Question

I have an Server, using asio with the following code, 100% copied from the examples. The code behaves well when there are no connections. however, as soon as the first client connects, handle accept starts to output vast amounts of system:9 errors and I am unable to connect any other clients. The single client that is connected can perform async read/writes just fine. I’m at a loss at what I’m doing incorrectly. I’ve tried the code with and without io_service in a thread pool with the same results.

Server::Server(io_service& io_service,
      const std::string address,
      const std::string port)
    : io_service_(io_service),
    //zmq_strand_(io_service),
      acceptor_(io_service) {

    ip::tcp::resolver resolver(io_service_);
    ip::tcp::resolver::query query(address, port);
    ip::tcp::endpoint endpoint = *resolver.resolve(query);
    acceptor_.open(endpoint.protocol());
    acceptor_.set_option(ip::tcp::acceptor::reuse_address(true));
    acceptor_.bind(endpoint);
    acceptor_.listen(); 
    start_accept();
  }

void Server::start_accept() {
    auto session = std::shared_ptr<Session>(new Session(io_service_,session_manager_));

    acceptor_.async_accept(session->socket()
        ,std::bind(&Server::handle_accept,this,session,std::placeholders::_1));
}

void Server::handle_accept(std::shared_ptr<Session> session, const system::error_code& error) {
  if (!error) {
    std::cout << "handle_accept" << std::endl;
    session->start();
  } else {
      std::cout << error << std::endl;
  }
  start_accept();
}

small update:
Instead of using the http3 example I used the example Denis proposed. It’s less convoluted and made the server behave correctly. I haven’t found a rational explanation to why the previous piece of code doesn’t work. This does:

Server::Server(
      io_service &service,
      const uint8_t num_threads,
      const std::string address,
      const uint16_t port)
    : 
      num_threads_(num_threads),
    //zmq_strand_(io_service),
      acceptor_(service,ip::tcp::endpoint(ip::tcp::v4(),port)) {

    //io_service::work work(io_service_);
    start_accept();
  }


void Server::start_accept() {
    auto session = std::shared_ptr<Session>(new Session(acceptor_.get_io_service(),session_manager_));

    acceptor_.async_accept(session->socket()
        ,std::bind(&Server::handle_accept,this,session,std::placeholders::_1));
}

void Server::handle_accept(std::shared_ptr<Session> session, const system::error_code& error) {
  if (!error) {
    std::cout << "handle_accept" << std::endl;
    session->start();
  }
  start_accept();
}

void Server::start() {
    std::set<std::unique_ptr<std::thread>> threads;

    for( unsigned int i = 0; i < num_threads_; ++i ) {
        threads.insert(std::unique_ptr<std::thread>(new std::thread( [&]() {
            acceptor_.get_io_service().run();
            }) 
        ));
    }
    std::cout << threads.size() << " threads started" << std::endl;
    for(auto &t:threads) {
        t->join();
    }
    std::cout << threads.size() << " threads joined" << std::endl;
}

Answer 1

Look precisely at this tutorial and compare your code with tutorial: Boost TCP async server example