Add a chapter on asio compliance - per operation cancellation

Summary: related to T12804

Reviewers: ivica

Reviewed By: ivica

Subscribers: miljen, iljazovic

Differential Revision: https://repo.mireo.local/D28516

example/timeout_with_awaitable_operators.cpp

@@ -0,0 +1,66 @@
+//[timeout_with_awaitable_operators
+#include <iostream>
+
+#include <boost/asio/as_tuple.hpp>
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/co_spawn.hpp>
+#include <boost/asio/detached.hpp>
+#include <boost/asio/steady_timer.hpp>
+#include <boost/asio/use_awaitable.hpp>
+#include <boost/asio/experimental/awaitable_operators.hpp>
+#include <boost/asio/experimental/parallel_group.hpp>
+#include <boost/asio/ip/tcp.hpp>
+
+#include <async_mqtt5.hpp>
+
+#ifdef BOOST_ASIO_HAS_CO_AWAIT
+
+// Modified completion token that will prevent co_await from throwing exceptions.
+constexpr auto use_nothrow_awaitable = boost::asio::as_tuple(boost::asio::use_awaitable);
+
+using client_type = async_mqtt5::mqtt_client<boost::asio::ip::tcp::socket>;
+
+boost::asio::awaitable<void> send_over_mqtt(client_type& client, const std::string& message) {
+	client.brokers("<your-mqtt-broker>", 1883)
+		.async_run(boost::asio::detached);
+
+	auto&& [pec, prc, puback_props] = co_await client.async_publish<async_mqtt5::qos_e::at_least_once>(
+		"<your-mqtt-topic>", message,
+		async_mqtt5::retain_e::no, async_mqtt5::publish_props {},
+		use_nothrow_awaitable
+	);
+
+	co_await client.async_disconnect(use_nothrow_awaitable);
+}
+
+int main() {
+	boost::asio::io_context ioc;
+
+	co_spawn(ioc, [&ioc]() -> boost::asio::awaitable<void> {
+		// Construct the Client.
+		async_mqtt5::mqtt_client<boost::asio::ip::tcp::socket> client(ioc);
+
+		// Construct the timer.
+		boost::asio::steady_timer timer(ioc, std::chrono::seconds(5));
+
+		using namespace boost::asio::experimental::awaitable_operators;
+		auto res = co_await (
+			send_over_mqtt(client, "Hello world!") ||
+			timer.async_wait(use_nothrow_awaitable)
+		);
+
+		// The timer expired first. The client is cancelled.
+		if (res.index() == 1)
+			std::cout << "Send over MQTT timed out!" << std::endl;
+		// send_over_mqtt completed first. The timer is cancelled.
+		else
+			std::cout << "Send over MQTT completed!" << std::endl;
+		
+	}, boost::asio::detached);
+
+	ioc.run();
+}
+
+#endif
+
+//]