c++ - 使用 boost-asio 时实时将缓冲区写入磁盘

Question

我有一个由 boost.asio 编写的服务器。该服务器从客户端获取文件并将其写入磁盘。我只是有一个问题。当服务器获取文件时，它会在完全接收到文件后将其写入磁盘。我希望服务器以实时方式将缓冲区写入磁盘。例如，服务器将它从客户端获取的文件每 100kb 大小写入磁盘。我已经编写了以下代码，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);
        });
}

Answer 1

首先，在这种情况下，读取明确大小的数据似乎比读取任何可用数据的 read_some 更好。

在这种模式下，跟踪“剩余可接收字节数”比 m_fileSize 更容易。

这里有一些小的重组，使您的代码成为一个独立的示例。它期望服务器发送一行文本，给出负载大小和输出文件名，然后是该文件的内容。可以使用 netcat 运行示例服务器，例如:

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

Live On Coliru

#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
}

测试

(stat -c '%soutput.dat' main.cpp; cat main.cpp) | netcat -l -p 6868&
./a.out
md5sum main.cpp output.dat

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

表明接收到的文件与原始文件相同。

注意事项:

• 数据包在我的系统上以未指定的大小传送，例如收到相同的文件:

   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
  

  Note that it starts out with 497 bytes already in the input buffer from the read_until.

• 协议(protocol)不安全:
  • 应该验证文件名。想象一下，如果文件是 '/home/sehe/myimportant_file.txt' 或更糟，比如/dev/sde1，并且我们有权进行原始 block 设备访问，会发生什么......
  • 您可能想为 streambuf 指定一个最大大小，这样如果您得到一个永远不会发送 '\n' 的模糊器，您就不会吞噬所有 RAM<
• 文件 IO 的错误处理非常粗糙。我使用了 io 异常，但你可能想在不同的地方检查 m_outputFile.good()