[网络协议]boost::beast::websocket 服务器端获取客户端请求的URL相对路径（URL RVA PATH）

?WebSocket，URL格式：

WS协议://{主机}:{端口}{RVA PATH}，比如：ws://121.40.165.18:8800

协议：WS（HTTP）、WSS（HTTPS），本文则提示 boost::beast::websocket 如何获取 {RVA PATH} 的内容，否则按照?boost::beast::websocket 提供的默认实现，会 accept 任何 RVA_PATH的WS连接请求，按照正常的WS服务器设计是不被允许的。

可用于调试 boost::beast::websocket 服务器端的在线测试网页。

websocket在线测试 (websocket-test.com)

#include <boost/beast/core.hpp>
#include <boost/beast/websocket.hpp>
#include <boost/beast.hpp>
#include <boost/beast/http.hpp>
#include <boost/beast/http/empty_body.hpp>
#include <boost/beast/http/fields.hpp>
#include <boost/beast/http/message.hpp>
#include <boost/beast/http/string_body.hpp>
#include <boost/asio/ip/tcp.hpp>

#include <cstdlib>
#include <functional>
#include <iostream>
#include <string>
#include <thread>

using tcp = boost::asio::ip::tcp;
namespace websocket = boost::beast::websocket;

// echoes back all received WebSocket messages
void do_session(tcp::socket& socket)
{
    try {
        // Construct the stream by moving in the socket
        websocket::stream<tcp::socket> ws{ std::move(socket) };

        // This buffer is used for reading and must be persisted
        boost::beast::flat_buffer buffer;

        // Declare a container to hold the response
        boost::beast::http::request<boost::beast::http::dynamic_body> req;

        // Receive the HTTP response
        boost::system::error_code ec;
        boost::beast::http::read(ws.next_layer(), buffer, req, ec);
        if (ec == boost::beast::http::error::end_of_stream)
            ec = boost::beast::websocket::error::closed;
        if (ec)
            return;

        // Accept the websocket handshake
        ws.accept(req);

        // Print path.
        printf("path: %s\n", std::string(req.target().data(), req.target().length()).data());
        for (;;) {
            // This buffer will hold the incoming message
            boost::beast::multi_buffer buffer;

            // Read a message
            ws.read(buffer);

            // Echo the message back
            ws.text(ws.got_text());
            ws.write(buffer.data());
        }
    }
    catch (boost::system::system_error const& se) {
        // This indicates that the session was closed
        if (se.code() != websocket::error::closed)
            std::cerr << "Error: " << se.code().message() << std::endl;
    }
    catch (std::exception const& e) {
        std::cerr << "Error: " << e.what() << std::endl;
    }
}

int main(int argc, char* argv[])
{
    try {
        // Check command line arguments
        if (argc != 3) {
            std::cerr <<
                "Usage: server <address> <port>\n" <<
                "Example:\n" <<
                "    server 0.0.0.0 8080\n";
            return EXIT_FAILURE;
        }
        auto const address = boost::asio::ip::make_address(argv[1]);
        auto const port = static_cast<unsigned short>(std::atoi(argv[2]));

        // The io_context is required for all I/O
        boost::asio::io_context ioc{ 1 };

        // The acceptor receives incoming connections
        tcp::acceptor acceptor{ ioc, {address, port} };
        for (;;) {
            // This will receive the new connection
            tcp::socket socket{ ioc };

            // Block until we get a connection
            acceptor.accept(socket);

            // Launch the session, transferring ownership of the socket
            std::thread{ std::bind(&do_session, std::move(socket)) }.detach();
        }
    }
    catch (const std::exception& e) {
        std::cerr << "Error: " << e.what() << std::endl;
        return EXIT_FAILURE;
    }
}