使用 boost-asio 時實時將緩沖區寫入磁盤

Question

我有一個由 boost.asio 編寫的服務器。 該服務器從客戶端獲取文件並將其寫入磁盤。 我只是有一個問題。 當服務器獲取文件時，它會在完全接收到文件后將其寫入磁盤。 我希望服務器以實時方式將緩沖區寫入磁盤。 例如，服務器將它從客戶端獲取的文件每 100kb 大小寫入磁盤。 我已經編寫了以下代碼，但我不知道如何編輯才能達到這個目標。

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，在這種情況下，讀取明確大小的數據似乎比讀取任何可用數據的read_some更好。

在這種模式下，跟蹤“剩余可接收字節數”比m_fileSize更容易。

這里有一些小的重新洗牌，使您的代碼成為一個獨立的示例。 它期望服務器發送一行文本，給出有效負載大小和 output 文件名，然后是該文件的內容。 示例服務器可以使用 .netcat 運行，例如：

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

生活在 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
}

測試用

(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

最后兩行

b4eec7203f6a1dcbfbf3d298c7ec0832  main.cpp
b4eec7203f6a1dcbfbf3d298c7ec0832  output.dat

表明接收到的文件與原始文件相同。

筆記：

• 數據包以未指定的大小傳送，在我的系統上，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

  請注意，它從read_until的輸入緩沖區中已有的 497 個字節開始。

• 該協議不安全：
  • 應驗證文件名。 想象一下，如果文件是 '/home/sehe/myimportant_file.txt' 或更糟，比如 /dev/sde1 並且我們有權進行原始塊設備訪問，會發生什么......
  • 你可能想為 streambuf 指定一個最大大小，這樣如果你得到一個永遠不會發送'\n'的模糊器，你就不會吞噬所有 RAM
• 文件 IO 的錯誤處理非常粗糙。 我使用了 io 異常，但您可能想在不同的地方檢查m_outputFile.good()