How to close async client connection in ASIO?

Question

I'm trying to create a client for the C++ 20 server example, the one that uses coroutines.

I'm not quite sure how I'm supposed to close the client connection. As far as I'm aware, there are two ways:

#1

This one seems to be closing it once it's ready/there is nothing else to do like read/write operations.

asio::signal_set signals(io_context, SIGINT, SIGTERM);
signals.async_wait([&](auto, auto) { io_context.stop(); });

#2

Force close?

asio::post(io_context_, [this]() { socket_.close(); });

Which one should I use?

Client code (unfinished)

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

#include <asio.hpp>

using asio::ip::tcp;
using asio::awaitable;
using asio::co_spawn;
using asio::detached;
using asio::redirect_error;
using asio::use_awaitable;

awaitable<void> connect(tcp::socket socket, const tcp::endpoint& endpoint)
{
    co_await socket.async_connect(endpoint, use_awaitable);
}

int main()
{
    try
    {
        asio::io_context io_context;
        tcp::endpoint endpoint(asio::ip::make_address("127.0.0.1"), 666);
        tcp::socket socket(io_context);

        co_spawn(io_context, connect(std::move(socket), endpoint), detached);

        io_context.run();
    }
    catch (std::exception& e)
    {
        std::cerr << "Exception: " << e.what() << "\n";
    }

    return 0;
}

Server code

#include <cstdlib>
#include <deque>
#include <iostream>
#include <list>
#include <memory>
#include <set>
#include <string>
#include <utility>
#include <asio/awaitable.hpp>
#include <asio/detached.hpp>
#include <asio/co_spawn.hpp>
#include <asio/io_context.hpp>
#include <asio/ip/tcp.hpp>
#include <asio/read_until.hpp>
#include <asio/redirect_error.hpp>
#include <asio/signal_set.hpp>
#include <asio/steady_timer.hpp>
#include <asio/use_awaitable.hpp>
#include <asio/write.hpp>

using asio::ip::tcp;
using asio::awaitable;
using asio::co_spawn;
using asio::detached;
using asio::redirect_error;
using asio::use_awaitable;

//----------------------------------------------------------------------

class chat_participant
{
public:
    virtual ~chat_participant() = default;
    virtual void deliver(const std::string& msg) = 0;
};

typedef std::shared_ptr<chat_participant> chat_participant_ptr;

//----------------------------------------------------------------------

class chat_room
{
public:
    void join(chat_participant_ptr participant)
    {
        participants_.insert(participant);
        for (const auto &msg : recent_msgs_)
            participant->deliver(msg);
    }

    void leave(chat_participant_ptr participant)
    {
        participants_.erase(participant);
    }

    void deliver(const std::string& msg)
    {
        recent_msgs_.push_back(msg);
        while (recent_msgs_.size() > max_recent_msgs)
            recent_msgs_.pop_front();

        for (const auto &participant : participants_)
            participant->deliver(msg);
    }

private:
    std::set<chat_participant_ptr> participants_;
    enum { max_recent_msgs = 100 };
    std::deque<std::string> recent_msgs_;
};

//----------------------------------------------------------------------

class chat_session
    : public chat_participant,
    public std::enable_shared_from_this<chat_session>
{
public:
    chat_session(tcp::socket socket, chat_room& room)
        : socket_(std::move(socket)),
        timer_(socket_.get_executor()),
        room_(room)
    {
        timer_.expires_at(std::chrono::steady_clock::time_point::max());
    }

    void start()
    {
        room_.join(shared_from_this());

        co_spawn(socket_.get_executor(),
            [self = shared_from_this()]{ return self->reader(); },
            detached);

        co_spawn(socket_.get_executor(),
            [self = shared_from_this()]{ return self->writer(); },
            detached);
    }

    void deliver(const std::string& msg) override
    {
        write_msgs_.push_back(msg);
        timer_.cancel_one();
    }

private:
    awaitable<void> reader()
    {
        try
        {
            for (std::string read_msg;;)
            {
                std::size_t n = co_await asio::async_read_until(socket_,
                    asio::dynamic_buffer(read_msg, 1024), "\n", use_awaitable);

                room_.deliver(read_msg.substr(0, n));
                read_msg.erase(0, n);
            }
        }
        catch (std::exception&)
        {
            stop();
        }
    }

    awaitable<void> writer()
    {
        try
        {
            while (socket_.is_open())
            {
                if (write_msgs_.empty())
                {
                    asio::error_code ec;
                    co_await timer_.async_wait(redirect_error(use_awaitable, ec));
                }
                else
                {
                    co_await asio::async_write(socket_,
                        asio::buffer(write_msgs_.front()), use_awaitable);
                    write_msgs_.pop_front();
                }
            }
        }
        catch (std::exception&)
        {
            stop();
        }
    }

    void stop()
    {
        room_.leave(shared_from_this());
        socket_.close();
        timer_.cancel();
    }

    tcp::socket socket_;
    asio::steady_timer timer_;
    chat_room& room_;
    std::deque<std::string> write_msgs_;
};

//----------------------------------------------------------------------

awaitable<void> listener(tcp::acceptor acceptor)
{
    chat_room room;

    for (;;)
    {
        std::make_shared<chat_session>(co_await acceptor.async_accept(use_awaitable), room)->start();
    }
}

//----------------------------------------------------------------------

int main()
{
    try
    {
        unsigned short port = 666;

        asio::io_context io_context(1);

        co_spawn(io_context,
            listener(tcp::acceptor(io_context, { tcp::v4(), port })),
            detached);

        asio::signal_set signals(io_context, SIGINT, SIGTERM);
        signals.async_wait([&](auto, auto) { io_context.stop(); });

        io_context.run();
    }
    catch (std::exception& e)
    {
        std::cerr << "Exception: " << e.what() << "\n";
    }

    return 0;
}

Answer 1

In the example provided by asio, the listener runs within the io_context thread/thread-pool, which is started by run() and given a thread-pool size when constructing the io_context(1 /* pool of 1 */).

The listener will use an acceptor to listen for new connections from within the io_context. The acceptor will create a new chat_session for each new socket connection and will hand it over to the chat_room.

Thus, to safely close a connection, you need to post a lambda to asio. The asio::post will queue the lambda to be done from within the io_context thread(s).

You need to provided the correct io_context and the socket owned by the chat_session. The connection MUST be closed from within the io_context as follows:

// Where "this" is the current chat_session owning the socket
asio::post(io_context_, [this]() { socket_.close(); });

The io_context wil then close the connection and also call any active registered async_read / async_write methods of the chat_session such as in the c++11 example:

void do_read()
  {
    asio::async_read(socket_,
        asio::buffer(read_msg_.data(), chat_message::header_length),
      /* You can provide a lambda to be called on a read / error */
        [this](std::error_code ec, std::size_t /*length read*/)
        {
          if (!ec)
          {
            do_read(); // No error -> Keep on reading
          }
          else
          {
          // You'll reach this point if an active async_read was stopped
          // due to an error or if you called socket_.close()

          // Error -> You can close the socket here as well, 
          // because it is called from within the io_context
            socket_.close(); 
          }
        });
  }

Your first option will actually stop the entire io_context. This should be used to gracefully exit your program or stop the asio io_context as a whole.

You should thus use the second option to "close an async client connection in ASIO".