How to close async client connection in ASIO? - c++

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".