C++ 使用boost asio时将缓冲区实时写入磁盘

我有一个由boost.asio编写的服务器。此服务器从客户端获取文件并将其写入磁盘。我只是有个问题。当服务器获取文件时，它会在完全接收文件时将其写入磁盘。我希望服务器以实时方式将缓冲区写入磁盘。例如，服务器从客户端获取的文件每100kb写入一次磁盘。我已经写了以下代码，但我不知道如何编辑才能达到这个目标

void Session::DoReadFileContent(size_t arg_bytes_transferred)
{
    if (arg_bytes_transferred > 0)
    {
        m_outputFile.write(m_buffer.data(), static_cast<std::streamsize>(arg_bytes_transferred));

        if (m_outputFile.tellp() >= static_cast<std::streamsize>(m_fileSize))
        {
            std::cout << "Received file: " << m_fileName << std::endl;
            return;
        }
    }

    auto self = shared_from_this();

    m_socket.async_read_some(boost::asio::buffer(m_buffer.data(), m_buffer.size()),
        [this, self](boost::system::error_code arg_error_code, size_t arg_bytes)
        {
            DoReadFileContent(arg_bytes);
        });
}

void Session:：DoReadFileContent（大小参数字节数）
{
如果（参数字节数>0）
{
m_outputFile.write（m_buffer.data（），静态_转换（arg_bytes_transfer））；
if（m_outputFile.tellp（）>=static_cast（m_fileSize））
{
std：：cout首先，在这种情况下，最好读取显式大小的数据，而不是读取任何可用数据的
read_some

在这种模式中，跟踪“剩余可接收字节”比跟踪m_fileSize

更容易

下面是一些小的重新洗牌，使您的代码成为一个独立的示例。它要求服务器发送一行文本，给出有效负载大小和输出文件名，然后是该文件的内容。示例服务器可以使用netcat运行，例如：

(stat -c '%soutput.dat' main.cpp; cat main.cpp) | netcat -l -p 6969

#include <boost/asio.hpp>
#include <fstream>
#include <iostream>

using boost::system::error_code;
using boost::asio::ip::tcp;

struct Session : std::enable_shared_from_this<Session> {

    Session(boost::asio::io_context& io, uint16_t port)
     : m_socket(io) 
    {
        m_socket.connect({{}, port});
    }

    void Start();
    void DoReadFileContent(size_t transferred = 0);

  private:
    std::array<char, 1024> m_buffer;
    std::streamsize m_remainingSize = 0;
    std::string     m_fileName      = "noname.dat";
    std::ofstream   m_outputFile;

    tcp::socket m_socket;
};

void Session::Start() {
    // Reading a size (in text for simplicity) and subsequently receive as many bytes
    //
    // I'm keeping this sync for simplicity, because you probably already have
    // this coded somehwere
    boost::asio::streambuf buf;
    error_code ec;
    auto n = read_until(m_socket, buf, "\n", ec);

    std::istream is(&buf);
    if (is >> m_remainingSize && getline(is, m_fileName)) {
        std::cerr << "Protocol trace: n:" << n << ", fileName:" << m_fileName << " payload_size:" << m_remainingSize << "\n";

        m_outputFile.exceptions(std::ios::failbit | std::ios::badbit);
        m_outputFile.open(m_fileName, std::ios::binary);

        // write excess buffer contents as part of payload
        if (buf.size()) {
            std::cerr << "Writing " << buf.size() << " bytes\n";
            m_remainingSize -= buf.size();
            m_outputFile << &buf;
        }

        DoReadFileContent();
    } else {
        std::cerr << "Protocol error, payload_size expected\n";
    }
}
void Session::DoReadFileContent(size_t transferred) {
    if (transferred > 0) {
        std::cerr << "Writing " << transferred << " bytes\n";
        m_remainingSize -= transferred;
        m_outputFile.write(m_buffer.data(), transferred);
    }
    if (m_remainingSize <= 0) {
        std::cout << "Completed file: " << m_fileName << std::endl;
        return;
    }

    auto self = shared_from_this();
    auto expect = std::min(size_t(m_remainingSize), m_buffer.size());
    std::cout << "Trying to receive next " << expect << " bytes" << std::endl;
    async_read(m_socket,
        boost::asio::buffer(m_buffer.data(), expect),
        [this, self](error_code ec, size_t arg_bytes) {
            std::cerr << "async_read: " << ec.message() << " - " << arg_bytes << " bytes\n";
            if (!ec) {
                DoReadFileContent(arg_bytes);
            }
        });
}

int main() {
    boost::asio::io_context io;

    std::make_shared<Session>(io, 6868) // download from port 6868
        ->Start();

    io.run(); // complete
}

打印，例如：

Protocol trace: n:15, fileName:output.dat payload_size:2654
Trying to receive next 1024 bytes
async_read: Success - 1024 bytes
Writing 1024 bytes
Trying to receive next 1024 bytes
async_read: Success - 1024 bytes
Writing 1024 bytes
Trying to receive next 606 bytes
async_read: Success - 606 bytes
Writing 606 bytes
Completed file: output.dat

最后两行

b4eec7203f6a1dcbfbf3d298c7ec0832  main.cpp
b4eec7203f6a1dcbfbf3d298c7ec0832  output.dat

指示接收到的文件与原始文件相同

笔记：

• 数据包在我的系统上以未指定的大小发送，例如，接收的文件与以下文件相同：

   Protocol trace: n:15, fileName:output.dat payload_size:2654
   Writing 497 bytes
   Trying to receive next 1024 bytes
   async_read: Success - 1024 bytes
   Writing 1024 bytes
   Trying to receive next 1024 bytes
   async_read: Success - 1024 bytes
   Writing 1024 bytes
   Trying to receive next 109 bytes
   async_read: Success - 109 bytes
   Writing 109 bytes
   Completed file: output.dat
   b4eec7203f6a1dcbfbf3d298c7ec0832  main.cpp
   b4eec7203f6a1dcbfbf3d298c7ec0832  output.dat
  

  请注意，它从

  read\u到

  ，在输入缓冲区中已经有497个字节开始

• 该协议不安全：
  • 应该验证文件名。想象一下，如果文件名为“/home/sehe/myimportant_file.txt”或更糟，比如/dev/sde1，并且我们有权访问原始块设备，会发生什么情况
  • 您可能希望为streambuf指定一个最大大小，这样，如果您得到一个fuzzer，它不会发送

    '\n'

    ，您就不会占用所有RAM
• 文件IO上的错误处理非常粗糙。我使用了IO异常，但您可能希望在不同的位置检查

  m\u outputFile.good（）

 Protocol trace: n:15, fileName:output.dat payload_size:2654
 Writing 497 bytes
 Trying to receive next 1024 bytes
 async_read: Success - 1024 bytes
 Writing 1024 bytes
 Trying to receive next 1024 bytes
 async_read: Success - 1024 bytes
 Writing 1024 bytes
 Trying to receive next 109 bytes
 async_read: Success - 109 bytes
 Writing 109 bytes
 Completed file: output.dat
 b4eec7203f6a1dcbfbf3d298c7ec0832  main.cpp
 b4eec7203f6a1dcbfbf3d298c7ec0832  output.dat