Fix ignored exceptions in MqttHostedServer startup

Author: beppemarazzi

Source/MQTTnet.AspnetCore/MqttHostedServer.cs

@@ -7,6 +7,8 @@
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
 using MQTTnet.Diagnostics.Logger;
 using MQTTnet.Server;
 
@@ -16,16 +18,20 @@ public sealed class MqttHostedServer : MqttServer, IHostedService
 {
     readonly IHostApplicationLifetime _hostApplicationLifetime;
     readonly MqttServerFactory _mqttFactory;
+    readonly ILogger _appLogger;
 
     public MqttHostedServer(
         IHostApplicationLifetime hostApplicationLifetime,
         MqttServerFactory mqttFactory,
         MqttServerOptions options,
         IEnumerable<IMqttServerAdapter> adapters,
-        IMqttNetLogger logger) : base(options, adapters, logger)
+        IMqttNetLogger logger,
+        ILogger<MqttHostedServer> appLogger = null
+        ) : base(options, adapters, logger)
     {
         _mqttFactory = mqttFactory ?? throw new ArgumentNullException(nameof(mqttFactory));
         _hostApplicationLifetime = hostApplicationLifetime;
+        _appLogger = appLogger ?? NullLogger<MqttHostedServer>.Instance;
     }
 
     public async Task StartAsync(CancellationToken cancellationToken)
@@ -43,6 +49,19 @@ public Task StopAsync(CancellationToken cancellationToken)
 
     void OnStarted()
     {
-        _ = StartAsync();
+        async Task DoStart()
+        {
+            try
+            {
+                await StartAsync();
+            }
+            catch (Exception e)
+            {
+                _appLogger.LogError(e, "Stopping application: failed to start MqttServer: {Error}", e.Message);
+                _hostApplicationLifetime.StopApplication();
+                throw;
+            }
+        }
+        _ = DoStart();
     }
 }