basic example with boost beast and coroutines how to co_await a response

There were just a few coro-beginner mistakes:

• missing co_await on the this_coro::executor transform keyword
• missing std::move on the task awaitable in co_await std::move(task)

I think that was basically that. So, then I removed the unncessary io_context& reference, and arrive at the following cleaned-up listing. Among the changes I made is replacing use_awaitable with the more efficient asio::deferred -- which is the default completion token in recent boost.

#include <boost/asio.hpp>
#include <boost/beast.hpp>
#include <boost/beast/ssl.hpp>
#include <boost/certify/extensions.hpp>         // added
#include <boost/certify/https_verification.hpp> // added
#include <iostream>
#include <syncstream> // added

namespace asio      = boost::asio;
namespace this_coro = asio::this_coro; // added
namespace beast     = boost::beast;    // from <boost/beast.hpp>
namespace http      = beast::http;     // from <boost/beast/http.hpp>
namespace ssl       = asio::ssl;       // from <boost/asio/ssl.hpp>
using asio::ip::tcp;                   // from <boost/asio/ip/tcp.hpp>
using beast::error_code;               // from <boost/beast/core.hpp>
using namespace std::chrono_literals;

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

using SessionResult = void;

// Performs an HTTP GET and prints the response
asio::awaitable<SessionResult> do_session( //
    std::string host, std::string port, std::string target, ssl::context& ctx) {

    auto ex = co_await this_coro::executor;

    // These objects perform our I/O
    tcp::resolver resolver(ex);
    beast::ssl_stream<beast::tcp_stream> stream(ex, ctx);

    // Set SNI Hostname (many hosts need this to handshake successfully)
    if (!::SSL_set_tlsext_host_name(stream.native_handle(), host.c_str())) {
        std::cerr << beast::error_code(static_cast<asio::error::ssl_errors>(::ERR_get_error())).message()
                  << "\n";
        co_return;
    }

    try {
        // Look up the domain name
        auto const results = co_await resolver.async_resolve(host, port);

        // Set the timeout.
        beast::get_lowest_layer(stream).expires_after(30s);

        // Make the connection on the IP address we get from a lookup
        co_await beast::get_lowest_layer(stream).async_connect(results);

        // Set the timeout.
        beast::get_lowest_layer(stream).expires_after(30s);

        // Perform the SSL handshake
        co_await stream.async_handshake(ssl::stream_base::client);

        // Set up an HTTP GET request message
        http::request<http::string_body> req{http::verb::get, target, 11};
        req.set(http::field::host, host);
        req.set(http::field::user_agent, BOOST_BEAST_VERSION_STRING);

        // Set the timeout.
        beast::get_lowest_layer(stream).expires_after(30s);

        // Send the HTTP request to the remote host
        co_await http::async_write(stream, req);

        // This buffer is used for reading and must be persisted
        beast::flat_buffer b;

        // Declare a container to hold the response
        http::response<http::dynamic_body> res;

        // Receive the HTTP response
        auto [ec, bytes] =
            co_await http::async_read(stream, b, res, asio::as_tuple(asio::deferred));

        // Write the message to standard out
        std::cout << res << std::endl;

        // Set the timeout.
        beast::get_lowest_layer(stream).expires_after(30s);

        if(ec == asio::error::eof)
        {
            // eof is to be expected for some services
            if(ec != asio::error::eof)
                throw beast::system_error(ec);
        }
        else
        {
            std::cout << res << std::endl;

            // Gracefully close the stream
            co_await stream.async_shutdown();
        }

    } catch (beast::system_error const& se) {
        //std::cerr << "Handled: " << se.code().message() << "\n";
        throw; // handled at the spawn site instead
    }

    // If we get here then the connection is closed gracefully
}

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

asio::awaitable<SessionResult> task_main_GET() {
    auto const host = "https://microsoftedge.github.io/Demos/json-dummy-data/64KB.json";
    auto const port = "443";
    auto const target = "/";

    std::osyncstream(std::cout) << "GET " << host << ":" << port << target << std::endl;
    // The SSL context is required, and holds certificates
    ssl::context ctx{ssl::context::tlsv12_client};

    // https://stackoverflow.com/a/61429566/2366975
    ctx.set_verify_mode(ssl::context::verify_peer);
    boost::certify::enable_native_https_server_verification(ctx);

    auto ex   = co_await this_coro::executor;
    auto task = co_spawn(
        ex,
        [&] -> asio::awaitable<std::string> {
            co_await do_session(host, port, target, ctx);
            co_return "some value, then";
        },
        asio::deferred);

    std::osyncstream(std::cout) << "response:\n" << co_await std::move(task) << std::endl;

    co_return; // TODO SessionResult
}

int main() try {
    asio::io_context ioc;

    co_spawn(ioc, task_main_GET, asio::detached);

    ioc.run();
} catch (std::exception const& e) {
    std::cerr << "Error: " << e.what() << "\n";
} catch (...) {
    std::cerr << "Unknown error\n";
}

This compiles (GCC 14, Boost 1.88). However, there are some questions I have:

• your eof check is never reached because it's in the opposite if branch

• your host is not a host but the entire https://.... url. oops, that cannot resolve. Fixing:

   auto const host   = "microsoftedge.github.io";
   auto const port   = "443";
   auto const target = "/Demos/json-dummy-data/64KB.json";
  

• why the co_spawn when you immediately co_await it anyways? You could simply replace

   auto ex   = co_await this_coro::executor;
   auto task = co_spawn(
       ex,
       [&] -> asio::awaitable<std::string> {
           co_await do_session(host, port, target, ctx);
           co_return "some value, then";
       },
       asio::deferred);
  
   std::osyncstream(std::cout) << "response:\n" << co_await std::move(task) << std::endl;
  

  With

  SessionResult data = co_await do_session(host, port, target, ctx);
  std::osyncstream(std::cout) << "response:\n" << data << std::endl;
  

• On my system that host's certificate doesn't verify with enable_native_https_server_verification. However, it does so with Asio's own

  // boost::certify::enable_native_https_server_verification(ctx);
  ctx.set_default_verify_paths();
  

Fixing those (and probably some more minor things I forgot):

Live On Coliru

#include <boost/asio.hpp>
#include <boost/beast.hpp>
#include <boost/beast/ssl.hpp>
#include <iostream>
#include <syncstream> // added

namespace asio      = boost::asio;
namespace this_coro = asio::this_coro; // added
namespace beast     = boost::beast;    // from <boost/beast.hpp>
namespace http      = beast::http;     // from <boost/beast/http.hpp>
namespace ssl       = asio::ssl;       // from <boost/asio/ssl.hpp>
using asio::ip::tcp;                   // from <boost/asio/ip/tcp.hpp>
using beast::error_code;               // from <boost/beast/core.hpp>
using namespace std::chrono_literals;

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

using SessionResult = std::string;

// Performs an HTTP GET and prints the response
asio::awaitable<SessionResult> do_session( //
    std::string host, std::string port, std::string target, ssl::context& ctx) {

    auto ex = co_await this_coro::executor;

    // These objects perform our I/O
    tcp::resolver resolver(ex);
    beast::ssl_stream<beast::tcp_stream> stream(ex, ctx);

    // Set SNI Hostname (many hosts need this to handshake successfully)
    if (!::SSL_set_tlsext_host_name(stream.native_handle(), host.c_str()))
        throw beast::system_error(static_cast<asio::error::ssl_errors>(::ERR_get_error()));

    // Look up the domain name
    auto const results = co_await resolver.async_resolve(host, port);

    // Set the timeout.
    beast::get_lowest_layer(stream).expires_after(30s);

    // Make the connection on the IP address we get from a lookup
    co_await beast::get_lowest_layer(stream).async_connect(results);

    // Perform the SSL handshake
    co_await stream.async_handshake(ssl::stream_base::client);

    // Set up an HTTP GET request message
    http::request<http::string_body> req{http::verb::get, target, 11};
    req.set(http::field::host, host);
    req.set(http::field::user_agent, BOOST_BEAST_VERSION_STRING);
    req.set(http::field::connection, "close");

    // Send the HTTP request to the remote host
    co_await http::async_write(stream, req);

    // This buffer is used for reading and must be persisted
    beast::flat_buffer b;

    // Declare a container to hold the response
    http::response<http::string_body> res;

    // Receive the HTTP response
    auto [ec, bytes] = co_await http::async_read(stream, b, res, asio::as_tuple);

    // eof is to be expected for some services
    if (ec && ec != asio::error::eof)
        throw beast::system_error(ec);

    // Write the message to standard out
    // std::cout << res << std::endl;

    // Gracefully close the stream
    std::tie(ec) = co_await stream.async_shutdown(asio::as_tuple);
    if (ec)
        std::cerr << "Shutdown: " << ec.message() << "\n";

    // If we get here then the connection is closed gracefully
    co_return std::move(res).body();
}

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

asio::awaitable<SessionResult> GET() {
    auto const host   = "microsoftedge.github.io";
    auto const port   = "443";
    auto const target = "/Demos/json-dummy-data/64KB.json";

    std::osyncstream(std::cerr) << "GET " << host << ":" << port << target << std::endl;
    // The SSL context is required, and holds certificates
    ssl::context ctx{ssl::context::tlsv12_client};

    // https://stackoverflow.com/a/61429566/2366975
    ctx.set_verify_mode(ssl::context::verify_peer);
    // boost::certify::enable_native_https_server_verification(ctx);
    ctx.set_default_verify_paths();

    co_return co_await do_session(host, port, target, ctx);
}

int main() try {
    asio::io_context ioc;

    co_spawn(ioc, GET, [](std::exception_ptr ep, SessionResult result) {
        if (ep)
            rethrow_exception(ep);
        std::cout << result << "\n";
    });

    ioc.run();
} catch (std::exception const& e) {
    std::cerr << "Main Error: " << e.what() << "\n";
} catch (...) {
    std::cerr << "Main Unknown error\n";
}

Local demo: