dh: Add Delegate after N requests handler

src/rm.DelegatingHandlers/DelegateAfterNRequestsHandler.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace rm.DelegatingHandlers;
+
+/// <summary>
+/// Runs delegates after N requests.
+/// </summary>
+public class DelegateAfterNRequestsHandler : DelegatingHandler
+{
+	private readonly IDelegateAfterNRequestsHandlerSettings delegateAfterNRequestsHandlerSettings;
+
+	private long n;
+
+	/// <inheritdoc cref="DelegateAfterNRequestsHandler" />
+	public DelegateAfterNRequestsHandler(
+		IDelegateAfterNRequestsHandlerSettings delegateAfterNRequestsHandlerSettings)
+	{
+		this.delegateAfterNRequestsHandlerSettings = delegateAfterNRequestsHandlerSettings
+			?? throw new ArgumentNullException(nameof(delegateAfterNRequestsHandlerSettings));
+	}
+
+	protected override async Task<HttpResponseMessage> SendAsync(
+		HttpRequestMessage request,
+		CancellationToken cancellationToken)
+	{
+		if (Interlocked.Read(ref n) >= delegateAfterNRequestsHandlerSettings.N)
+		{
+			await delegateAfterNRequestsHandlerSettings.PreDelegate(request)
+				.ConfigureAwait(false);
+		}
+
+		var response = await base.SendAsync(request, cancellationToken)
+			.ConfigureAwait(false);
+
+		if (Interlocked.Read(ref n) < delegateAfterNRequestsHandlerSettings.N)
+		{
+			Interlocked.Increment(ref n);
+		}
+
+		return response;
+	}
+}
+
+public interface IDelegateAfterNRequestsHandlerSettings
+{
+	long N { get; }
+	Func<HttpRequestMessage, Task> PreDelegate { get; }
+}
+
+public record class DelegateAfterNRequestsHandlerSettings : IDelegateAfterNRequestsHandlerSettings
+{
+	public long N { get; init; }
+	public Func<HttpRequestMessage, Task> PreDelegate { get; init; }
+}

tests/rm.DelegatingHandlersTest/DelegateAfterNRequestsHandlerTests.cs

@@ -0,0 +1,99 @@
+using System.Net.Http;
+using AutoFixture;
+using AutoFixture.AutoMoq;
+using NUnit.Framework;
+using rm.DelegatingHandlers;
+
+namespace rm.DelegatingHandlersTest;
+
+[TestFixture]
+public class DelegateAfterNRequestsHandlerTests
+{
+	[Test]
+	public async Task Runs_PreDelegate()
+	{
+		var fixture = new Fixture().Customize(new AutoMoqCustomization());
+
+		var n = 5;
+		var preDelegateExecutedCount = 0;
+		var delegateAfterNRequestsHandler = new DelegateAfterNRequestsHandler(
+			new DelegateAfterNRequestsHandlerSettings
+			{
+				N = n,
+				PreDelegate = (request) =>
+				{
+					preDelegateExecutedCount++;
+					return Task.CompletedTask;
+				}
+			});
+
+		using var invoker = HttpMessageInvokerFactory.Create(
+			fixture.Create<HttpMessageHandler>(), delegateAfterNRequestsHandler);
+
+		for (int i = 0; i < n; i++)
+		{
+			using var _ = await invoker.SendAsync(fixture.Create<HttpRequestMessage>(), CancellationToken.None);
+		}
+		using var __ = await invoker.SendAsync(fixture.Create<HttpRequestMessage>(), CancellationToken.None);
+
+		Assert.AreEqual(1, preDelegateExecutedCount);
+	}
+
+	[Test]
+	public async Task Runs_PreDelegate_After()
+	{
+		var fixture = new Fixture().Customize(new AutoMoqCustomization());
+
+		var n = 5;
+		var preDelegateExecutedCount = 0;
+		var delegateAfterNRequestsHandler = new DelegateAfterNRequestsHandler(
+			new DelegateAfterNRequestsHandlerSettings
+			{
+				N = n,
+				PreDelegate = (request) =>
+				{
+					preDelegateExecutedCount++;
+					return Task.CompletedTask;
+				}
+			});
+
+		using var invoker = HttpMessageInvokerFactory.Create(
+			fixture.Create<HttpMessageHandler>(), delegateAfterNRequestsHandler);
+
+		for (int i = 0; i < n + 1; i++)
+		{
+			using var _ = await invoker.SendAsync(fixture.Create<HttpRequestMessage>(), CancellationToken.None);
+		}
+		using var __ = await invoker.SendAsync(fixture.Create<HttpRequestMessage>(), CancellationToken.None);
+
+		Assert.AreEqual(2, preDelegateExecutedCount);
+	}
+
+	[Test]
+	public async Task Does_Not_Run_PreDelegate()
+	{
+		var fixture = new Fixture().Customize(new AutoMoqCustomization());
+
+		var n = 5;
+		var preDelegateExecutedCount = 0;
+		var delegateAfterNRequestsHandler = new DelegateAfterNRequestsHandler(
+			new DelegateAfterNRequestsHandlerSettings
+			{
+				N = n,
+				PreDelegate = (request) =>
+				{
+					preDelegateExecutedCount++;
+					return Task.CompletedTask;
+				}
+			});
+
+		using var invoker = HttpMessageInvokerFactory.Create(
+			fixture.Create<HttpMessageHandler>(), delegateAfterNRequestsHandler);
+
+		for (int i = 0; i < n; i++)
+		{
+			using var _ = await invoker.SendAsync(fixture.Create<HttpRequestMessage>(), CancellationToken.None);
+		}
+		Assert.AreEqual(0, preDelegateExecutedCount);
+	}
+}