Boost beast::websocket callback functions

Question

I am experimenting with the Boost beast::websocket websocket_client_async.cpp example, in conjunction with websocket_server_async.cpp.

As given, the client example just makes a connection, sends a string to the server (which simply echoes back), prints the reply, closes, and exists.

I'm trying to modify the client to keep the session alive, so that I can repeatedly send/receive strings. So, whereas the example code's on_handshake function immediately sends the string via ws_.async_write(...), I separate that out into its own write(...) function.

Here is my modified session class:

using tcp = boost::asio::ip::tcp;
namespace websocket = boost::beast::websocket;

void fail(boost::system::error_code ec, char const* what)
{
    std::cerr << what << ": " << ec.message() << "\n";
}

// Sends a WebSocket message and prints the response
class session : public std::enable_shared_from_this<session>
{
    tcp::resolver resolver_;
    websocket::stream<tcp::socket> ws_;
    std::atomic<bool> io_in_progress_;
    boost::beast::multi_buffer buffer_;
    std::string host_;

public:
    // Resolver and socket require an io_context
    explicit session(boost::asio::io_context& ioc) : resolver_(ioc), ws_(ioc) {
        io_in_progress_ = false;
    }

    bool io_in_progress() const {
        return io_in_progress_;
    }

    // +---------------------+
    // | The "open" sequence |
    // +---------------------+
    void open(char const* host, char const* port)
    {
        host_ = host;

        // Look up the domain name
        resolver_.async_resolve(host, port,
            std::bind( &session::on_resolve, shared_from_this(),
                std::placeholders::_1, std::placeholders::_2 )
        );
    }

    void on_resolve(boost::system::error_code ec, tcp::resolver::results_type results)
    {
        if (ec)
            return fail(ec, "resolve");

        boost::asio::async_connect(
            ws_.next_layer(), results.begin(), results.end(),
            std::bind( &session::on_connect, shared_from_this(),
                std::placeholders::_1 )
        );
    }

    void on_connect(boost::system::error_code ec)
    {
        if (ec)
            return fail(ec, "connect");

        ws_.async_handshake(host_, "/",
            std::bind( &session::on_handshake, shared_from_this(),
                std::placeholders::_1 )
        );
    }

    void on_handshake(boost::system::error_code ec)
    {
        if (ec)
            return fail(ec, "handshake");
        else {
            std::cout << "Successful handshake with server.\n";
        }
    }

    // +---------------------------+
    // | The "write/read" sequence |
    // +---------------------------+
    void write(const std::string &text)
    {
        io_in_progress_ = true;
        ws_.async_write(boost::asio::buffer(text),
            std::bind( &session::on_write, shared_from_this(),
                std::placeholders::_1, std::placeholders::_2 )
        );
    }

    void on_write(boost::system::error_code ec, std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);
        if (ec)
            return fail(ec, "write");

        ws_.async_read(buffer_,
            std::bind( &session::on_read, shared_from_this(),
                std::placeholders::_1, std::placeholders::_2 )
        );
    }

    void on_read(boost::system::error_code ec, std::size_t bytes_transferred)
    {
        io_in_progress_ = false; // end of write/read sequence
        boost::ignore_unused(bytes_transferred);
        if (ec)
            return fail(ec, "read");

        std::cout << boost::beast::buffers(buffer_.data()) << std::endl;
    }

    // +----------------------+
    // | The "close" sequence |
    // +----------------------+
    void close()
    {
        io_in_progress_ = true;
        ws_.async_close(websocket::close_code::normal,
            std::bind( &session::on_close, shared_from_this(),
                std::placeholders::_1)
        );
    }

    void on_close(boost::system::error_code ec)
    {
        io_in_progress_ = false; // end of close sequence
        if (ec)
            return fail(ec, "close");

        std::cout << "Socket closed successfully.\n";
    }
};

The problem is that, while the connection works fine and I can send a string, the on_read callback is never hit (unless I do an ugly hack described below).

My main looks like this:

void wait_for_io(std::shared_ptr<session> psession, boost::asio::io_context &ioc)
{
    // Continually try to run the ioc until the callbacks are finally
    // triggered (as indicated by the session::io_in_progress_ flag)
    while (psession->io_in_progress()) {
        std::this_thread::sleep_for(std::chrono::milliseconds(1));
        ioc.run();
    }
}

int main(int argc, char** argv)
{
    // Check command line arguments.
    if (argc != 3) {
        std::cerr << "usage info goes here...\n";
        return EXIT_FAILURE;
    }
    const char *host = argv[1], *port = argv[2];

    boost::asio::io_context ioc;
    std::shared_ptr<session> p = std::make_shared<session>(ioc);
    p->open(host, port);
    ioc.run(); // This works. Connection is established and all callbacks are executed.

    p->write("Hello world"); // String is sent & received by server,
                             // even before calling ioc.run()
                             // However, session::on_read callback is never
                             // reached.

    ioc.run();               // This seems to be ignored and returns immediately, so
    wait_for_io(p, ioc);     // <-- so this hack is necessary

    p->close();              // session::on_close is never reached
    ioc.run();               // Again, this seems to be ignored and returns immediately, so
    wait_for_io(p, ioc);     // <-- this is necessary

    return EXIT_SUCCESS;
}

If I do this:

p->write("Hello world");
while(1) {
    std::this_thread::sleep_for(std::chrono::milliseconds(1));
}

I can confirm that the string is sent & received¹ by the server, and that the session::on_read callback is not reached.

Same thing occurs with p->close().

But, if I add my weird wait_for_io() function, everything works. I'm positive this a terrible hack, but I can't figure out what's going on.

¹ Note: I can confirm that the message does reach the server, as I modified the server example to print any received strings to the console. This was the only modified I made. The echo-to-client functionality was not changed.

Answer 1

io_context::run will only return when there is no more pending work. If you simply make sure that there is a pending call to websocket::stream::async_read active at all times, then run will never return and the hacks will not be needed. Also, you will receive all messages sent by the server.

Answer 2

The reason the calls to io_context::run() are not working after the first call (shown here):

boost::asio::io_context ioc;
std::shared_ptr<session> p = std::make_shared<session>(ioc);
p->open(host, port);
ioc.run(); // This works. Connection is established and all callbacks are executed.

is because the function io_context::restart() must be called prior to any subsequent calls of io_context::run.

From the documentation:

io_context::restart

Restart the io_context in preparation for a subsequent run() invocation.

This function must be called prior to any second or later set of invocations of the run(), run_one(), poll() or poll_one() functions when a previous invocation of these functions returned due to the io_context being stopped or running out of work. After a call to restart(), the io_context object's stopped() function will return false.