在chromium中如何创建线程

base::Thread thread("test");
thread.Start();
//thread.StartWithOptions(base::Thread::Options(base::MessageLoop::TYPE_IO, 0));

创建线程对象后，可以调用Start，或者StartWithOptions指定创建线程的一些选项。

Thread::Thread(const std::string& name)
    : id_event_(WaitableEvent::ResetPolicy::MANUAL,
                WaitableEvent::InitialState::NOT_SIGNALED),
      name_(name),
      start_event_(WaitableEvent::ResetPolicy::MANUAL,
                   WaitableEvent::InitialState::NOT_SIGNALED) {
}

Thread的构造函数里创建了一个id_event，和start_event的WaitableEvent对象，同时把name赋值给name_变量。

bool Thread::Start() {
  DCHECK(owning_sequence_checker_.CalledOnValidSequence());

  Options options;
#if defined(OS_WIN)
  if (com_status_ == STA)
    options.message_loop_type = MessageLoop::TYPE_UI;
#endif
  return StartWithOptions(options);
}

调用Start会创建一个默认Option，调用StartWithOptions.

bool Thread::StartWithOptions(const Options& options) {
  // Reset |id_| here to support restarting the thread.
  id_event_.Reset();
  id_ = kInvalidThreadId;

  SetThreadWasQuitProperly(false);

  MessageLoop::Type type = options.message_loop_type;
  if (!options.message_pump_factory.is_null())
    type = MessageLoop::TYPE_CUSTOM;

  message_loop_timer_slack_ = options.timer_slack;
  std::unique_ptr<MessageLoop> message_loop_owned =
      MessageLoop::CreateUnbound(type, options.message_pump_factory);
  message_loop_ = message_loop_owned.get();
  start_event_.Reset();

  // Hold |thread_lock_| while starting the new thread to synchronize with
  // Stop() while it's not guaranteed to be sequenced (until crbug/629139 is
  // fixed).
  {
    AutoLock lock(thread_lock_);
    bool success =
        options.joinable
            ? PlatformThread::CreateWithPriority(options.stack_size, this,
                                                 &thread_, options.priority)
            : PlatformThread::CreateNonJoinableWithPriority(
                  options.stack_size, this, options.priority);
    if (!success) {
      DLOG(ERROR) << "failed to create thread";
      message_loop_ = nullptr;
      return false;
    }
  }

  joinable_ = options.joinable;

  // The ownership of |message_loop_| is managed by the newly created thread
  // within the ThreadMain.
  ignore_result(message_loop_owned.release());

  DCHECK(message_loop_);
  return true;
}

StartWithOptions中做了两件事，1.调用MessageLoop::CreateUnbound，2.调用PlatformThread::CreateWithPriority。

1.创建MessageLoop

std::unique_ptr<MessageLoop> MessageLoop::CreateUnbound(
    Type type,
    MessagePumpFactoryCallback pump_factory) {
  return WrapUnique(new MessageLoop(type, std::move(pump_factory)));
}

MessageLoop::MessageLoop(Type type, MessagePumpFactoryCallback pump_factory)
    : MessageLoopCurrent(this),
      type_(type),
      pump_factory_(std::move(pump_factory)),
      incoming_task_queue_(new internal::IncomingTaskQueue(this)),
      unbound_task_runner_(
          new internal::MessageLoopTaskRunner(incoming_task_queue_)),
      task_runner_(unbound_task_runner_) {
}

MessageLoop::CreateUnbound静态函数，创建一个MessageLoop，MessageLoop中含有一个IncomingTaskQueue，MessageLoopTaskRunner。

2.调用PlatformThread::CreateWithPriority

这部分是由各个平台各自实现的, Posix平台的实现如下

// static
bool PlatformThread::CreateWithPriority(size_t stack_size, Delegate* delegate,
                                        PlatformThreadHandle* thread_handle,
                                        ThreadPriority priority) {
  return CreateThread(stack_size, true /* joinable thread */, delegate,
                      thread_handle, priority);
}

bool CreateThread(size_t stack_size,
                  bool joinable,
                  PlatformThread::Delegate* delegate,
                  PlatformThreadHandle* thread_handle,
                  ThreadPriority priority) {
  base::InitThreading();

  pthread_attr_t attributes;
  pthread_attr_init(&attributes);

  // Pthreads are joinable by default, so only specify the detached
  // attribute if the thread should be non-joinable.
  if (!joinable)
    pthread_attr_setdetachstate(&attributes, PTHREAD_CREATE_DETACHED);

  // Get a better default if available.
  if (stack_size == 0)
    stack_size = base::GetDefaultThreadStackSize(attributes);

  if (stack_size > 0)
    pthread_attr_setstacksize(&attributes, stack_size);

  std::unique_ptr<ThreadParams> params(new ThreadParams);
  params->delegate = delegate;
  params->joinable = joinable;
  params->priority = priority;

  pthread_t handle;
  int err = pthread_create(&handle, &attributes, ThreadFunc, params.get());
  bool success = !err;
  if (success) {
    // ThreadParams should be deleted on the created thread after used.
    ignore_result(params.release());
  } else {
    // Value of |handle| is undefined if pthread_create fails.
    handle = 0;
    errno = err;
    PLOG(ERROR) << "pthread_create";
  }
  *thread_handle = PlatformThreadHandle(handle);

  pthread_attr_destroy(&attributes);

  return success;
}

调用posix平台的pthread_create创建一个线程，然后调用ThreadFunc

void* ThreadFunc(void* params) {
  PlatformThread::Delegate* delegate = nullptr;

  {
    std::unique_ptr<ThreadParams> thread_params(
        static_cast<ThreadParams*>(params));

    delegate = thread_params->delegate;
    if (!thread_params->joinable)
      base::ThreadRestrictions::SetSingletonAllowed(false);

#if !defined(OS_NACL)
    // Threads on linux/android may inherit their priority from the thread
    // where they were created. This explicitly sets the priority of all new
    // threads.
    PlatformThread::SetCurrentThreadPriority(thread_params->priority);
#endif
  }

  ThreadIdNameManager::GetInstance()->RegisterThread(
      PlatformThread::CurrentHandle().platform_handle(),
      PlatformThread::CurrentId());

  delegate->ThreadMain();

  ThreadIdNameManager::GetInstance()->RemoveName(
      PlatformThread::CurrentHandle().platform_handle(),
      PlatformThread::CurrentId());

  base::TerminateOnThread();
  return nullptr;
}

delegate为Thread, 会在线程中调用Thread::ThreadMain，退出这个方法后会调用base::TerminateOnThread()

void Thread::ThreadMain() {
  id_event_.Signal();

  // Complete the initialization of our Thread object.
  PlatformThread::SetName(name_.c_str());
  ANNOTATE_THREAD_NAME(name_.c_str());  // Tell the name to race detector.

  // Lazily initialize the |message_loop| so that it can run on this thread.
  DCHECK(message_loop_);
  std::unique_ptr<MessageLoop> message_loop(message_loop_);
  message_loop_->BindToCurrentThread();
  message_loop_->SetTimerSlack(message_loop_timer_slack_);

#if defined(OS_POSIX) && !defined(OS_NACL)
  // Allow threads running a MessageLoopForIO to use FileDescriptorWatcher API.
  std::unique_ptr<FileDescriptorWatcher> file_descriptor_watcher;
  if (MessageLoopForIO::IsCurrent()) {
    file_descriptor_watcher.reset(new FileDescriptorWatcher(
        static_cast<MessageLoopForIO*>(message_loop_)));
  }
#endif

  // Let the thread do extra initialization.
  Init();

  {
    AutoLock lock(running_lock_);
    running_ = true;
  }

  start_event_.Signal();

  RunLoop run_loop;
  run_loop_ = &run_loop;
  Run(run_loop_);

  {
    AutoLock lock(running_lock_);
    running_ = false;
  }

  // Let the thread do extra cleanup.
  CleanUp();

  // We can't receive messages anymore.
  // (The message loop is destructed at the end of this block)
  message_loop_ = nullptr;
  run_loop_ = nullptr;
}

ThreadMain中会调用MessageLoop的bindToCurrentThread

void MessageLoop::BindToCurrentThread() {
  DCHECK_CALLED_ON_VALID_THREAD(bound_thread_checker_);

  DCHECK(!pump_);
  if (!pump_factory_.is_null())
    pump_ = std::move(pump_factory_).Run();
  else
    pump_ = CreateMessagePumpForType(type_);

  DCHECK(!MessageLoopCurrent::IsSet())
      << "should only have one message loop per thread";
  MessageLoopCurrent::BindToCurrentThreadInternal(this);

  incoming_task_queue_->StartScheduling();
  unbound_task_runner_->BindToCurrentThread();
  unbound_task_runner_ = nullptr;
  SetThreadTaskRunnerHandle();
  thread_id_ = PlatformThread::CurrentId();

  scoped_set_sequence_local_storage_map_for_current_thread_ = std::make_unique<
      internal::ScopedSetSequenceLocalStorageMapForCurrentThread>(
      &sequence_local_storage_map_);

  RunLoop::RegisterDelegateForCurrentThread(this);
}

其中会创建MessageLoopPump，IncomingTaskQueue::StartScheduing, TaskRunner::BindToCurrentThread。

ThreadMain中还会调用Run(RunLoop* run_loop)

void Thread::Run(RunLoop* run_loop) {
  // Overridable protected method to be called from our |thread_| only.
  DCHECK(id_event_.IsSignaled());
  DCHECK_EQ(id_, PlatformThread::CurrentId());

  run_loop->Run();
}

void RunLoop::Run() {
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);

  if (!BeforeRun())
    return;

  const bool application_tasks_allowed =
      delegate_->active_run_loops_.size() == 1U ||
      type_ == Type::kNestableTasksAllowed;
  delegate_->Run(application_tasks_allowed);

  // Rebind this RunLoop to the current thread after Run().
  DETACH_FROM_SEQUENCE(sequence_checker_);
  DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);

  AfterRun();
}

delegate为MessageLoop，RunLoop::Run调用MessageLoop::Run

void MessageLoop::Run(bool application_tasks_allowed) {
  DCHECK_CALLED_ON_VALID_THREAD(bound_thread_checker_);
  if (application_tasks_allowed && !task_execution_allowed_) {
    // Allow nested task execution as explicitly requested.
    DCHECK(RunLoop::IsNestedOnCurrentThread());
    task_execution_allowed_ = true;
    pump_->Run(this);
    task_execution_allowed_ = false;
  } else {
    pump_->Run(this);
  }
}

最后会调用MessageLoopPump的Run方法, MessageLoopPump根据MessageLoop的不同type会创建不同的子类，默认是MessagePumpDefault。

void MessagePumpDefault::Run(Delegate* delegate) {
  AutoReset<bool> auto_reset_keep_running(&keep_running_, true);

  for (;;) {
    bool did_work = delegate->DoWork();
    if (!keep_running_)
      break;

    did_work |= delegate->DoDelayedWork(&delayed_work_time_);
    if (!keep_running_)
      break;

    if (did_work)
      continue;

    did_work = delegate->DoIdleWork();
    if (!keep_running_)
      break;

    if (did_work)
      continue;

    ThreadRestrictions::ScopedAllowWait allow_wait;
    if (delayed_work_time_.is_null()) {
      event_.Wait();
    } else {
      // No need to handle already expired |delayed_work_time_| in any special
      // way. When |delayed_work_time_| is in the past TimeWaitUntil returns
      // promptly and |delayed_work_time_| will re-initialized on a next
      // DoDelayedWork call which has to be called in order to get here again.
      event_.TimedWaitUntil(delayed_work_time_);
    }
    // Since event_ is auto-reset, we don't need to do anything special here
    // other than service each delegate method.
  }
}

MessageLoopPump的作用就是启动一个消息循环，然后去schedule delegate去做一些work。delegate是MessageLoop