TcpConnection

TcpConnection是muduo库中最重要的一个类。是整个网络库的核心，封装一次TCP连接。而且该类的生命期由用户和库共同控制，因为其生命期的管理更加复杂，所以该类使用shared_ptr管理，并继承了std::enable_shared_from_this<TcpConnection>。关于enable_shared_from_this可以参考这篇博客：https://blog.youkuaiyun.com/caoshangpa/article/details/79392878 。

void muduo::net::defaultConnectionCallback(const TcpConnectionPtr& conn)
{
  LOG_TRACE << conn->localAddress().toIpPort() << " -> "
            << conn->peerAddress().toIpPort() << " is "
            << (conn->connected() ? "UP" : "DOWN");
  // do not call conn->forceClose(), because some users want to register message callback only.
}

void muduo::net::defaultMessageCallback(const TcpConnectionPtr&,
                                        Buffer* buf,
                                        Timestamp)
{
  buf->retrieveAll();
}

默认的建立连接回调函数和消息处理回调函数。

TcpConnection::TcpConnection(EventLoop* loop,
                             const string& nameArg,
                             int sockfd,
                             const InetAddress& localAddr,
                             const InetAddress& peerAddr)
  : loop_(CHECK_NOTNULL(loop)),
    name_(nameArg),
    state_(kConnecting),
    reading_(true),
    socket_(new Socket(sockfd)),
    channel_(new Channel(loop, sockfd)),
    localAddr_(localAddr),
    peerAddr_(peerAddr),
    highWaterMark_(64*1024*1024)
{
  channel_->setReadCallback(
      std::bind(&TcpConnection::handleRead, this, _1));
  channel_->setWriteCallback(
      std::bind(&TcpConnection::handleWrite, this));
  channel_->setCloseCallback(
      std::bind(&TcpConnection::handleClose, this));
  channel_->setErrorCallback(
      std::bind(&TcpConnection::handleError, this));
  LOG_DEBUG << "TcpConnection::ctor[" <<  name_ << "] at " << this
            << " fd=" << sockfd;
  socket_->setKeepAlive(true);
}

TcpConnection::~TcpConnection()
{
  LOG_DEBUG << "TcpConnection::dtor[" <<  name_ << "] at " << this
            << " fd=" << channel_->fd()
            << " state=" << stateToString();
  assert(state_ == kDisconnected);
}

构造和析构函数，构造函数相当复杂，将套接字描述符封装为Socket，设置该连接的本端和对端地址信息，设置可读、可写、关闭、错误发生时的回调函数，开启心跳。

bool TcpConnection::getTcpInfo(struct tcp_info* tcpi) const
{
  return socket_->getTcpInfo(tcpi);
}

string TcpConnection::getTcpInfoString() const
{
  char buf[1024];
  buf[0] = '\0';
  socket_->getTcpInfoString(buf, sizeof buf);
  return buf;
}

获取本TCP连接的信息，关于该信息可以参看之前的博客。

void TcpConnection::send(const void* data, int len)
{
  send(StringPiece(static_cast<const char*>(data), len));
}

void TcpConnection::send(const StringPiece& message)
{
  if (state_ == kConnected)
  {
    if (loop_->isInLoopThread())
    {
      sendInLoop(message);
    }
    else
    {
      void (TcpConnection::*fp)(const StringPiece& message) = &TcpConnection::sendInLoop;
      loop_->runInLoop(
          std::bind(fp,
                    this,     // FIXME
                    message.as_string()));
                    //std::forward<string>(message)));
    }
  }
}

// FIXME efficiency!!!
void TcpConnection::send(Buffer* buf)
{
  if (state_ == kConnected)
  {
    if (loop_->isInLoopThread())
    {
      sendInLoop(buf->peek(), buf->readableBytes());
      buf->retrieveAll();
    }
    else
    {
      void (TcpConnection::*fp)(const StringPiece& message) = &TcpConnection::sendInLoop;
      loop_->runInLoop(
          std::bind(fp,
                    this,     // FIXME
                    buf->retrieveAllAsString()));
                    //std::forward<string>(message)));
    }
  }
}

void TcpConnection::sendInLoop(const StringPiece& message)
{
  sendInLoop(message.data(), message.size());
}

void TcpConnection::sendInLoop(const void* data, size_t len)
{
  loop_->assertInLoopThread();
  ssize_t nwrote = 0;
  size_t remaining = len;
  bool faultError = false;
  if (state_ == kDisconnected)
  {
    LOG_WARN << "disconnected, give up writing";
    return;
  }
  // if no thing in output queue, try writing directly
  if (!channel_->isWriting() && outputBuffer_.readableBytes() == 0)
  {
    nwrote = sockets::write(channel_->fd(), data, len);
    if (nwrote >= 0)
    {
      remaining = len - nwrote;
      if (remaining == 0 && writeCompleteCallback_)
      {
        loop_->queueInLoop(std::bind(writeCompleteCallback_, shared_from_this()));
      }
    }
    else // nwrote < 0
    {
      nwrote = 0;
      if (errno != EWOULDBLOCK)
      {
        LOG_SYSERR << "TcpConnection::sendInLoop";
        if (errno == EPIPE || errno == ECONNRESET) // FIXME: any others?
        {
          faultError = true;
        }
      }
    }
  }

  assert(remaining <= len);
  if (!faultError && remaining > 0)
  {
    size_t oldLen = outputBuffer_.readableBytes();
    if (oldLen + remaining >= highWaterMark_
        && oldLen < highWaterMark_
        && highWaterMarkCallback_)
    {
      loop_->queueInLoop(std::bind(highWaterMarkCallback_, shared_from_this(), oldLen + remaining));
    }
    outputBuffer_.append(static_cast<const char*>(data)+nwrote, remaining);
    if (!channel_->isWriting())
    {
      channel_->enableWriting();
    }
  }
}

发送接口。TcpConnection::send()比较有趣的地方是调用EventLoop::runInLoop()时并没有像其他地方那样直接绑定函数名，而是使用了函数指针。是因为不能bind重载函数，所以这里使用了函数指针。

TcpConnection::sendInLoop()是最终的实现接口，比较复杂。if (!channel_->isWriting() && outputBuffer_.readableBytes() == 0)判断是否在之前存在没有发送完的数据，如果没有在写并且当前的数据为空，那么可以直接发送。如果之前有数据还没有发送完，必须将新的数据放到之前的数据后面，不能直接发送，否则会出现乱序。在可以直接发送的情况下，只尝试写一次，如果一次性发送完，调用writeCompleteCallback_发送完成后的回调，由于该回调是用户注册的接口，所以在调用这个接口时，使用了shared_from_this()。如果一次性发送不完，那么就把还剩的数据放到发送缓冲区中。在添加到缓冲区的那部分代码中，有一个高水位的判断

    if (oldLen + remaining >= highWaterMark_
        && oldLen < highWaterMark_
        && highWaterMarkCallback_)
    {
      loop_->queueInLoop(std::bind(highWaterMarkCallback_, shared_from_this(), oldLen + remaining));
    }

如果输出缓冲区内的数据量达到一定的值，会尝试调用高水位回调函数，并且只会调用一次。

void TcpConnection::shutdown()
{
  // FIXME: use compare and swap
  if (state_ == kConnected)
  {
    setState(kDisconnecting);
    // FIXME: shared_from_this()?
    loop_->runInLoop(std::bind(&TcpConnection::shutdownInLoop, this));
  }
}

void TcpConnection::shutdownInLoop()
{
  loop_->assertInLoopThread();
  if (!channel_->isWriting())
  {
    // we are not writing
    socket_->shutdownWrite();
  }
}

关闭连接，不过只关闭写端。这样对端的读会返回0，继而对端关闭。本端的读也会返回0，执行关闭。

void TcpConnection::forceClose()
{
  // FIXME: use compare and swap
  if (state_ == kConnected || state_ == kDisconnecting)
  {
    setState(kDisconnecting);
    loop_->queueInLoop(std::bind(&TcpConnection::forceCloseInLoop, shared_from_this()));
  }
}

void TcpConnection::forceCloseWithDelay(double seconds)
{
  if (state_ == kConnected || state_ == kDisconnecting)
  {
    setState(kDisconnecting);
    loop_->runAfter(
        seconds,
        makeWeakCallback(shared_from_this(),
                         &TcpConnection::forceClose));  // not forceCloseInLoop to avoid race condition
  }
}

void TcpConnection::forceCloseInLoop()
{
  loop_->assertInLoopThread();
  if (state_ == kConnected || state_ == kDisconnecting)
  {
    // as if we received 0 byte in handleRead();
    handleClose();
  }
}

强制关闭的接口

void TcpConnection::setTcpNoDelay(bool on)
{
  socket_->setTcpNoDelay(on);
}

设置Nagle算法，一般是禁止该算法。默认情况下该算法是启动的。Nagle算法的目的在于减少广域网上小分组的数目。该算法指出：如果某个给定连接上有待确认数据，那么原本应该作为用户写操作之响应的在该连接上立即发送相应小分组的行为就不会发生，直到现有数据被确认为止。这里“小”分组的定义就是小于MSS的任何分组。TCP总是尽可能地发送最大大小的分组，Nagle算法的目的在于防止一个连接在任何时刻有多个小分组待确认。

void TcpConnection::startRead()
{
  loop_->runInLoop(std::bind(&TcpConnection::startReadInLoop, this));
}

void TcpConnection::startReadInLoop()
{
  loop_->assertInLoopThread();
  if (!reading_ || !channel_->isReading())
  {
    channel_->enableReading();
    reading_ = true;
  }
}

void TcpConnection::stopRead()
{
  loop_->runInLoop(std::bind(&TcpConnection::stopReadInLoop, this));
}

void TcpConnection::stopReadInLoop()
{
  loop_->assertInLoopThread();
  if (reading_ || channel_->isReading())
  {
    channel_->disableReading();
    reading_ = false;
  }
}

开启读和关闭读，在muduo库中没见使用，应该是为了一些特殊的目的。

void TcpConnection::connectEstablished()
{
  loop_->assertInLoopThread();
  assert(state_ == kConnecting);
  setState(kConnected);
  channel_->tie(shared_from_this());
  channel_->enableReading();

  connectionCallback_(shared_from_this());
}

void TcpConnection::connectDestroyed()
{
  loop_->assertInLoopThread();
  if (state_ == kConnected)
  {
    setState(kDisconnected);
    channel_->disableAll();

    connectionCallback_(shared_from_this());
  }
  channel_->remove();
}

连接建立和连接断开，这两个函数执行的功能相反。TcpConnection::connectEstablished()设置连接建立状态、使能可读、并回调connectionCallback_。TcpConnection::connectDestroyed()设置连接关闭状态、移除关心的事件、并回调connectionCallback_。如果用户对建立连接和关闭连接事件感兴趣，会将处理函数注册到TcpServer，TcpServer再注册到TcpConnection，最终由TcpConnection调用。

void TcpConnection::handleRead(Timestamp receiveTime)
{
  loop_->assertInLoopThread();
  int savedErrno = 0;
  ssize_t n = inputBuffer_.readFd(channel_->fd(), &savedErrno);
  if (n > 0)
  {
    messageCallback_(shared_from_this(), &inputBuffer_, receiveTime);
  }
  else if (n == 0)
  {
    handleClose();
  }
  else
  {
    errno = savedErrno;
    LOG_SYSERR << "TcpConnection::handleRead";
    handleError();
  }
}

void TcpConnection::handleWrite()
{
  loop_->assertInLoopThread();
  if (channel_->isWriting())
  {
    ssize_t n = sockets::write(channel_->fd(),
                               outputBuffer_.peek(),
                               outputBuffer_.readableBytes());
    if (n > 0)
    {
      outputBuffer_.retrieve(n);
      if (outputBuffer_.readableBytes() == 0)
      {
        channel_->disableWriting();
        if (writeCompleteCallback_)
        {
          loop_->queueInLoop(std::bind(writeCompleteCallback_, shared_from_this()));
        }
        if (state_ == kDisconnecting)
        {
          shutdownInLoop();
        }
      }
    }
    else
    {
      LOG_SYSERR << "TcpConnection::handleWrite";
      // if (state_ == kDisconnecting)
      // {
      //   shutdownInLoop();
      // }
    }
  }
  else
  {
    LOG_TRACE << "Connection fd = " << channel_->fd()
              << " is down, no more writing";
  }
}

void TcpConnection::handleClose()
{
  loop_->assertInLoopThread();
  LOG_TRACE << "fd = " << channel_->fd() << " state = " << stateToString();
  assert(state_ == kConnected || state_ == kDisconnecting);
  // we don't close fd, leave it to dtor, so we can find leaks easily.
  setState(kDisconnected);
  channel_->disableAll();

  TcpConnectionPtr guardThis(shared_from_this());
  connectionCallback_(guardThis);
  // must be the last line
  closeCallback_(guardThis);
}

void TcpConnection::handleError()
{
  int err = sockets::getSocketError(channel_->fd());
  LOG_ERROR << "TcpConnection::handleError [" << name_
            << "] - SO_ERROR = " << err << " " << strerror_tl(err);
}

注册到Channel的可读、可写、关闭、出错的回调函数。当事件发生时，由Channel调用。TcpConnection::handleRead()将数据读到输入缓冲区，只要读到数据，就回调用户注册的回调函数；如果读到的数据为长度为0，表示对端关闭了写，所以本端执行关闭操作；如果出错，执行错误处理函数。注意，这里的四个函数和上面的两个，因为都是由TcpServer注册的，所以都使用的shared_from_this()传递的指针。

TcpConnection::handleWrite()也就是在上面的send操作中没有发送完的数据，在这里继续发送。在TcpConnection::handleWrite()中，如果发送完成会立刻取消可写属性，避免一直触发可写事件。还有一个判断，是如果这时候该连接被关闭了，在写完之后会将该连接关闭。也就是说只有在写完存在的数据后，才真正意义上关闭连接。

TcpConnection::handleError()只是单纯的获取了错误信息，并打印。

TcpConnection::handleClose()实际上在设置完状态后，最终调用的closeCallback_(guardThis)是由TcpServer设置的。会执行如下程序：

void TcpServer::removeConnection(const TcpConnectionPtr& conn)
{
  // FIXME: unsafe
  loop_->runInLoop(std::bind(&TcpServer::removeConnectionInLoop, this, conn));
}

void TcpServer::removeConnectionInLoop(const TcpConnectionPtr& conn)
{
  loop_->assertInLoopThread();
  LOG_INFO << "TcpServer::removeConnectionInLoop [" << name_
           << "] - connection " << conn->name();
  size_t n = connections_.erase(conn->name());
  (void)n;
  assert(n == 1);
  EventLoop* ioLoop = conn->getLoop();
  ioLoop->queueInLoop(
      std::bind(&TcpConnection::connectDestroyed, conn));
}

会在TcpServer中将该连接删除，并调用TcpConnection::connectDestroyed()。