boost::asio: how can I make some clients listen to server and other client read/write to server at the same time - boost-asio

Question

I am a novice about boost::asio, I write a server, some clients can connect to it and keep listening.
class socket_server {
public:
~socket_server() { io_context.stop(); };
int server_process();
private:
boost::asio::io_context io_context;
};
int socket_server::server_process() {
try {
unlink("/var/run/socket");
server s(io_context, "/var/run/socket");
INFO("server_process, start run\n");
io_context.run();
} catch (std::exception &e) {
std::cerr << "Exception: " << e.what() << "\n";
}
return 0;
}
class server {
public:
server(boost::asio::io_context &io_context, const std::string &file)
: acceptor_(io_context, stream_protocol::endpoint(file)), socket_id_(0) {
do_accept();
}
private:
void do_accept();
stream_protocol::acceptor acceptor_;
int socket_id_;
};
void server::do_accept() {
INFO("do accept\n");
acceptor_.async_accept(
[this](std::error_code ec, stream_protocol::socket socket) {
if (!ec) {
INFO("new session create\n");
std::make_shared<session>(std::move(socket), socket_id_++)->start();
}
do_accept();
});
}
class session : public std::enable_shared_from_this<session> {
public:
session(stream_protocol::socket sock, int socket_id)
: socket_(std::move(sock)), socket_id_(socket_id) {}
~session() { socket_id_--; }
void start();
private:
void do_read();
void do_write(std::array<char, 1024> data);
int get_id() { return socket_id_; }
// The socket used to communicate with the client.
stream_protocol::socket socket_;
// Buffer used to store data received from the client.
std::array<char, 1024> data_;
int socket_id_;
};
void session::start() { do_read(); }
void session::do_read() {
INFO("in do_read\n");
auto self(shared_from_this());
socket_.async_read_some(
boost::asio::buffer(data_),
[this, self](std::error_code ec, std::size_t length) {
if (!ec) {
if (request.find("listen") != std::string::npos) {
std::unique_lock<std::mutex> lock(unsol_mutex);
unsol_cond.wait(lock)
do_write(get_unsol_data());
} else {
std::unique_lock<std::mutex> lock(send_mutex);
if (send_cond.wait_for(lock, std::chrono::seconds(2)) ==
std::cv_status::timeout) {
ERROR("response time out\n");
}
do_write(get_write_data());
}
}
});
}
In do_read(), I found when a client is listening (block in unsol_cond.wait(lock)), another client can not go to do_read().
Is it due to make_shared session? Is there a better implementation suggestion?
Thanks~

Answer 1

You're using blocking synchronization primitives in async code. That's an anti-pattern.
Firstly, as you noticed, the blocking operations will prevent the event loop from progressing.
Secondly, holding locks across async calls is often a bug (it doesn't guard the critical execution during execution of the async operation).
For simple integration with Asio proactor model, you can often
use a strand instead.
Under the hood, it will end up using mutexes, just like now, but only
if the concurrency model requires it. That mainly depends on the
execution context used and/or how many threads are running the
services.
Use a queue with a async send-chain. I have quite a few answers on this site that show you how to do that.
I would gladly demonstrate, but the code is too incomplete, and the naming doesn't really give me an idea what things mean ("listen"/"unsol"?, nothing ever signals those conditions so... hard to guess what they do in reality)