boost async_accept不能与boost asio use_future选项一起使用

Question

我想在超时的情况下听boost::asio::ip::tcp::socket。为此，我使用了std::future::wait_for函数。下面是我的代码：

std::optional<boost::asio::ip::tcp::socket> server::listen()
{
    boost::asio::ip::tcp::socket sock(io_service);
    std::future<void> accept_status = acceptor.async_accept(
        sock, boost::asio::use_future);
    if (accept_status.wait_for(std::chrono::seconds(10)) == std::future_status::timeout)
    {
        // I hope there's no race-condition between
        // accepting a connection and calling cancel
        acceptor.cancel();
        std::cerr << "Timeout" << std::endl;
        return {};
    }
    std::cerr << "Accepted a connection" << std::endl;
    return {std::move(sock)};
}

但这不起作用:客户端可以连接，但我仍然超时。这意味着将来的对象和异步接受函数不会通信。我遗漏了什么？

我正在使用Boost版本的1.65。

对于Explorer_N，以下是一个完整的程序，它的工作方式与我预期的不同：

#include <boost/asio.hpp>
#include <boost/asio/use_future.hpp>
#include <chrono>
#include <future>
#include <iostream>
#include <thread>

using namespace std;

void server_listen() {
    boost::asio::io_service io_service;
    boost::asio::ip::tcp::endpoint endpoint(boost::asio::ip::tcp::v4(), 31132);
    boost::asio::ip::tcp::acceptor acceptor(io_service, endpoint);
    boost::asio::ip::tcp::socket socket(io_service);
    std::future<void> accept_status = acceptor.async_accept(
            socket, boost::asio::use_future);
    while(true) {
        if(accept_status.wait_for(std::chrono::seconds(10)) == std::future_status::timeout) {
            acceptor.cancel();
            std::cerr << "Timeout\n";
        } else {
            break;
        }
    }
    // if I replace the lines starting from the async_accept call
    // by just the following, everything works as expected
    // acceptor.accept(socket);
    std::cout << "Accepted a connection\n";
    while(true) {
    }
}

void client_connect() {
    boost::asio::io_service io_service;
    boost::asio::ip::tcp::resolver resolver(io_service);
    boost::asio::ip::tcp::socket socket(io_service);
    boost::asio::ip::tcp::endpoint endpoint(*resolver.resolve({"127.0.0.1", std::to_string(31132)}));
    socket.connect(endpoint);
    std::cout << "Connected to server\n";
    while(true) {
    }
}

int main() {
    std::thread server(server_listen);
    std::this_thread::sleep_for(std::chrono::seconds(2));
    std::thread client(client_connect);
    while(true) {
    }
}

由g++ -std=c++17 <program>.cpp -lpthread -lboost_system -o <program>编译。

我得到的输出是：

Connected to server
Timeout
Timeout
Timeout
Timeout
Timeout
Timeout
Timeout
Timeout
Timeout
Timeout
Timeout
Timeout
Timeout
...

Answer 1

要回答您的申请，请执行以下操作：

“未来的对象和异步接受函数无法通信”--这是不可能的。

“客户端能够连接，但我仍然获得超时。”，--您的客户端连接到侦听器是一个事件，而执行完成处理程序(设置promise)是另一个事件。

因此，连接可以在第9秒接受，而回调将安排在第11秒运行(例如)。

请记住，我们正在处理异步操作，因此对未来事件进行绝对预测是不正确的。

除此之外，

// I hope there's no race-condition between
        // accepting a connection and calling cancel
        acceptor.cancel();
        std::cerr << "Timeout" << std::endl;
        return {};

acceptor.cancel();只是收集挂起的等待程序，并在ec设置为operation_aborted的情况下完成它们，如果处理程序已经到达完成事件队列，那么cancel()就是一个no-op

根据OP最近的编辑扩展我的答案：

using namespace std;

void server_listen() {
    boost::asio::io_service io_service;
    boost::asio::ip::tcp::endpoint endpoint(boost::asio::ip::tcp::v4(), 31132);
    boost::asio::ip::tcp::acceptor acceptor(io_service, endpoint);
    boost::asio::ip::tcp::socket socket(io_service);
    auto work = make_work_guard(io_service);
    using type= std::decay_t<decltype(work)>;
    std::thread io([&](){io_service.run();});
    std::future<void> accept_status = acceptor.async_accept(
            socket, boost::asio::use_future);
    if(accept_status.wait_for(std::chrono::seconds(10)) == std::future_status::timeout) {
        acceptor.cancel();
        std::cerr << "Timeout\n";
        work.~type();
        //break;
    } else {
        std::cout<<"future is ready\n";
        work.~type();
       // break;
    }

    io.join();
    // if I replace the lines starting from the async_accept call
    // by just the following, everything works as expected
    // acceptor.accept(socket);
    std::cout << "Accepted a connection\n";

}

void client_connect() {
    boost::asio::io_service io_service;
    boost::asio::ip::tcp::resolver resolver(io_service);
    boost::asio::ip::tcp::socket socket(io_service);
    boost::asio::ip::tcp::endpoint endpoint(*resolver.resolve({"127.0.0.1", std::to_string(31132)}));
    socket.connect(endpoint);
    std::cout << "Connected to server\n";

}
enter code here
int main() {
    std::thread server(server_listen);
    std::this_thread::sleep_for(std::chrono::seconds(2));
    std::thread client(client_connect);
    server.join(); client.join();
}

在你的程序中有许多事情需要注意(避免不必要的自旋循环，不要忘记加入或分离std::thread，并确保在使用async*版本时调用io_service::run )

Start
Connected to server
future is ready
Accepted a connection
0
Finish