Bind(...));
//   pool.PostSequencedWorkerTask(token, SequencedWorkerPool::SKIP_ON_SHUTDOWN,
//                                FROM_HERE, base::Bind(...));
//
// You can make named sequence tokens to make it easier to share a token
// across different components.
//
// You can also post tasks to the pool without ordering using PostWorkerTask.
// These will be executed in an unspecified order. The order of execution
// between tasks with different sequence tokens is also unspecified.
//
// This class is designed to be leaked on shutdown to allow the
// CONTINUE_ON_SHUTDOWN behavior to be implemented. To enforce the
// BLOCK_SHUTDOWN behavior, you must call Shutdown() which will wait until
// the necessary tasks have completed.
//
// Implementation note: This does not use a base::WorkerPool since that does
// not enforce shutdown semantics or allow us to specify how many worker
// threads to run. For the typical use case of random background work, we don't
// necessarily want to be super aggressive about creating threads.
class BASE_EXPORT SequencedWorkerPool {
 public:
  // Defines what should happen to a task posted to the worker pool on shutdown.
  enum WorkerShutdown {
    // Tasks posted with this mode which have not run at shutdown will be
    // deleted rather than run, and any tasks with this mode running at
    // shutdown will be ignored (the worker thread will not be joined).
    //
    // This option provides a nice way to post stuff you don't want blocking
    // shutdown. For example, you might be doing a slow DNS lookup and if it's
    // blocked on the OS, you may not want to stop shutdown, since the result
    // doesn't really matter at that point.
    //
    // However, you need to be very careful what you do in your callback when
    // you use this option. Since the thread will continue to run until the OS
    // terminates the process, the app can be in the process of tearing down
    // when you're running. This means any singletons or global objects you
    // use may suddenly become invalid out from under you. For this reason,
    // it's best to use this only for slow but simple operations like the DNS
    // example.
    CONTINUE_ON_SHUTDOWN,

    // Tasks posted with this mode that have not started executing at shutdown
    // will be deleted rather than executed. However, tasks already in progress
    // will be completed.
    SKIP_ON_SHUTDOWN,

    // Tasks posted with this mode will block browser shutdown until they're
    // executed. Since this can have significant performance implications, use
    // sparingly.
    //
    // Generally, this should be used only for user data, for example, a task
    // writing a preference file.
    //
    // If a task is posted during shutdown, it will not get run since the
    // workers may already be stopped. In this case, the post operation will
    // fail (return false) and the task will be deleted.
    BLOCK_SHUTDOWN,
  };

  // Opaque identifier that defines sequencing of tasks posted to the worker
  // pool. See NewSequenceToken().
  class SequenceToken {
   public:
    explicit SequenceToken() : id_(0) {}
    ~SequenceToken() {}

    bool Equals(const SequenceToken& other) const {
      return id_ == other.id_;
    }

   private:
    friend class SequencedWorkerPool;

    SequenceToken(int id) : id_(id) {}

    int id_;
  };

  // Allows tests to perform certain actions.
  class TestingObserver {
   public:
    virtual ~TestingObserver() {}
    virtual void WillWaitForShutdown() = 0;
  };

  // Pass the maximum number of threads (they will be lazily created as needed)
  // and a prefix for the thread name to ad in debugging.
  SequencedWorkerPool(size_t max_threads,
                      const std::string& thread_name_prefix);
  ~SequencedWorkerPool();

  // Returns a unique token that can be used to sequence tasks posted to
  // PostSequencedWorkerTask(). Valid tokens are alwys nonzero.
  SequenceToken GetSequenceToken();

  // Returns the sequence token associated with the given name. Calling this
  // function multiple times with the same string will always produce the
  // same sequence token. If the name has not been used before, a new token
  // will be created.
  SequenceToken GetNamedSequenceToken(const std::string& name);

  // Posts the given task for execution in the worker pool. Tasks posted with
  // this function will execute in an unspecified order on a background thread.
  // Returns true if the task was posted. If your tasks have ordering
  // requirements, see PostSequencedWorkerTask().
  //
  // This class will attempt to delete tasks that aren't run
  // (non-block-shutdown semantics) but can't guarantee that this happens. If
  // all worker threads are busy running CONTINUE_ON_SHUTDOWN tasks, there
  // will be no workers available to delete these tasks. And there may be
  // tasks with the same sequence token behind those CONTINUE_ON_SHUTDOWN
  // tasks. Deleting those tasks before the previous one has completed could
  // cause nondeterministic crashes because the task could be keeping some
  // objects alive which do work in their destructor, which could voilate the
  // assumptions of the running task.
  //
  // The task will be guaranteed to run to completion before shutdown
  // (BLOCK_SHUTDOWN semantics).
  //
  // Returns true if the task was posted successfully. This may fail during
  // shutdown regardless of the specified ShutdownBehavior.
  bool PostWorkerTask(const tracked_objects::Location& from_here,
                      const base::Closure& task);

  // Same as PostWorkerTask but allows specification of the shutdown behavior.
  bool PostWorkerTaskWithShutdownBehavior(
      const tracked_objects::Location& from_here,
      const base::Closure& task,
      WorkerShutdown shutdown_behavior);

  // Like PostWorkerTask above, but provides sequencing semantics. This means
  // that tasks posted with the same sequence token (see GetSequenceToken())
  // are guaranteed to execute in order. This is useful in cases where you're
  // doing operations that may depend on previous ones, like appending to a
  // file.
  //
  // The task will be guaranteed to run to completion before shutdown
  // (BLOCK_SHUTDOWN semantics).
  //
  // Returns true if the task was posted successfully. This may fail during
  // shutdown regardless of the specified ShutdownBehavior.
  bool PostSequencedWorkerTask(SequenceToken sequence_token,
                               const tracked_objects::Location& from_here,
                               const base::Closure& task);

  // Same as PostSequencedWorkerTask but allows specification of the shutdown
  // behavior.
  bool PostSequencedWorkerTaskWithShutdownBehavior(
      SequenceToken sequence_token,
      const tracked_objects::Location& from_here,
      const base::Closure& task,
      WorkerShutdown shutdown_behavior);

  // Implements the worker pool shutdown. This should be called during app
  // shutdown, and will discard/join with appropriate tasks before returning.
  // After this call, subsequent calls to post tasks will fail.
  void Shutdown();

  // Called by tests to set the testing observer. This is NULL by default
  // and ownership of the pointer is kept with the caller.
  void SetTestingObserver(TestingObserver* observer);

 private:
  class Inner;
  class Worker;

  friend class Inner;
  friend class Worker;

  scoped_refptr<Inner> inner_;

  DISALLOW_COPY_AND_ASSIGN(SequencedWorkerPool);
};

}  // namespace base

#endif  // BASE_THREADING_SEQUENCED_WORKER_POOL_H_