[MqttServer] Subscription to similar base and subtopic with wildcards fails

[grobmiller]

We encountered a bug which was not present in earlier major versions of MQTTnet, but we met it since we had a major upgrade to 4.1.2.350. Same applies for the newest version 4.1.2.436

We start an MQTTnet server. Then a client. We add a subscription to these to topics:

house/+/room
house/+/room/+

Then we send messages with these two topics:

house/1/room
house/1/room/bed

We would expect to receive both messages, but only the first one arrives. When testing this with other servers (older MQTTnet server or Moquitto) both messages arrive.

As you can see in the sample code below, we also made some slight variations (# instead of +), but no success. When we only use one subscription (#) everything is fine again. Replacing the mid-level wildcard with a concrete house id (1) also makes it work again.

It is not an ultra-critical bug, since there is a workaround. But we lost data in production since we did not realize this problem immediately. Might be that some others may stumble over this, too.

using System;
using System.Threading;
using System.Threading.Tasks;
using MQTTnet;
using MQTTnet.Client;
using MQTTnet.Server;

namespace MqttServerTest
{
	class Program
	{
		static void Main(string[] args)
		{
			Task.Run(StartMqttServer).Wait();

			var client = Task.Run(StartClient).Result;

			client.SubscribeAsync("house/+/room");
			client.SubscribeAsync("house/+/room/+");

			//You can try these two subscriptions instead, but this will also fail
			//client.SubscribeAsync("house/+/room");
			//client.SubscribeAsync("house/+/room/#");

			//This works
			//client.SubscribeAsync("house/1/room");
			//client.SubscribeAsync("house/1/room/+");

			//This works, too
			//client.SubscribeAsync("house/1/room/#");

			Publish(client, "house/1/room");
			Publish(client, "house/1/room/bed");

			//Result with first two examples: Only first message will be shown

			Console.ReadLine();
		}

		static async Task<MqttServer> StartMqttServer()
		{
			var mqttFactory = new MqttFactory();
			
			var mqttServerOptions = 
				mqttFactory
					.CreateServerOptionsBuilder()
					.WithDefaultEndpoint()
					.Build();
			var server = mqttFactory.CreateMqttServer(mqttServerOptions);
			await server.StartAsync();
			return server;
		}
		
		static async Task<IMqttClient> StartClient()
		{
			var mqttFactory = new MqttFactory();

			var mqttClientOptions =
				mqttFactory
					.CreateClientOptionsBuilder()
					.WithTcpServer("localhost")
					.WithClientId("test")
					.Build();

			var client = mqttFactory.CreateMqttClient();

			client.ApplicationMessageReceivedAsync += args =>
			{
				Console.WriteLine(args.ApplicationMessage.Topic);
				return Task.CompletedTask;
			};

			await client.ConnectAsync(mqttClientOptions, CancellationToken.None);
			
			return client;
		}
		
		private static Task<MqttClientPublishResult> Publish(IMqttClient client, string topic)
		{
			return client.PublishAsync(new MqttApplicationMessage
			{
				Topic = topic,
				Payload = new byte[] { 1, 2, 3, 4 }
			});
		}
	}
}

[logicaloud]

That indeed looks to be a bug. @chkr1011 I think this one is for me; I'll be looking into it if you like.

[chkr1011]

@logicaloud Yes please have look. I will also have a look tomorrow or so. We can then join with our findings.

[chkr1011]

@logicaloud Please us the branch created for this ticket.

[logicaloud]

@chkr1011 Will do. I have found the issue and will add a pull request into the branch soon. @grobmiller, thanks for reporting this.

[grobmiller]

You're welcome, thanx for your great effort and product!