Asio12-HandlePacketStickingProblemSimply
在Asio8中我们处理过粘包的问题，下面的部分的源码：
void CSession::HandleRead(const boost::system::error_code& error, size_t bytes_transferred, shared_ptr<CSession> _self_shared)
{if (!error) {PrintRecvData(_data, bytes_transferred);std::chrono::milliseconds dura(2000);std::this_thread::sleep_for(dura);// 已经移动的字节数:在消息体中int copy_len = 0;while (bytes_transferred > 0) {// 如果还未解析头部if (!_head_parsed) {// 确保头部信息体+待处理的也不会溢出：收到的比头部小if (bytes_transferred + _recv_head_node->_cur_len < HEAD_LENGTH) {memcpy(_recv_head_node->_data + _recv_head_node->_cur_len, _data + copy_len, bytes_transferred);_recv_head_node->_cur_len += bytes_transferred;::memset(_data, 0, MAX_LENGTH);_socket.async_read_some(boost::asio::buffer(_data, MAX_LENGTH), std::bind(&CSession::HandleRead, this, std::placeholders::_1, std::placeholders::_2, _self_shared));return;}// 收到的比头部大，解析头部int head_remain = HEAD_LENGTH - _recv_head_node->_cur_len;memcpy(_recv_head_node->_data + _recv_head_node->_cur_len, _data + copy_len, head_remain);copy_len += head_remain;bytes_transferred -= head_remain;short data_len = 0;memcpy(&data_len, _recv_head_node->_data, HEAD_LENGTH);// 网络字节序转成本地字节序data_len = boost::asio::detail::socket_ops::network_to_host_short(data_len);// data_len = ntohs(data_len);if (data_len > MAX_LENGTH) {std::cout << "data_len" << data_len << std::endl;std::cout << "Data length is too long" << std::endl;_server->ClearSession(_uuid);return;}_recv_msg_node = make_shared<MsgNode>(data_len);// 这时候发现消息长度小于规定长度，数据未收全，先存放到接受节点中if (bytes_transferred < data_len) {memcpy(_recv_msg_node->_data + _recv_msg_node->_cur_len, _data + copy_len, bytes_transferred);_recv_msg_node->_cur_len += bytes_transferred;::memset(_data, 0, MAX_LENGTH);// 头部处理完成_head_parsed = true;_socket.async_read_some(boost::asio::buffer(_data, MAX_LENGTH), std::bind(&CSession::HandleRead, this, std::placeholders::_1, std::placeholders::_2, _self_shared));return;}// 这时候消息长度大于规定长度，数据收全，直接处理memcpy(_recv_msg_node->_data + _recv_msg_node->_cur_len, _data + copy_len, data_len);_recv_msg_node->_cur_len += data_len;copy_len += data_len;bytes_transferred -= data_len;_recv_msg_node->_data[_recv_msg_node->_total_len] = '\0';std::cout << "Received message: " << _recv_msg_node->_data << std::endl;Send(_recv_msg_node->_data, _recv_msg_node->_total_len);_head_parsed = false;_recv_head_node->Clear();if (bytes_transferred <= 0) {::memset(_data, 0, MAX_LENGTH);_socket.async_read_some(boost::asio::buffer(_data, MAX_LENGTH), std::bind(&CSession::HandleRead, this, std::placeholders::_1, std::placeholders::_2, _self_shared));return;}continue;}std::cout << "--------------------" << std::endl;// 已经处理完头部，继续上次未处理完的消息int remain_msg = _recv_msg_node->_total_len - _recv_msg_node->_cur_len;if (bytes_transferred < remain_msg) {memcpy(_recv_msg_node->_data + _recv_msg_node->_cur_len, _data + copy_len, bytes_transferred);_recv_msg_node->_cur_len += bytes_transferred;::memset(_data, 0, MAX_LENGTH);_socket.async_read_some(boost::asio::buffer(_data, MAX_LENGTH), std::bind(&CSession::HandleRead, this, std::placeholders::_1, std::placeholders::_2, _self_shared));return;}memcpy(_recv_msg_node->_data + _recv_msg_node->_cur_len, _data + copy_len, remain_msg);_recv_msg_node->_cur_len += remain_msg;bytes_transferred -= remain_msg;copy_len += remain_msg;_recv_msg_node->_data[_recv_msg_node->_total_len - 1] = '\0';std::cout << "Received message2: " << _recv_msg_node->_data << std::endl;Send(_recv_msg_node->_data, _recv_msg_node->_total_len);_head_parsed = false;_recv_head_node->Clear();if (bytes_transferred <= 0) {::memset(_data, 0, MAX_LENGTH);_socket.async_read_some(boost::asio::buffer(_data, MAX_LENGTH), std::bind(&CSession::HandleRead, this, std::placeholders::_1, std::placeholders::_2, _self_shared));return;}continue;}} else {std::cout << "handle read failed, error is " << error.what() << endl;_server->ClearSession(_uuid);}
}
很明显，将所有不同的情况全部放置在一个函数处理，既复杂，同时也容易出错。原因在于我们异步读的时候使用的是async_read_some,只要接收到信息，asio框架就会触发回调，而这时候就有可能读取到支离破碎（不整体规整）的内容。
因此我们改进我们的做法，我们异步读取的时候使用的是async_read.区别在于这个读的内部实际会多次调用async_read_some,但是只有读取到指定的字节数的时候，才会触发一次回调。这样既解决了多次回调的效率问题，也解决了粘包的问题。
我们的思路是，开始，我们先读取HEAD_LENGTH的字节，用于填充_recv_head_node,解析消息体的长度，然后我们根据这个长度，async_read去接受消息体的内容。读取到指定的字节数之后，触发回调，填充_recv_msg_node,这时候内部的_data就是消息体的内容了。
接下来是代码:
// 这时候我们改动Start函数，开始先要读取两个字节用于解析头
void CSession::Start()
{boost::asio::async_read(_socket, boost::asio::buffer(_recv_head_node->_data, HEAD_LENGTH), std::bind(&CSession::HandleHeader, this, std::placeholders::_1, std::placeholders::_2, SharedSelf()));
}
// HandleHead : 解析头部
void CSession::HandleHeader(const boost::system::error_code& error, size_t bytes_transferred, std::shared_ptr<CSession> shared_self)
{if (error) {if (error == boost::asio::error::eof) {std::cout << "Connection closed by peer" << std::endl;} else {std::cout << "Error reading header: " << error.message()<< ", bytes transferred: " << bytes_transferred << std::endl;}_server->ClearSession(_uuid);return;}short data_len = 0;memcpy(&data_len, _recv_head_node->_data, HEAD_LENGTH);if (data_len > MAX_LENGTH) {std::cout << "invalid data length" << std::endl;_server->ClearSession(_uuid);return;}std::cout << "data_len:" << data_len;_recv_head_node->Clear();_recv_msg_node = make_shared<MsgNode>(data_len);boost::asio::async_read(_socket, boost::asio::buffer(_recv_msg_node->_data, data_len), std::bind(&CSession::HandleMsg, this, std::placeholders::_1, std::placeholders::_2, shared_self));
}// HandleMsg:用于存储消息体
void CSession::HandleMsg(const boost::system::error_code& error, size_t bytes_transferred, std::shared_ptr<CSession> shared_self)
{if (!error) {std::cout << "=" << bytes_transferred << std::endl;PrintRecvData(_recv_msg_node->_data, bytes_transferred);std::this_thread::sleep_for(std::chrono::milliseconds(2000));_recv_msg_node->_data[bytes_transferred] = '\0';std::cout << "Received message: " << _recv_msg_node->_data << std::endl;for (int i = 0; i < bytes_transferred; i++) {_recv_msg_node->_data[i] = toupper(_recv_msg_node->_data[i]);}Send(_recv_msg_node->_data, _recv_msg_node->_total_len);_recv_head_node->Clear();boost::asio::async_read(_socket, boost::asio::buffer(_recv_head_node->_data, HEAD_LENGTH), std::bind(&CSession::HandleHeader, this, std::placeholders::_1, std::placeholders::_2, shared_self));}
}