Issues #2091 Improve the performance of RetainedMessages without subs…

[zhaowgit]

Issues #2091 Improve the performance of RetainedMessages without subscribing to RetainedMessageChangedEvent.

[chkr1011]

Could you please add a Unit Tests which covers your situation? I am curious if we can make it fast even with an attached event handler (which is often the case) by converting the List to a ReadOnlyCollection etc. I am also wondering if the copy of the items to the list causes the issue or the fact that the async lock is acquired before (trying) to invoke the event.

[zhaowgit]

using MQTTnet;
using MQTTnet.Server;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;
using System.Threading.Tasks;

namespace DotNet5WpfTest
{
    public class MQTTnet_Test
    {
        public static async Task RetainedMessages_Test(bool retain, bool subscribe, int amount)
        {
            var serverFactory = new MqttServerFactory();
            var options = serverFactory.CreateServerOptionsBuilder()
                .WithDefaultEndpoint()
                .WithDefaultEndpointPort(12346)
                .Build();
            var server = serverFactory.CreateMqttServer(options);
            await server.StartAsync();

            if (subscribe)
            {
                server.RetainedMessageChangedAsync += Server_RetainedMessageChangedAsync;
            }

            var testData = new Dictionary<string, string>();

            for (int i = 0; i < 3; i++)
            {
                for (int j = 0; j < amount; j++)
                {
                    var topic = $"RetainTest/{j:D6}";
                    testData[topic] = ($"{{\"topic\": \"{topic}\", \"value\": \"{j + i}\"}}");
                }

                var sw = Stopwatch.StartNew();
                foreach (var data in testData)
                {
                    await server.InjectApplicationMessage(new(new MqttApplicationMessageBuilder().WithTopic(data.Key).WithPayload(Encoding.UTF8.GetBytes(data.Value)).WithRetainFlag(retain).Build()));
                }
                sw.Stop();
                Debug.WriteLine($"Count:{i} Retain: {retain}, Subscribe:{subscribe}, Amount: {amount}, Time: {sw.ElapsedMilliseconds} ms");

                await Task.Delay(1000);
            }

            await server.StopAsync();
        }

        private static async Task Server_RetainedMessageChangedAsync(RetainedMessageChangedEventArgs arg)
        {
            return;
        }
    }
}

The output before this commit.
Count:0 Retain: True, Subscribe:True, Amount: 100000, Time: 40254 ms
Count:1 Retain: True, Subscribe:True, Amount: 100000, Time: 105772 ms
Count:2 Retain: True, Subscribe:True, Amount: 100000, Time: 106438 ms

Count:0 Retain: False, Subscribe:True, Amount: 100000, Time: 36 ms
Count:1 Retain: False, Subscribe:True, Amount: 100000, Time: 66 ms
Count:2 Retain: False, Subscribe:True, Amount: 100000, Time: 50 ms

Count:0 Retain: True, Subscribe:False, Amount: 100000, Time: 41163 ms
Count:1 Retain: True, Subscribe:False, Amount: 100000, Time: 103599 ms
Count:2 Retain: True, Subscribe:False, Amount: 100000, Time: 107543 ms

Count:0 Retain: False, Subscribe:False, Amount: 100000, Time: 49 ms
Count:1 Retain: False, Subscribe:False, Amount: 100000, Time: 51 ms
Count:2 Retain: False, Subscribe:False, Amount: 100000, Time: 56 ms

The output after this commit.
Count:0 Retain: True, Subscribe:True, Amount: 100000, Time: 39066 ms
Count:1 Retain: True, Subscribe:True, Amount: 100000, Time: 100003 ms
Count:2 Retain: True, Subscribe:True, Amount: 100000, Time: 99984 ms

Count:0 Retain: False, Subscribe:True, Amount: 100000, Time: 45 ms
Count:1 Retain: False, Subscribe:True, Amount: 100000, Time: 52 ms
Count:2 Retain: False, Subscribe:True, Amount: 100000, Time: 53 ms

Count:0 Retain: True, Subscribe:False, Amount: 100000, Time: 122 ms
Count:1 Retain: True, Subscribe:False, Amount: 100000, Time: 79 ms
Count:2 Retain: True, Subscribe:False, Amount: 100000, Time: 96 ms

Count:0 Retain: False, Subscribe:False, Amount: 100000, Time: 49 ms
Count:1 Retain: False, Subscribe:False, Amount: 100000, Time: 54 ms
Count:2 Retain: False, Subscribe:False, Amount: 100000, Time: 63 ms

ValueCollection is not IList, cannot use the constructor of ReadOnlyCollection.
I think we can optimize the performance when Retain: True, Subscribe:False first.