token->wait() hangs when publishing from 2 threads

[elzbieta-nowak-kubat]

Hello

Consider the following example

#include <iostream>
#include <cstdlib>
#include <string>
#include <thread>
#include <atomic>
#include <chrono>
#include <cstring>
#include "mqtt/async_client.h"

using namespace std;

const string DFLT_SERVER_ADDRESS	{ "mqtt://localhost:1883" };
const string CLIENT_ID				{ "paho_cpp_publish" };

const string TOPIC { "hello" };

const char* PAYLOAD1 = "Hello World!";

const int  QOS = 0;

/////////////////////////////////////////////////////////////////////////////

/**
 * A callback class for use with the main MQTT client.
 */
class callback : public virtual mqtt::callback
{
public:
	void connection_lost(const string& cause) override {
		cout << "\nConnection lost" << endl;
		if (!cause.empty())
			cout << "\tcause: " << cause << endl;
	}

	void delivery_complete(mqtt::delivery_token_ptr tok) override {
		cout << "\tDelivery complete for token: "
			<< (tok ? tok->get_message_id() : -1) << endl;
	}
};



/////////////////////////////////////////////////////////////////////////////

int main(int argc, char* argv[])
{
	string	address  = (argc > 1) ? string(argv[1]) : DFLT_SERVER_ADDRESS,
			clientID = (argc > 2) ? string(argv[2]) : CLIENT_ID;

	cout << "Initializing for server '" << address << "'..." << endl;
	auto createOptions = mqtt::create_options_builder()
		.mqtt_version(MQTTVERSION_5)
		.send_while_disconnected(false, false)
		.max_buffered_messages(1)
		.delete_oldest_messages(false)
		.restore_messages(false)
		.persist_qos0(false)
		.finalize();

	mqtt::async_client client(address, clientID, createOptions);

	callback cb;
	client.set_callback(cb);

	auto connOpts = mqtt::connect_options::v5();
	connOpts.set_keep_alive_interval(60);
	connOpts.set_connect_timeout(30);
	connOpts.set_clean_start(true);
	connOpts.set_automatic_reconnect(false);

	cout << "  ...OK" << endl;

	try {
		cout << "\nConnecting..." << endl;
		mqtt::token_ptr conntok = client.connect(connOpts);
		cout << "Waiting for the connection..." << endl;
		conntok->wait();
		cout << "  ...OK" << endl;
	}
	catch (const mqtt::exception& exc) {
		cerr << exc.what() << endl;
		return 1;
	}

	auto publish = [&client](int id) {
		atomic_int counter = 0;

		for (auto i = 0; i < 100; i++) {
			try {
				cout << "\nSending message, thread id : " << id << endl;
				mqtt::message_ptr pubmsg = mqtt::make_message(TOPIC + std::to_string(id), PAYLOAD1 + std::to_string(counter));
				pubmsg->set_qos(QOS);
				pubmsg->set_retained(false);
				auto token = client.publish(pubmsg);
				token->wait();
				cout << "  ...OK, thread id : "<< id << endl;
				counter++;
			}
			catch (const mqtt::exception& exc) {
				cerr << "Publish error : " << exc.what() << endl;
			}
		}
	};

	auto t1 = std::thread(publish, 111);
	auto t2 = std::thread(publish, 222);

	t1.join();
	t2.join();

	try {
		cout << "\nDisconnecting..." << endl;
		client.disconnect()->wait();
		cout << "  ...OK" << endl;
	}
	catch (const mqtt::exception& exc) {
		cerr << exc.what() << endl;
		return 1;
	}

 	return 0;
}

We publish messages in loop from two threads. Publish is synchronous. Almost every time we observe that one of threads hangs on token->wait(), while other is publishing. From time to time we see also error MQTT error [-12]: No more messages can be buffered for some publish. Our observation is that this is related to setting max_buffered_messages to 1 (increasing this number helps).

Is it a bug that max_buffered_messages set to 1 causes hang or we just misconfigured something? If the second case please explain, how to set it (e.g. in relation to number of threads).

[fpagliughi]

I'll have a look and maybe put up new example with two threads publishing.

[elzbieta-nowak-kubat]

That's great.

In meantime I have investigated it a bit and it seems that the problem is caused by wrong synchronization of noBufferedMessages member of MQTTAsync on pahoc side. In can happen that two threads can simultaneously accesses the check on this member, both commands are added to the list and increase this number, so in effect in MQTTAsync_add function processing enters condition

/* delete oldest message if buffer is full.  We wouldn't be here if delete newest was in operation */
if (command->client->createOptions && (command->client->noBufferedMessages >= command->client->createOptions->maxBufferedMessages))

where it should never enter for our create options (delete_oldest_messages(false)) and then ListDetach is called which causes removing the first publish command from the processing. So on pahocpp side wait corresponding to the publish never gets released.

[fpagliughi]

OK. That's what I was afraid of. This should be an issue in the Paho C repo. I will copy and paste the issue there.

[icraggs]

I think this is because of the lack of synchronization in MQTTAsync_send, the conflict arising here:

else if (m->createOptions &&
			(m->createOptions->struct_version < 2 || m->createOptions->deleteOldestMessages == 0) &&
			(MQTTAsync_getNoBufferedMessages(m) >= m->createOptions->maxBufferedMessages))
		rc = MQTTASYNC_MAX_BUFFERED_MESSAGES;

I've taken a C library trace of the occurrence:

lll.zip

and tried out adding a lock/unlock of mqttasync_mutex to MQTTAsync_send, which appears to work for this case. I'll need to check it doesn't affect anything else.