一句话讲懂：muduo网络库——Poller

OK了兄弟们。
今天咱们分析一手，muduo 网络库的——Poller 类

我们的 Poller 类其实是搞抽象的啊，一个抽象类（没实例呀，牢弟）。这个类主要是为了派生 EpollPoller 类和 PollPoller 类。这两个派生类 也很顾名思义，分别是为了封装 epoll 和 poll 两个模型。

Poller.h 文件：

class Poller : noncopyable
{
 public:
  typedef std::vector<Channel*> ChannelList;

  Poller(EventLoop* loop);
  virtual ~Poller();

  virtual Timestamp poll(int timeoutMs, ChannelList* activeChannels) = 0; // timeoutMs 是超时时间，activeChannels 是一个传出参数

  virtual void updateChannel(Channel* channel) = 0; //向模型中加入新的文件描述符，或更新文件描述符关心的事件

  virtual void removeChannel(Channel* channel) = 0; //从模型中移除文件描述符

  virtual bool hasChannel(Channel* channel) const; //看看是不是已经有这个channel

  static Poller* newDefaultPoller(EventLoop* loop); //给 EventLoop 构造属于他的 poller 用

  void assertInLoopThread() const
  {
    ownerLoop_->assertInLoopThread();
  }

 protected:
  typedef std::map<int, Channel*> ChannelMap;
  ChannelMap channels_; //文件描述符和对应的 channel

 private:
  EventLoop* ownerLoop_; //指向所属的 EventLoop
};

EpollPoller 类：

相比于基类，多了两个变量：

  typedef std::vector<struct epoll_event> EventList;

  int epollfd_;
  EventList events_;

新增了一个函数用于找出有事件发生的文件描述符：

void EPollPoller::fillActiveChannels(int numEvents, ChannelList* activeChannels) const
{
  assert(implicit_cast<size_t>(numEvents) <= events_.size());
  for (int i = 0; i < numEvents; ++i)
  {
    Channel* channel = static_cast<Channel*>(events_[i].data.ptr);
#ifndef NDEBUG
    int fd = channel->fd();
    ChannelMap::const_iterator it = channels_.find(fd);
    assert(it != channels_.end());
    assert(it->second == channel);
#endif
    channel->set_revents(events_[i].events);
    activeChannels->push_back(channel);
  }
}

一句注释也没写。
没啥可写的。

还有一个函数，用于调用 epoll_ctl 函数：

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)
  {
    if (operation == EPOLL_CTL_DEL)
    {
      LOG_SYSERR << "epoll_ctl op =" << operationToString(operation) << " fd =" << fd;
    }
    else
    {
      LOG_SYSFATAL << "epoll_ctl op =" << operationToString(operation) << " fd =" << fd;
    }
  }
}

其实还有一个函数，输出用的：

const char* EPollPoller::operationToString(int op)
{
  switch (op)
  {
    case EPOLL_CTL_ADD:
      return "ADD";
    case EPOLL_CTL_DEL:
      return "DEL";
    case EPOLL_CTL_MOD:
      return "MOD";
    default:
      assert(false && "ERROR op");
      return "Unknown Operation";
  }
}

这回真没有新增的函数了，剩下的都是重写基类的函数了。也是比较重要的部分。

看看 EPollPoller::poll 函数吧：

Timestamp EPollPoller::poll(int timeoutMs, ChannelList* activeChannels)
{
  LOG_TRACE << "fd total count " << channels_.size();
  int numEvents = ::epoll_wait(epollfd_,
                               &*events_.begin(),
                               static_cast<int>(events_.size()),
                               timeoutMs);
  int savedErrno = errno;
  Timestamp now(Timestamp::now());
  if (numEvents > 0) // 有事发生
  {
    LOG_TRACE << numEvents << " events happened";
    fillActiveChannels(numEvents, activeChannels);
    if (implicit_cast<size_t>(numEvents) == events_.size())
    {
      events_.resize(events_.size()*2);// 扩容
    }
  }
  else if (numEvents == 0) // 无事发生
  {
    LOG_TRACE << "nothing happened";
  }
  else // 有错发生
  {
    // error happens, log uncommon ones
    if (savedErrno != EINTR)
    {
      errno = savedErrno;
      LOG_SYSERR << "EPollPoller::poll()";
    }
  }
  return now;
}

EPollPoller::updateChannel 函数：

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) // 新来的或者曾经注册过的，需要再次注册到 epoll 中
  {
    int fd = channel->fd();
    if (index == kNew)
    {
      assert(channels_.find(fd) == channels_.end());
      channels_[fd] = channel; // 登记到表中
    }
    else // index == kDeleted
    {
      assert(channels_.find(fd) != channels_.end());
      assert(channels_[fd] == channel);
    }

    channel->set_index(kAdded); // 更新状态
    update(EPOLL_CTL_ADD, channel); // 加入 epoll
  }
  else // 已经在 epoll 中
  {
    int fd = channel->fd();
    (void)fd;
    assert(channels_.find(fd) != channels_.end());
    assert(channels_[fd] == channel);
    assert(index == kAdded);
    if (channel->isNoneEvent()) // 没什么关注的事
    {
      update(EPOLL_CTL_DEL, channel); // 删掉
      channel->set_index(kDeleted);
    }
    else // 有所留恋
    {
      update(EPOLL_CTL_MOD, channel); 
    }
  }
}

EPollPoller::removeChannel 函数：

void EPollPoller::removeChannel(Channel* channel)
{
  Poller::assertInLoopThread();
  int fd = channel->fd();
  LOG_TRACE << "fd = " << fd;
  assert(channels_.find(fd) != channels_.end());
  assert(channels_[fd] == channel);
  assert(channel->isNoneEvent());
  int index = channel->index();
  assert(index == kAdded || index == kDeleted);
  size_t n = channels_.erase(fd); // 从表里面开除
  (void)n;
  assert(n == 1);

  if (index == kAdded) //如果注册在 epoll 中
  {
    update(EPOLL_CTL_DEL, channel); //从 epoll 中开除
  }
  channel->set_index(kNew); //更新状态
}

PollPoller 类：

不想讲。
把代码贴上吧，和 epoll 大差不差的

class PollPoller : public Poller
{
 public:

  PollPoller(EventLoop* loop);
  ~PollPoller() override;

  Timestamp poll(int timeoutMs, ChannelList* activeChannels) override;
  void updateChannel(Channel* channel) override;
  void removeChannel(Channel* channel) override;

 private:
  void fillActiveChannels(int numEvents,
                          ChannelList* activeChannels) const;

  typedef std::vector<struct pollfd> PollFdList;
  PollFdList pollfds_;
};

PollPoller::PollPoller(EventLoop* loop)
  : Poller(loop)
{
}

PollPoller::~PollPoller() = default;

Timestamp PollPoller::poll(int timeoutMs, ChannelList* activeChannels)
{
  // XXX pollfds_ shouldn't change
  int numEvents = ::poll(&*pollfds_.begin(), pollfds_.size(), timeoutMs);
  int savedErrno = errno;
  Timestamp now(Timestamp::now());
  if (numEvents > 0)
  {
    LOG_TRACE << numEvents << " events happened";
    fillActiveChannels(numEvents, activeChannels);
  }
  else if (numEvents == 0)
  {
    LOG_TRACE << " nothing happened";
  }
  else
  {
    if (savedErrno != EINTR)
    {
      errno = savedErrno;
      LOG_SYSERR << "PollPoller::poll()";
    }
  }
  return now;
}

void PollPoller::fillActiveChannels(int numEvents,
                                    ChannelList* activeChannels) const
{
  for (PollFdList::const_iterator pfd = pollfds_.begin();
      pfd != pollfds_.end() && numEvents > 0; ++pfd)
  {
    if (pfd->revents > 0)
    {
      --numEvents;
      ChannelMap::const_iterator ch = channels_.find(pfd->fd);
      assert(ch != channels_.end());
      Channel* channel = ch->second;
      assert(channel->fd() == pfd->fd);
      channel->set_revents(pfd->revents);
      // pfd->revents = 0;
      activeChannels->push_back(channel);
    }
  }
}

void PollPoller::updateChannel(Channel* channel)
{
  Poller::assertInLoopThread();
  LOG_TRACE << "fd = " << channel->fd() << " events = " << channel->events();
  if (channel->index() < 0)
  {
    // a new one, add to pollfds_
    assert(channels_.find(channel->fd()) == channels_.end());
    struct pollfd pfd;
    pfd.fd = channel->fd();
    pfd.events = static_cast<short>(channel->events());
    pfd.revents = 0;
    pollfds_.push_back(pfd);
    int idx = static_cast<int>(pollfds_.size())-1;
    channel->set_index(idx);
    channels_[pfd.fd] = channel;
  }
  else
  {
    // update existing one
    assert(channels_.find(channel->fd()) != channels_.end());
    assert(channels_[channel->fd()] == channel);
    int idx = channel->index();
    assert(0 <= idx && idx < static_cast<int>(pollfds_.size()));
    struct pollfd& pfd = pollfds_[idx];
    assert(pfd.fd == channel->fd() || pfd.fd == -channel->fd()-1);
    pfd.fd = channel->fd();
    pfd.events = static_cast<short>(channel->events());
    pfd.revents = 0;
    if (channel->isNoneEvent())
    {
      // ignore this pollfd
      pfd.fd = -channel->fd()-1;
    }
  }
}

void PollPoller::removeChannel(Channel* channel)
{
  Poller::assertInLoopThread();
  LOG_TRACE << "fd = " << channel->fd();
  assert(channels_.find(channel->fd()) != channels_.end());
  assert(channels_[channel->fd()] == channel);
  assert(channel->isNoneEvent());
  int idx = channel->index();
  assert(0 <= idx && idx < static_cast<int>(pollfds_.size()));
  const struct pollfd& pfd = pollfds_[idx]; (void)pfd;
  assert(pfd.fd == -channel->fd()-1 && pfd.events == channel->events());
  size_t n = channels_.erase(channel->fd());
  assert(n == 1); (void)n;
  if (implicit_cast<size_t>(idx) == pollfds_.size()-1)
  {
    pollfds_.pop_back();
  }
  else
  {
    int channelAtEnd = pollfds_.back().fd;
    iter_swap(pollfds_.begin()+idx, pollfds_.end()-1);
    if (channelAtEnd < 0)
    {
      channelAtEnd = -channelAtEnd-1;
    }
    channels_[channelAtEnd]->set_index(idx);
    pollfds_.pop_back();
  }
}