TCP网络库：Acceptor、TcpServer、TcpConnection

class Acceptor : boost::noncopyable
{
 public:
  typedef boost::function<void (int sockfd,
                                const InetAddress&)> NewConnectionCallback;

  Acceptor(EventLoop* loop, const InetAddress& listenAddr, bool reuseport);
  ~Acceptor();

  void setNewConnectionCallback(const NewConnectionCallback& cb)
  { newConnectionCallback_ = cb; }

  bool listenning() const { return listenning_; }
  void listen();

 private:
  //调用accept()接受新连接，并回调用户callback
  void handleRead();

  EventLoop* loop_;
  //此socket是listen socket
  Socket acceptSocket_;
  //channel对象观察上述socket上的readable事件，并回调handleRead()
  Channel acceptChannel_;
  NewConnectionCallback newConnectionCallback_;
  bool listenning_;
  int idleFd_;
};

//构造函数调用socket() 、bind()，即创建TCP服务端的传统步骤
//socket() bind() listen()任何一个步骤出错都会造成程序终止，故没有错误处理
//sockets::createNonblockingOrDie创建非阻塞的socket
Acceptor::Acceptor(EventLoop* loop, const InetAddress& listenAddr, bool reuseport)
  : loop_(loop),
    acceptSocket_(sockets::createNonblockingOrDie(listenAddr.family())),
    acceptChannel_(loop, acceptSocket_.fd()),
    listenning_(false),
    idleFd_(::open("/dev/null", O_RDONLY | O_CLOEXEC))
{
  assert(idleFd_ >= 0);
  acceptSocket_.setReuseAddr(true);
  acceptSocket_.setReusePort(reuseport);
  acceptSocket_.bindAddress(listenAddr);
  acceptChannel_.setReadCallback(
      boost::bind(&Acceptor::handleRead, this));
}
//调用listen()，当socket可读时调用handleRead()
void Acceptor::listen()
{
  loop_->assertInLoopThread();
  listenning_ = true;
  acceptSocket_.listen();
  acceptChannel_.enableReading();
}
//accept策略
//参考acceptable strategies for improving web server performance
void Acceptor::handleRead()
{
  loop_->assertInLoopThread();
  InetAddress peerAddr;
  //FIXME loop until no more
  int connfd = acceptSocket_.accept(&peerAddr);
  if (connfd >= 0)
  {
    // string hostport = peerAddr.toIpPort();
    // LOG_TRACE << "Accepts of " << hostport;
    if (newConnectionCallback_)
    {
      newConnectionCallback_(connfd, peerAddr);
    }
    else
    {
      sockets::close(connfd);
    }
  }
  else
  {
    LOG_SYSERR << "in Acceptor::handleRead";
    // Read the section named "The special problem of
    // accept()ing when you can‘t" in libev‘s doc.
    // By Marc Lehmann, author of livev.
    //本进程的文件描述符已经达到上限，由于没有socket文件描述符来表示这个连接，就无法close它.若epoll_wait是LT，则每次调用都会立刻返回，因为新连接还等待处理
    //准备一个空闲的文件描述符，在这种情况下，先关闭这个空闲的fd，然后accept拿到新socket连接的描述符，随后close它，再重新打开一个空闲文件给该空闲文件描述符
    if (errno == EMFILE)
    {
      ::close(idleFd_);
      idleFd_ = ::accept(acceptSocket_.fd(), NULL, NULL);
      ::close(idleFd_);
      idleFd_ = ::open("/dev/null", O_RDONLY | O_CLOEXEC);
    }
  }
}