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

doc/qbk/00_main.qbk

@@ -42,6 +42,7 @@
 [def __Asio__ [@boost:/libs/asio/index.html Boost.Asio]]
 [def __Beast__ [@boost:/libs/beast/index.html Boost.Beast]]
 [def __ASIO_PER_OP_CANCELLATION__ [@boost:/doc/html/boost_asio/overview/core/cancellation.html Per-Operation Cancellation]]
+[def __ASIO_PARALLEL_GROUP__ [@boost:/doc/html/boost_asio/overview/composition/parallel_group.html Co-ordinating Parallel Operations]]
 [def __IOC__ [@boost:doc/html/boost_asio/reference/io_context.html `boost::asio::io_context`]]
 [def __POST__ [@boost:doc/html/boost_asio/reference/post.html `boost::asio::post`]]
 [def __CO_SPAWN__ [@boost:/doc/html/boost_asio/reference/co_spawn.html `boost::asio::co_spawn`]]

doc/qbk/07_asio_compliance.qbk

@@ -6,9 +6,14 @@ Every asynchronous operation in __Asio__ has associated characteristics that spe
 * A *cancellation slot* determines how the asynchronous operations support cancellation.
 * An *executor* determines the queuing and execution strategy for completion handlers.
 
-This section expands further into the roles of allocators, cancellation slots, and [link async_mqtt5.asio_compliance.executors Executors],
-highlighting their integration and usage within the __Client__.
+This section expands further into the roles of allocators,
+cancellation slots, and executors, highlighting their integration and usage within the __Client__.
+
+* See [link async_mqtt5.asio_compliance.per_op_cancellation Per-Operation Cancellation] for more information about how
+asynchronous operations within the __Client__ support cancellation.
+* See [link async_mqtt5.asio_compliance.executors Executors] for more information about executors.
 
 [include 08_executors.qbk]
+[include 09_per_op_cancellation.qbk]
 
 [endsect] [/asio_compliance]

doc/qbk/09_per_op_cancellation.qbk

@@ -0,0 +1,89 @@
+[section:per_op_cancellation Per-Operation Cancellation]
+[nochunk]
+
+In __Asio__, various objects such as sockets and timers offer the ability to terminate all ongoing asynchronous operations
+globally through their `close` or `cancel` member functions.
+Beyond this global cancellation capability, __Asio__ also provides mechanisms for cancelling specific asynchronous
+operations on an individual basis.
+This individual per-operation cancellation is enabled by specifying that a completion handler has an associated cancellation slot.
+If the asynchronous operation supports cancellation, then it will install a cancellation handler into the associated slot.
+The cancellation handler will be invoked when the user emits a cancellation signal into the cancellation slot. 
+See __ASIO_PER_OP_CANCELLATION__ for more information.
+
+The utility of per-operation cancellation becomes particularly evident when using `boost::asio::experimental::parallel_group` to launch
+work performed in parallel and wait for one or all of the operations to complete.
+For instance, in a parallel group awaiting the completion of any one operation among several,
+completing one operation will trigger individual cancellation of all the remaining operations.
+The same concept applies to the `awaitable operator ||`, which runs two awaitables in parallel,
+waiting for one to complete and subsequently cancelling the remaining one.
+See __ASIO_PARALLEL_GROUP__ for more information.
+
+Within the __Client__, every asynchronous function is designed to support individual per-operation cancellation.
+This allows for associating of a cancellation slot with any `async_xxx` function call, enabling the emission of a cancellation signal as needed.
+The impact of emitting a cancellation signal varies depending on the signal type (terminal, total, partial) and the operation being cancelled.
+Detailed descriptions of how cancellation signals affect each `async_xxx` function
+are provided in the ['Per-Operation Cancellation] paragraph in their respective sections of the __Client__ reference documentation.
+
+[heading Example: associating a cancellation slot with an asynchronous operation]
+
+This example illustrates associating a cancellation slot with a [refmem mqtt_client async_publish] operation
+and emitting a terminal cancellation signal.
+Executing this sequence effectively results in the immediate cancellation of the entire client operation,
+mirroring the outcome of invoking [refmem mqtt_client cancel] directly.
+
+If a total or partial cancellation signal were issued instead of a terminal one, the implications would be less severe. 
+In such cases, the cancellation would specifically target resending the __PUBLISH__ packet,
+preventing it from being retransmitted should the client reconnect during the ongoing operation.
+
+```
+	async_mqtt5::mqtt_client<boost::asio::ip::tcp::socket> client(ioc);
+	
+	client.brokers("<your-mqtt-broker>", 1883)
+		.async_run(boost::asio::detached);
+	
+	boost::asio::cancellation_signal signal;
+
+	client.async_publish<async_mqtt5::qos_e::at_least_once>(
+		"<topic>", "Hello world!",
+		async_mqtt5::retain_e::no, async_mqtt5::publish_props {},
+		boost::asio::bind_cancellation_slot(
+			signal.slot(),
+			[&client](async_mqtt5::error_code ec, async_mqtt5::reason_code rc, async_mqtt5::puback_props props ) {
+				std::cout << ec.message() << std::endl;
+			}
+		)
+	);
+
+	signal.emit(boost::asio::cancellation_type_t::terminal);
+```
+
+[section:parallel_group parallel_group/operator || and asynchronous functions in the mqtt_client]
+As a result of supporting per-operation cancellation,
+all the asynchronous functions with the __Client__ can be used in `parallel_group` or with `awaitable operator ||`.
+This feature is especially beneficial for executing operations that require a timeout mechanism.
+
+Below are two examples illustrating how to implement a timeout:
+
+* [link async_mqtt5.timeout_with_parallel_group timeout_with_parallel_group.cpp]
+* [link async_mqtt5.timeout_with_awaitable_operators timeout_with_awaitable_operators.cpp]
+
+[endsect] [/parallel_group]
+
+[section:protocol_level_cancellation About protocol-level cancellation]
+In the context of __Client__, the handling of cancellation signals varies across different asynchronous operations.
+Except for [refmem mqtt_client async_receive], all other `async_xxx` operations respond to a terminal cancellation signal by invoking [refmem mqtt_client cancel].
+These operations will halt the resending of certain packets for total and partial cancellation signals.
+
+It is worth noting that cancelling an `async_xxx` operation during an ongoing protocol exchange is not implemented because of a design decision
+to prevent protocol breaches.
+For example, if [refmem mqtt_client async_publish] with QoS 2 is in the middle of a communication with the Broker
+[footnote Publishing with QoS 2 involves sending a __PUBLISH__ packet, receiving a __PUBREC__ acknowledgement from the Broker, transmitting a __PUBREL__ packet, and finally receiving a __PUBCOMP__ packet.] 
+and an attempt to cancel it is made, it could lead to a protocol violation.
+For instance, if the operation is cancelled after a __PUBREC__ packet has been received from the Broker but before sending the __PUBREL__ packet,
+that would breach the MQTT protocol by failing to send a necessary packet and leave the connection with the Broker in an invalid state.
+
+Therefore, the design of __Client__'s cancellation strategy carefully avoids these pitfalls to ensure continuous protocol compliance.
+
+[endsect] [/protocol_level_cancellation]
+
+[endsect] [/per_op_cancellation]

doc/qbk/10_examples.qbk

@@ -32,6 +32,20 @@ The following list contains all the examples that showcase how to use the __Clie
 		[[link async_mqtt5.multiflight_client multiflight_client.cpp]]
 		[Shows how to use the __Client__ to simultaneously dispatch multiple requests.]
 	]
+	[
+		[[link async_mqtt5.timeout_with_parallel_group timeout_with_parallel_group.cpp]]
+		[
+			Shows how to use the __Client__ with its support for per-operation cancellation to perform operations under a time constraint
+			using parallel group.
+		]
+	]
+	[
+		[[link async_mqtt5.timeout_with_awaitable_operators timeout_with_awaitable_operators.cpp]]
+		[
+			Shows how to use the __Client__ with its support for per-operation cancellation to perform operations under a time constraint
+			using awaitable operators.
+		]
+	]
 ]
 
 [endsect][/examples]

doc/qbk/examples/Examples.qbk

@@ -25,28 +25,46 @@ This example illustrates the process of setting up the Client to connect to the
 [endsect] [/hello_world_over_websocket_tls]
 
 [section:publisher The publisher]
-This example will show how to use __Client__ as a publisher that publishes sensor readings every `5` seconds.
-The __Client__ will use TCP to connect to the Broker and modified __USE_AWAITABLE__ as the completion token.
+This example shows how to use __Client__ as a publisher that publishes sensor readings every `5` seconds.
+The __Client__ uses TCP to connect to the Broker and modified __USE_AWAITABLE__ as the completion token.
 
 [import ../../../example/publisher.cpp]
 [publisher]
 [endsect]
 
 [section:receiver The receiver]
-This example will show how to use __Client__ as a receiver.
+This example shows how to use __Client__ as a receiver.
 The __Client__ subscribes and indefinitely receives Application Messages from the Broker.
-The __Client__ will use TCP to connect to the Broker and modified __USE_AWAITABLE__ as the completion token.
+The __Client__ uses TCP to connect to the Broker and modified __USE_AWAITABLE__ as the completion token.
 
 [import ../../../example/receiver.cpp]
 [receiver]
 [endsect]
 
 [section:multiflight_client The multiflight Client]
-This example will show how to use __Client__ to simultaneously dispatch multiple
-requests.
+This example shows how to use __Client__ to simultaneously dispatch multiple requests.
 
 [import ../../../example/multiflight_client.cpp]
 [multiflight_client]
 [endsect]
 
+[section:timeout_with_parallel_group Timed MQTT operations with parallel group]
+This example demonstrates how to use the __Client__ with its support for per-operation cancellation to perform operations under a time constraint
+using parallel group.
+Specifically, in this example, the __Client__ will subscribe to a Topic and try to receive a message from the Topic within `5 seconds`.
+
+[import ../../../example/timeout_with_parallel_group.cpp]
+[timeout_with_parallel_group]
+[endsect]
+
+[section:timeout_with_awaitable_operators Timed MQTT operations with awaitable operators]
+This example demonstrates how to use the __Client__ with its support for per-operation cancellation to perform operations under a time constraint
+using awaitable operators.
+Specifically, in this example, a call to [refmem mqtt_client async_publish] and [refmem mqtt_client async_disconnect] must complete
+within `5 seconds`. Otherwise, they will be cancelled.
+
+[import ../../../example/timeout_with_awaitable_operators.cpp]
+[timeout_with_awaitable_operators]
+[endsect]
+
 [block'''</part>''']

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
+
+//]

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();
+}
+
+//]