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_parallel_group.cpp

@@ -0,0 +1,63 @@
+//[timeout_with_parallel_group
+#include <iostream>
+
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/deferred.hpp>
+#include <boost/asio/steady_timer.hpp>
+#include <boost/asio/experimental/parallel_group.hpp>
+#include <boost/asio/ip/tcp.hpp>
+
+#include <async_mqtt5.hpp>
+
+int main() {
+	boost::asio::io_context ioc;
+
+	// 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));
+
+	client.brokers("<your-mqtt-broker>", 1883)
+		.async_run(boost::asio::detached);
+
+	// Subscribe to a Topic.
+	client.async_subscribe(
+		{ "<your-mqtt-topic>" }, async_mqtt5::subscribe_props {},
+		[](async_mqtt5::error_code ec, std::vector<async_mqtt5::reason_code> rcs, async_mqtt5::suback_props) {
+			std::cout << "[subscribe ec]: " << ec.message() << std::endl;
+			std::cout << "[subscribe rc]: " << rcs[0].message() << std::endl;
+		}
+	);
+
+	// Create a parallel group to wait up to 5 seconds to receive a message
+	// using client.async_receive(...).
+	boost::asio::experimental::make_parallel_group(
+		timer.async_wait(boost::asio::deferred),
+		client.async_receive(boost::asio::deferred)
+	).async_wait(
+		boost::asio::experimental::wait_for_one(),
+		[&client](
+			std::array<std::size_t, 2> ord, // Completion order
+			async_mqtt5::error_code /* timer_ec */, // timer.async_wait(...) handler signature
+			// client.async_receive(...) handler signature
+			async_mqtt5::error_code receive_ec, 
+			std::string topic, std::string payload, async_mqtt5::publish_props /* props */
+		) {
+			if (ord[0] == 1) {
+				std::cout << "Received a message!" << std::endl;
+				std::cout << "[receive ec]: " << receive_ec.message() << std::endl;
+				std::cout << "[receive topic]: " << topic << std::endl;
+				std::cout << "[receive payload]: " << payload << std::endl;
+			}
+			else
+				std::cout << "Timed out! Did not receive a message within 5 seconds." << std::endl;
+
+			client.cancel();
+		}
+	);
+
+	ioc.run();
+}
+
+//]