tests work

tests/IntegrationUtils/PeriodicTaskTestClasses.cs

@@ -0,0 +1,33 @@
+namespace BetterHostedServices.Test.IntegrationUtils
+{
+    using System;
+    using System.Threading;
+    using System.Threading.Tasks;
+
+    public class CrashingPeriodicTask: IPeriodicTask
+    {
+        public async Task ExecuteAsync(CancellationToken stoppingToken)
+        {
+            await Task.Delay(50, stoppingToken); // Just to yield control
+            throw new Exception("oh no");
+        }
+    }
+
+    public class IncrementingThenCrashingPeriodicTask: IPeriodicTask
+    {
+        private readonly SingletonStateHolder singletonStateHolder;
+
+        public IncrementingThenCrashingPeriodicTask(SingletonStateHolder singletonStateHolder) => this.singletonStateHolder = singletonStateHolder;
+
+        public async Task ExecuteAsync(CancellationToken stoppingToken)
+        {
+            this.singletonStateHolder.Count += 1;
+            throw new Exception("oh no");
+        }
+    }
+
+    public class SingletonStateHolder
+    {
+        public int Count { get; set; }
+    }
+}

tests/PeriodicTasksTest.cs

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+namespace BetterHostedServices.Test
+{
+    using System;
+    using System.Threading.Tasks;
+    using FluentAssertions;
+    using IntegrationUtils;
+    using Microsoft.AspNetCore.TestHost;
+    using Microsoft.Extensions.DependencyInjection;
+    using Xunit;
+
+    public class PeriodicTasksTest
+    {
+        private readonly CustomWebApplicationFactory<DummyStartup> _factory;
+
+        public PeriodicTasksTest()
+        {
+            _factory = new CustomWebApplicationFactory<DummyStartup>();
+        }
+
+        [Fact]
+        public async Task PeriodicTask_ShouldEndApplication_IfFailureModeIsSetToCrash()
+        {
+            // Arrange
+            var applicationEnder = new ApplicationEnderMock();
+
+            var factory = this._factory.WithWebHostBuilder(builder =>
+            {
+                builder.ConfigureTestServices(services =>
+                {
+                    services.AddTransient<IApplicationEnder>(s => applicationEnder);
+                    services.AddPeriodicTask<CrashingPeriodicTask>(PeriodicTaskFailureMode.CrashApplication, TimeSpan.FromSeconds(1));
+                });
+            });
+
+            var client = factory.CreateClient();
+            // Act & assert - we should crash here at some point
+
+            // Task is hella flaky because it depends on the internals of the IHostedService - try yielding a bunch of times
+            // to hope that it's done requesting application shutdown at this point
+            for (int i = 0; i < 10; i++)
+            {
+                await Task.Delay(50);
+                await Task.Yield();
+            }
+
+            // due to https://github.com/dotnet/aspnetcore/issues/25857 we can't test if the process is closed directly
+            applicationEnder.ShutDownRequested.Should().BeTrue();
+        }
+
+        [Fact]
+        public async Task PeriodicTask_ShouldContinueRunningTasks_IfFailureModeIsSetToRetry()
+        {
+            // Arrange
+            var applicationEnder = new ApplicationEnderMock();
+            var stateHolder = new SingletonStateHolder();
+
+            var factory = this._factory.WithWebHostBuilder(builder =>
+            {
+                builder.ConfigureTestServices(services =>
+                {
+                    services.AddTransient<IApplicationEnder>(s => applicationEnder);
+                    services.AddPeriodicTask<IncrementingThenCrashingPeriodicTask>(PeriodicTaskFailureMode.RetryLater, TimeSpan.FromMilliseconds(50));
+                    services.AddSingleton<SingletonStateHolder>(s => stateHolder);
+                });
+            });
+
+            var client = factory.CreateClient();
+            // Act & assert - we crash here after each invocation, but we truck on. The stateHolder should keep being incremented
+
+            // Task is hella flaky because it depends on the internals of the IHostedService - try yielding a bunch of times
+            // to hope that it's done requesting application shutdown at this point
+            for (int i = 0; i < 10; i++)
+            {
+                await Task.Delay(100); // 1s ms all in all
+                await Task.Yield();
+            }
+
+            stateHolder.Count.Should().BeGreaterThan(5);
+
+            // due to https://github.com/dotnet/aspnetcore/issues/25857 we can't test if the process is closed directly
+            applicationEnder.ShutDownRequested.Should().BeFalse();
+        }
+    }
+}