Managed MQTT client hangs when trying to disconnect using `StopAsync()` in Unity Engine

[Joe-Heffer-Shef]

I am using

• MQTTnet 4.3.5
• MQTTnet.Extensions.ManagedClient 4.3.5

Describe the bug

When I run ManagedMqttClient.StopAsync() the task hangs on this line:

https://github.com/dotnet/MQTTnet/blob/4e3ede42c60a6beafbc446efe13ee34a2d87ba75/Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs#L286

Which component is your bug related to?

ManagedMqttClient

To Reproduce

var mqttFactory = new MqttFactory();
var client = mqttFactory.CreateManagedMqttClient();
var task = client.StartAsync(ClientOptions);
task.Wait();
var task = Client.StopAsync(cleanDisconnect: false);
task.Wait();
# Hangs

Expected behavior

I expect this to disconnect in under a second.

[chkr1011]

I created a Unit Test with your code and it runs fine. Does this also occur with later versions?

[Joe-Heffer-Shef]

I am using version 4.3.5 which I believe is the latest version which I installed from NuGet.

Thank you for running that test. I also ran the code above as a unit test and it does indeed work fine. I think the problem is happening when I run this same code inside Unity Editor for the Unity Game Engine. (I should've explained this context before, but I assumed the problem was with the MQTTnet library itself.)

It seems like there's something in StopAsync() or MaintainConnectionAsync() that works differently in the context of a Unity Engine environment.

Is there a way to set a disconnect timeout so that StopAsync() will throw an exception after x seconds?

The code examples for the two unit tests are below:

This works:

// NOT in Unity Engine
using MQTTnet;
using MQTTnet.Client;
using MQTTnet.Extensions.ManagedClient;

namespace TestProject1;

public class Tests
{
    [SetUp]
    public void Setup()
    {
    }

    [Test]
    public void Test1()
    {
        // https://github.com/dotnet/MQTTnet/wiki/Client#client-options
        var options = new MqttClientOptionsBuilder()
            .WithTcpServer("broker.emqx.io", 1883)
            .Build();

        var managedOptions = new ManagedMqttClientOptionsBuilder()
            .WithClientOptions(options)
            .Build();

        var mqttFactory = new MqttFactory();
        var client = mqttFactory.CreateManagedMqttClient();
        var task = client.StartAsync(managedOptions);
        task.Wait();
        task = client.StopAsync(cleanDisconnect: false);
        task.Wait();

        Assert.Pass();
    }
}

This Unity code hangs:

// Unity Engine testing with NUnit
using NUnit.Framework;
using UnityEngine;
using MQTTnet;
using MQTTnet.Client;
using MQTTnet.Extensions.ManagedClient;


namespace MyProject.Core.Tests
{
    public class MqttNetTests
    {
        /// <summary>
        /// Test the mqttnet library on its own.
        /// </summary>
        [Test]
        public void MqttNetStopAsync()
        {
            // Set up MQTT client
            var options = new MqttClientOptionsBuilder()
                .WithTcpServer("broker.emqx.io", 1883)
                .Build();

            var managedOptions = new ManagedMqttClientOptionsBuilder()
                .WithClientOptions(options)
                .Build();

            var mqttFactory = new MqttFactory();
            var client = mqttFactory.CreateManagedMqttClient();

            // Connect
            var task = client.StartAsync(managedOptions);
            task.Wait();
            Debug.Log("MQTT client started.");

            // Disconnect
            task = client.StopAsync(cleanDisconnect: false);
            task.Wait(); // HANGS
            Debug.Log("MQTT client stopped.");

            Assert.Pass();
        }
    }
}

[chkr1011]

My recommendation is to avoid the managed client in this case. The regular client gives you more flexibility. But it requires manual reconnecting etc. Please let me know if the regular client has the same issues in Unity.

[Joe-Heffer-Shef]

The unmanaged client works fine with Unity. Thanks for your help Christian @chkr1011

[xjekali]

I am seeing a similar hang when calling StopAsync from a SpecFlow test (no Unity involved, but I may be doing stuff to paint myself into a similar corner).

My problem can be alleviated if I add a .ConfigureAwait(false) to the Task.WhenAny call on line 286 here: https://github.com/dotnet/MQTTnet/blob/25c1da6a213b91a536ce7b7ed26f3ffe39e61ca3/Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs#L284-L288