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发布于 2021-01-07 17:46:45

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发布于 2021-01-07 17:46:45

文章被收录于专栏：后端技术

正文

遵循Reactor模型，封装了EventLoop。有以下特点：

利用MutexLockGuard->MutexLock->pthread_mutex_t控制EventLoop::pendingFunctors_的线程安全。
回调函数, std::function和std::bind的大量应用。用于回调读/写/错误/关闭等事件。
高度抽象。用TcpConnection封装Channel，后者封装了socket fd。用EventLoop封装Reactor线程。用TcpServer封装Acceptor线程和Event线程池。
异步函数队列。利用EventLoop::pendingFunctors_的异步函数队列机制，让Reactor线程能向ioLoop传递回调函数的方式，让ioLoop注册新的socket。
利用wakupFd唤醒线程。EventLoop在构造函数中就注册监听了wakupFd的读事件。Reactor线程通过向wakeupFd_写8字节的方式，触发ioLoop监听的wakeup_fd，从epoll_wait唤醒。

正文

遵从Reactor模型，简单阅读了下。来到目录examples/ace/loggin/server.cc server.cc main->LogServer->EventLoop和TcpServer

image.png

当新连接到来时，会调用TcpServer::newConnection

void TcpServer::newConnection(int sockfd, const InetAddress& peerAddr)
{
  ...
  EventLoop* ioLoop = threadPool_->getNextLoop();
  ...
  TcpConnectionPtr conn(new TcpConnection(ioLoop,
                                          connName,
                                          sockfd,
                                          localAddr,
                                          peerAddr));
  ...
  ioLoop->runInLoop(std::bind(&TcpConnection::connectEstablished, conn));
}

Reactor线程调用ioLoop->runInLoop,传入回调函数TcpConnection::connectEstablished

在此尝试调用回调函数。

如果调用者就是ioLoop本身，就直接调用cb
如果不是，则放入异步队列，等待ioLoop后续调用

// EventLoop.cc
void EventLoop::runInLoop(Functor cb)
{
  if (isInLoopThread())
  {
    cb();
  }
  else
  {
    queueInLoop(std::move(cb));
  }
}

回调函数的异步调用

先锁住pendingFunctors_, 传入回调函数cb, 然后如果不是loop线程，要将其唤醒

void EventLoop::queueInLoop(Functor cb)
{
  {
  MutexLockGuard lock(mutex_);
  pendingFunctors_.push_back(std::move(cb));
  }

  if (!isInLoopThread() || callingPendingFunctors_)
  {
    wakeup();
  }
}

利用事件机制唤醒线程

函数向wakupFd写了8个字节，触发其写事件，从而唤醒ioLoop线程。这里利用了epoll的事件机制。

void EventLoop::wakeup()
{
  uint64_t one = 1;
  ssize_t n = sockets::write(wakeupFd_, &one, sizeof one);
  if (n != sizeof one)
  {
    LOG_ERROR << "EventLoop::wakeup() writes " << n << " bytes instead of 8";
  }
}

回调函数调用

当回调函数被调用后，一路

void TcpConnection::connectEstablished()
{
  ...
  channel_->enableReading();

  connectionCallback_(shared_from_this());
}

// Channel.h
void enableReading() { events_ |= kReadEvent; update(); }

// Channel.cc
void Channel::update()
{
  addedToLoop_ = true;
  loop_->updateChannel(this);
}

void EventLoop::updateChannel(Channel* channel)
{
  assert(channel->ownerLoop() == this);
  assertInLoopThread();
  poller_->updateChannel(channel);
}

channel->index()标志了该Channel的状态，根据当前状态决定执行注册/注销/修改事件

void EPollPoller::updateChannel(Channel* channel)
{
  Poller::assertInLoopThread();
  const int index = channel->index();
  LOG_TRACE << "fd = " << channel->fd()
    << " events = " << channel->events() << " index = " << index;
  if (index == kNew || index == kDeleted)
  {
    // a new one, add with EPOLL_CTL_ADD
    int fd = channel->fd();
    if (index == kNew)
    {
      assert(channels_.find(fd) == channels_.end());
      channels_[fd] = channel;
    }
    else // index == kDeleted
    {
      assert(channels_.find(fd) != channels_.end());
      assert(channels_[fd] == channel);
    }

    channel->set_index(kAdded);
    update(EPOLL_CTL_ADD, channel);
  }
  else
  {
    // update existing one with EPOLL_CTL_MOD/DEL
    int fd = channel->fd();
    (void)fd;
    assert(channels_.find(fd) != channels_.end());
    assert(channels_[fd] == channel);
    assert(index == kAdded);
    if (channel->isNoneEvent())
    {
      update(EPOLL_CTL_DEL, channel);
      channel->set_index(kDeleted);
    }
    else
    {
      update(EPOLL_CTL_MOD, channel);
    }
  }
}

void EPollPoller::update(int operation, Channel* channel)
{
  struct epoll_event event;
  memZero(&event, sizeof event);
  event.events = channel->events();
  event.data.ptr = channel;
  int fd = channel->fd();
  LOG_TRACE << "epoll_ctl op = " << operationToString(operation)
    << " fd = " << fd << " event = { " << channel->eventsToString() << " }";
  if (::epoll_ctl(epollfd_, operation, fd, &event) < 0)  // 为epollfd_注册事件
  {
    ...
  }
}

EventLoop主循环

void EventLoop::loop()
{
  assert(!looping_);
  assertInLoopThread();
  looping_ = true;
  quit_ = false;  // FIXME: what if someone calls quit() before loop() ?
  LOG_TRACE << "EventLoop " << this << " start looping";

  while (!quit_)
  {
    activeChannels_.clear();
    pollReturnTime_ = poller_->poll(kPollTimeMs, &activeChannels_);  // 获取活跃的Channel
    ++iteration_;
    if (Logger::logLevel() <= Logger::TRACE)
    {
      printActiveChannels();
    }
    // TODO sort channel by priority
    eventHandling_ = true;
    for (Channel* channel : activeChannels_)
    {
      currentActiveChannel_ = channel;
      currentActiveChannel_->handleEvent(pollReturnTime_);  // 逐个执行Channel对应的事件
    }
    currentActiveChannel_ = NULL;
    eventHandling_ = false;
    doPendingFunctors();  // 处理异步队列，Reactor收到的socket在此被注册到epoll_fd
  }

  LOG_TRACE << "EventLoop " << this << " stop looping";
  looping_ = false;
}

EventLoop在构造函数监听wakeup_fd

EventLoop::EventLoop()
  : looping_(false),
    quit_(false),
    eventHandling_(false),
    callingPendingFunctors_(false),
    iteration_(0),
    threadId_(CurrentThread::tid()),
    poller_(Poller::newDefaultPoller(this)),
    timerQueue_(new TimerQueue(this)),
    wakeupFd_(createEventfd()),
    wakeupChannel_(new Channel(this, wakeupFd_)),
    currentActiveChannel_(NULL)
{
  LOG_DEBUG << "EventLoop created " << this << " in thread " << threadId_;
  if (t_loopInThisThread)
  {
    LOG_FATAL << "Another EventLoop " << t_loopInThisThread
              << " exists in this thread " << threadId_;
  }
  else
  {
    t_loopInThisThread = this;
  }

  // 这两句话使得EventLoop监听了wakup_fd
  wakeupChannel_->setReadCallback(
      std::bind(&EventLoop::handleRead, this));
  // we are always reading the wakeupfd
  wakeupChannel_->enableReading();
}

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