Muduo网络库源码分析 EventLoop事件循环(Poller和Channel)

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从这一篇博文起,我们开始剖析Muduo网络库的源码,主要结合《Linux多线程服务端编程》和网上的一些学习资料!

(一)TCP网络编程的本质:三个半事件

1. 连接的建立,包括服务端接受(accept) 新连接和客户端成功发起(connect) 连接。TCP 连接一旦建立,客户端和服务端是平等的,可以各自收发数据。

2. 连接的断开,包括主动断开(close 或shutdown) 和被动断开(read(2) 返回0)。

3. 消息到达,文件描述符可读。这是最为重要的一个事件,对它的处理方式决定了网络编程的风格(阻塞还是非阻塞,如何处理分包,应用层的缓冲如何设计等等)。

3.5 消息发送完毕,这算半个。对于低流量的服务,可以不必关心这个事件;另外,这里“发送完毕”是指将数据写入操作系统的缓冲区,将由TCP 协议栈负责数据的发送与重传,不代表对方已经收到数据。


这其中,最主要的便是第三点: 消息到达,文件描述符可读。下面我们来仔细分析(顺便分析消息发送完毕):

技术分享

(1)消息到达,文件可读:

内核接收-> 网络库可读事件触发--> 将数据从内核转至应用缓冲区(并且回调函数OnMessage根据协议判断是否是完整的数据包,如果不是立即返回)-->如果完整就取出读走、解包、处理、发送(read decode compute encode write)

(2)消息发送完毕:

应用缓冲区-->内核缓冲区(可全填)--->触发发送完成的事件,回调Onwrite。如果内核缓冲区不足以容纳数据(高流量的服务),要把数据追加到应用层发送缓冲区中内核数据发送之后,触发socket可写事件,应用层-->内核;当全发送至内核时,又会回调Onwrite(可继续写)

(二)事件循环类图

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EventLoop类:

EventLoop是对Reactor模式的封装,由于Muduo的并发原型是 Multiple reactors + threadpool  (one loop per thread + threadpool),所以每个线程最多只能有一个EventLoop对象。EventLoop对象构造的时候,会检查当前线程是否已经创建了其他EventLoop对象,如果已创建,终止程序(LOG_FATAL),EventLoop类的构造函数会记录本对象所属线程(threadld_),创建了EventLoop对象的线程称为IO线程,其功能是运行事件循环(EventLooploop),啥也不干==

下面是简化版的EventLoop(内部的Poller尚未实现,只是一个框架)

EventLoop.h

#ifndef MUDUO_NET_EVENTLOOP_H
#define MUDUO_NET_EVENTLOOP_H

#include <boost/noncopyable.hpp>
#include <muduo/base/CurrentThread.h>
#include <muduo/base/Thread.h>

namespace muduo
{
namespace net
{
/// Reactor, at most one per thread.
/// This is an interface class, so don't expose too much details.
class EventLoop : boost::noncopyable
{
 public:
  EventLoop();
  ~EventLoop();  // force out-line dtor, for scoped_ptr members.
  /// Loops forever.
  /// Must be called in the same thread as creation of the object.
  void loop();
  void assertInLoopThread()
  {
    if (!isInLoopThread())
    {
      abortNotInLoopThread();
    }
  }
  bool isInLoopThread() const { return threadId_ == CurrentThread::tid(); }

  static EventLoop* getEventLoopOfCurrentThread();

 private:
  void abortNotInLoopThread();
  
  bool looping_; /* atomic */
  const pid_t threadId_;		// 当前对象所属线程ID
};

}
}
#endif  // MUDUO_NET_EVENTLOOP_H


EventLoop.c

#include <muduo/net/EventLoop.h>
#include <muduo/base/Logging.h>
#include <poll.h>
using namespace muduo;
using namespace muduo::net;

namespace
{
// 当前线程EventLoop对象指针
// 线程局部存储
__thread EventLoop* t_loopInThisThread = 0;
}

EventLoop* EventLoop::getEventLoopOfCurrentThread()
{
  return t_loopInThisThread;
}

EventLoop::EventLoop()
  : looping_(false),
    threadId_(CurrentThread::tid())
{
  LOG_TRACE << "EventLoop created " << this << " in thread " << threadId_;
  // 如果当前线程已经创建了EventLoop对象,终止(LOG_FATAL)
  if (t_loopInThisThread)
  {
    LOG_FATAL << "Another EventLoop " << t_loopInThisThread
              << " exists in this thread " << threadId_;
  }
  else
  {
    t_loopInThisThread = this;
  }
}

EventLoop::~EventLoop()
{
  t_loopInThisThread = NULL;
}

// 事件循环,该函数不能跨线程调用
// 只能在创建该对象的线程中调用
void EventLoop::loop()
{
  assert(!looping_);
  // 断言当前处于创建该对象的线程中
  assertInLoopThread();
  looping_ = true;
  LOG_TRACE << "EventLoop " << this << " start looping";

  ::poll(NULL, 0, 5*1000);

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

void EventLoop::abortNotInLoopThread()
{
  LOG_FATAL << "EventLoop::abortNotInLoopThread - EventLoop " << this
            << " was created in threadId_ = " << threadId_
            << ", current thread id = " <<  CurrentThread::tid();
}


Poller类:

时序图:

技术分享

Poller是个抽象类,具体可以是EPollPoller(默认) 或者PollPoller,需要去实现(唯一使用面向对象的一个类)

对于PollPoller来说,存在一个map,用来关联fd和channel的,我们可以根据fd快速找到对应的channel。一个fd对应一个struct pollfd(pollfd.fd),一个fd 对应一个channel*;这个fd 可以是socket, eventfd, timerfd, signalfd。

Poller的作用是更新IO复用中的channel(IO事件),添加、删除Channel。我们看一下PollPoller的实现:

PollPoller.h

#ifndef MUDUO_NET_POLLER_POLLPOLLER_H
#define MUDUO_NET_POLLER_POLLPOLLER_H

#include <muduo/net/Poller.h>
#include <map>
#include <vector>

struct pollfd;

namespace muduo
{
namespace net
{

/// IO Multiplexing with poll(2).
class PollPoller : public Poller
{
 public:

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

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

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

  typedef std::vector<struct pollfd> PollFdList;
  typedef std::map<int, Channel*> ChannelMap;	// key是文件描述符,value是Channel*
  PollFdList pollfds_;
  ChannelMap channels_;
};

}
}
#endif  // MUDUO_NET_POLLER_POLLPOLLER_H

PollPoller.c

#include <muduo/net/poller/PollPoller.h>
#include <muduo/base/Logging.h>
#include <muduo/base/Types.h>
#include <muduo/net/Channel.h>
#include <assert.h>
#include <poll.h>

using namespace muduo;
using namespace muduo::net;

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

PollPoller::~PollPoller()
{
}

Timestamp PollPoller::poll(int timeoutMs, ChannelList* activeChannels)
{
  // XXX pollfds_ shouldn't change
  int numEvents = ::poll(&*pollfds_.begin(), pollfds_.size(), timeoutMs);
  Timestamp now(Timestamp::now());
  if (numEvents > 0)
  {
    LOG_TRACE << numEvents << " events happended";
    fillActiveChannels(numEvents, activeChannels);
  }
  else if (numEvents == 0)
  {
    LOG_TRACE << " nothing happended";
  }
  else
  {
    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)
  {
	// index < 0说明是一个新的通道
    // 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.events = static_cast<short>(channel->events());
    pfd.revents = 0;
	// 将一个通道暂时更改为不关注事件,但不从Poller中移除该通道
    if (channel->isNoneEvent())
    {
      // ignore this pollfd
	  // 暂时忽略该文件描述符的事件
	  // 这里pfd.fd 可以直接设置为-1
      pfd.fd = -channel->fd()-1;	// 这样子设置是为了removeChannel优化
    }
  }
}

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
  {
	// 这里移除的算法复杂度是O(1),将待删除元素与最后一个元素交换再pop_back
    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();
  }
}
代码中的几个技巧都在注释中标出。

Channel类:

Channel是selectable IO channel,负责注册与响应IO 事件,它不拥有file descriptor。

Channel是Reactor结构中的“事件”,它自始至终都属于一个EventLoop(一个EventLoop对应多个Channel,处理多个IO),负责一个文件描述符的IO事件,它包含又文件描述符fd_,但实际上它不拥有fd_,不用负责将其关闭。在Channel类中保存这IO事件的类型以及对应的回调函数,当IO事件发生时,最终会调用到Channel类中的回调函数。Channel类一般不单独使用,它常常包含在其他类中(Acceptor、Connector、EventLoop、TimerQueue、TcpConnection)使用。Channel类有EventLoop的指针 loop_,通过这个指针可以向EventLoop中添加当前Channel事件。事件类型用events_表示,不同事件类型对应不同回调函数。

以下两个都由Channel注册:

Acceptor是被动连接的抽象--->关注监听套接字的可读事件,回调handleRead

Connector对主动连接的抽象。

时序图:

技术分享

Channel.h

#ifndef MUDUO_NET_CHANNEL_H
#define MUDUO_NET_CHANNEL_H

#include <boost/function.hpp>
#include <boost/noncopyable.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>
#include <muduo/base/Timestamp.h>

namespace muduo
{
namespace net
{

class EventLoop;

/// A selectable I/O channel.
/// This class doesn't own the file descriptor.
/// The file descriptor could be a socket,
/// an eventfd, a timerfd, or a signalfd
class Channel : boost::noncopyable
{
 public:
  typedef boost::function<void()> EventCallback;
  typedef boost::function<void(Timestamp)> ReadEventCallback;

  Channel(EventLoop* loop, int fd);
  ~Channel();

  void handleEvent(Timestamp receiveTime);
  void setReadCallback(const ReadEventCallback& cb)
  { readCallback_ = cb; }
  void setWriteCallback(const EventCallback& cb)
  { writeCallback_ = cb; }
  void setCloseCallback(const EventCallback& cb)
  { closeCallback_ = cb; }
  void setErrorCallback(const EventCallback& cb)
  { errorCallback_ = cb; }

  /// Tie this channel to the owner object managed by shared_ptr,
  /// prevent the owner object being destroyed in handleEvent.
  void tie(const boost::shared_ptr<void>&);

  int fd() const { return fd_; }
  int events() const { return events_; }
  void set_revents(int revt) { revents_ = revt; } // used by pollers
  // int revents() const { return revents_; }
  bool isNoneEvent() const { return events_ == kNoneEvent; }

  void enableReading() { events_ |= kReadEvent; update(); }
  // void disableReading() { events_ &= ~kReadEvent; update(); }
  void enableWriting() { events_ |= kWriteEvent; update(); }
  void disableWriting() { events_ &= ~kWriteEvent; update(); }
  void disableAll() { events_ = kNoneEvent; update(); }
  bool isWriting() const { return events_ & kWriteEvent; }

  // for Poller
  int index() { return index_; }
  void set_index(int idx) { index_ = idx; }

  // for debug
  string reventsToString() const;

  void doNotLogHup() { logHup_ = false; }

  EventLoop* ownerLoop() { return loop_; }
  void remove();

 private:
  void update();
  void handleEventWithGuard(Timestamp receiveTime);

  static const int kNoneEvent;
  static const int kReadEvent;
  static const int kWriteEvent;

  EventLoop* loop_;			// 所属EventLoop
  const int  fd_;			// 文件描述符,但不负责关闭该文件描述符
  int        events_;		// 关注的事件
  int        revents_;		// poll/epoll返回的事件
  int        index_;		// used by Poller.表示在poll的事件数组中的序号
  bool       logHup_;		// for POLLHUP

  boost::weak_ptr<void> tie_;
  bool tied_;
  bool eventHandling_;		// 是否处于处理事件中
  ReadEventCallback readCallback_;
  EventCallback writeCallback_;
  EventCallback closeCallback_;
  EventCallback errorCallback_;
};

}
}
#endif  // MUDUO_NET_CHANNEL_H
Channel.c

#include <muduo/base/Logging.h>
#include <muduo/net/Channel.h>
#include <muduo/net/EventLoop.h>
#include <sstream>
#include <poll.h>

using namespace muduo;
using namespace muduo::net;

const int Channel::kNoneEvent = 0;
const int Channel::kReadEvent = POLLIN | POLLPRI;
const int Channel::kWriteEvent = POLLOUT;

Channel::Channel(EventLoop* loop, int fd__)
  : loop_(loop),
    fd_(fd__),
    events_(0),
    revents_(0),
    index_(-1),
    logHup_(true),
    tied_(false),
    eventHandling_(false)
{
}

Channel::~Channel()
{
  assert(!eventHandling_);
}

void Channel::tie(const boost::shared_ptr<void>& obj)
{
  tie_ = obj;
  tied_ = true;
}

void Channel::update()
{
  loop_->updateChannel(this);
}

// 调用这个函数之前确保调用disableAll
void Channel::remove()
{
  assert(isNoneEvent());
  loop_->removeChannel(this);
}

void Channel::handleEvent(Timestamp receiveTime)
{
  boost::shared_ptr<void> guard;
  if (tied_)
  {
    guard = tie_.lock();
    if (guard)
    {
      handleEventWithGuard(receiveTime);
    }
  }
  else
  {
    handleEventWithGuard(receiveTime);
  }
}

void Channel::handleEventWithGuard(Timestamp receiveTime)
{
  eventHandling_ = true;
  if ((revents_ & POLLHUP) && !(revents_ & POLLIN))
  {
    if (logHup_)
    {
      LOG_WARN << "Channel::handle_event() POLLHUP";
    }
    if (closeCallback_) closeCallback_();
  }

  if (revents_ & POLLNVAL)
  {
    LOG_WARN << "Channel::handle_event() POLLNVAL";
  }

  if (revents_ & (POLLERR | POLLNVAL))
  {
    if (errorCallback_) errorCallback_();
  }
  if (revents_ & (POLLIN | POLLPRI | POLLRDHUP))
  {
    if (readCallback_) readCallback_(receiveTime);
  }
  if (revents_ & POLLOUT)
  {
    if (writeCallback_) writeCallback_();
  }
  eventHandling_ = false;
}

string Channel::reventsToString() const
{
  std::ostringstream oss;
  oss << fd_ << ": ";
  if (revents_ & POLLIN)
    oss << "IN ";
  if (revents_ & POLLPRI)
    oss << "PRI ";
  if (revents_ & POLLOUT)
    oss << "OUT ";
  if (revents_ & POLLHUP)
    oss << "HUP ";
  if (revents_ & POLLRDHUP)
    oss << "RDHUP ";
  if (revents_ & POLLERR)
    oss << "ERR ";
  if (revents_ & POLLNVAL)
    oss << "NVAL ";

  return oss.str().c_str();
}
这三个类之间的关系不难理解,其实本质就是一个Poll/Epoll,只不过进行了更高的抽象后划分出来的这些类,重点理解博客开头的那张类图即可。

参考:

《Muduo使用手册》

《Linux多线程服务端编程》






































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