real-time writes buffer to the disk when using boost-asio

First off, in that case it seems better to read explicit sizes of data instead of read_some which reads whatever is available.

In this pattern, it becomes easier to track "remaining bytes receivable" than m_fileSize.

Here's some minor re-shufflings that made your code into a self-contained example. It expects a server to send a line of text giving the payload size and output filename, followed by the contents of that file. An example server can be run with netcat e.g.:

(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
}

Testing with

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

Prints, e.g.:

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

The last two lines

b4eec7203f6a1dcbfbf3d298c7ec0832  main.cpp
b4eec7203f6a1dcbfbf3d298c7ec0832  output.dat

indicate that the received file is identical to the original.

Notes:

• packets are delivered in unspecified sizes, on my system e.g. the same file is received as:

   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.

• The protocol is not secure:
  • the file names should be validated. Just imagine what happens if the file would be '/home/sehe/myimportant_file.txt' or worse, say /dev/sde1 and we have permissions to do raw block device access...
  • you might want to specify a amximum size for streambuf, so that if you get a fuzzer that doesn't ever send a '\n' you wouldn't just gobble up all RAM
• the error handling on file IO is very rough. I used io exceptions, but you probably want to check for m_outputFile.good() instead at various places