1 files changed, 249 insertions, 0 deletions
diff --git a/tests/Cepis.Tests/RetryTests.cs b/tests/Cepis.Tests/RetryTests.cs
new file mode 100644
index 0000000..2114a12
--- /dev/null
+++ b/tests/Cepis.Tests/RetryTests.cs
@@ -0,0 +1,249 @@
+namespace Cepis.Tests;
+
+/// <summary>The retry policy: what is retried, how long it waits, what it never retries.</summary>
+public sealed class RetryTests : HttpTestBase
+{
+    [Fact]
+    public async Task ATransientFiveHundredIsRetried()
+    {
+        Server.Respond("/01001000",
+            Reply.Of(500, Fixtures.InternalError),
+            Reply.Of(200, Fixtures.SeAddress));
+
+        var address = await Client.LookupAsync("01001000");
+
+        Assert.Equal("São Paulo", address.Localidade);
+        Assert.Equal(2, Server.Count("/01001000"));
+    }
+
+    [Theory]
+    [InlineData(500)]
+    [InlineData(502)]
+    [InlineData(503)]
+    [InlineData(504)]
+    public async Task EveryTransientStatusIsRetried(int status)
+    {
+        Server.Respond("/01001000", Reply.Of(status, Fixtures.InternalError), Reply.Of(200, Fixtures.SeAddress));
+
+        await Client.LookupAsync("01001000");
+
+        Assert.Equal(2, Server.Count("/01001000"));
+    }
+
+    [Fact]
+    public async Task ThreeAttemptsIsTheDefault()
+    {
+        Server.Respond("/01001000", Reply.Of(503, Fixtures.InternalError));
+
+        var failure = await Assert.ThrowsAsync<ServerErrorException>(
+            () => Client.LookupAsync("01001000"));
+
+        Assert.Equal(503, failure.Status);
+        Assert.Equal(3, Server.Count("/01001000"));
+    }
+
+    [Fact]
+    public async Task OneAttemptDisablesRetrying()
+    {
+        using var single = new CepisClient(new CepisClientOptions
+        {
+            BaseUrl = Server.Url,
+            MaxAttempts = 1,
+        });
+        Server.Respond("/01001000", Reply.Of(500, Fixtures.InternalError));
+
+        await Assert.ThrowsAsync<ServerErrorException>(() => single.LookupAsync("01001000"));
+
+        Assert.Equal(1, Server.Count("/01001000"));
+    }
+
+    [Fact]
+    public async Task A429BecomesRateLimitedAndCarriesRetryAfter()
+    {
+        Server.Respond("/01001000", Reply.Of(429, "{}", "Retry-After", "7"));
+
+        var failure = await Assert.ThrowsAsync<RateLimitedException>(
+            () => Client.LookupAsync("01001000"));
+
+        Assert.Equal(TimeSpan.FromSeconds(7), failure.RetryAfter);
+    }
+
+    [Fact]
+    public async Task A429WithoutRetryAfterLeavesNothingToRescheduleWith()
+    {
+        Server.Respond("/01001000", Reply.Of(429, "{}"));
+
+        var failure = await Assert.ThrowsAsync<RateLimitedException>(
+            () => Client.LookupAsync("01001000"));
+
+        Assert.Null(failure.RetryAfter);
+    }
+
+    [Fact]
+    public void AnAbsurdRetryAfterIsCappedAtBackoffMax()
+    {
+        using var capped = new CepisClient(new CepisClientOptions
+        {
+            BaseUrl = Server.Url,
+            BackoffMax = TimeSpan.FromMilliseconds(100),
+        });
+
+        Assert.Equal(TimeSpan.FromMilliseconds(100), capped.Backoff(1, TimeSpan.FromHours(1)));
+    }
+
+    [Fact]
+    public void ARetryAfterUnderTheCapIsHonouredAsAsked()
+    {
+        using var client = new CepisClient(new CepisClientOptions
+        {
+            BaseUrl = Server.Url,
+            BackoffMax = TimeSpan.FromSeconds(8),
+        });
+
+        Assert.Equal(TimeSpan.FromSeconds(2), client.Backoff(1, TimeSpan.FromSeconds(2)));
+    }
+
+    [Fact]
+    public void TheBackoffGrowsExponentiallyAndNeverPassesTheCap()
+    {
+        using var client = new CepisClient(new CepisClientOptions
+        {
+            BaseUrl = Server.Url,
+            BackoffBase = TimeSpan.FromMilliseconds(100),
+            BackoffMax = TimeSpan.FromMilliseconds(800),
+        });
+
+        for (var attempt = 1; attempt <= 8; attempt++)
+        {
+            var ceiling = TimeSpan.FromMilliseconds(Math.Min(100 * Math.Pow(2, attempt - 1), 800));
+            for (var draw = 0; draw < 200; draw++)
+            {
+                Assert.InRange(client.Backoff(attempt, null), TimeSpan.Zero, ceiling);
+            }
+        }
+    }
+
+    [Fact]
+    public void TheBackoffIsJitteredRatherThanTheCeiling()
+    {
+        using var client = new CepisClient(new CepisClientOptions
+        {
+            BaseUrl = Server.Url,
+            BackoffBase = TimeSpan.FromSeconds(1),
+            BackoffMax = TimeSpan.FromSeconds(8),
+        });
+
+        var draws = Enumerable.Range(0, 50).Select(_ => client.Backoff(3, null)).Distinct().Count();
+
+        Assert.True(draws > 1, $"full jitter draws a random wait, got {draws} distinct value(s)");
+    }
+
+    [Fact]
+    public async Task AnAbsurdRetryAfterDoesNotParkTheCaller()
+    {
+        using var capped = new CepisClient(new CepisClientOptions
+        {
+            BaseUrl = Server.Url,
+            BackoffMax = TimeSpan.FromMilliseconds(50),
+        });
+        Server.Respond("/01001000", Reply.Of(429, "{}", "Retry-After", "3600"));
+
+        var failure = await Assert.ThrowsAsync<RateLimitedException>(
+            () => capped.LookupAsync("01001000").WaitAsync(TimeSpan.FromSeconds(60)));
+
+        Assert.Equal(TimeSpan.FromSeconds(3600), failure.RetryAfter);
+        Assert.Equal(3, Server.Count("/01001000"));
+    }
+
+    [Theory]
+    [InlineData("120", 120)]
+    [InlineData(" 120 ", 120)]
+    [InlineData("0", 0)]
+    public void RetryAfterInSeconds(string header, int expected) =>
+        Assert.Equal(TimeSpan.FromSeconds(expected), CepisClient.ParseRetryAfter(header));
+
+    [Fact]
+    public void RetryAfterAsAnHttpDate()
+    {
+        var parsed = CepisClient.ParseRetryAfter("Wed, 21 Oct 2099 07:28:00 GMT");
+
+        Assert.NotNull(parsed);
+        Assert.True(parsed > TimeSpan.Zero);
+    }
+
+    [Fact]
+    public void AnHttpDateInThePastIsZeroRatherThanNegative()
+    {
+        var parsed = CepisClient.ParseRetryAfter("Wed, 21 Oct 1998 07:28:00 GMT");
+
+        Assert.Equal(TimeSpan.Zero, parsed);
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    [InlineData("not a date at all")]
+    public void NonsenseRetryAfterIsIgnored(string? header) =>
+        Assert.Null(CepisClient.ParseRetryAfter(header));
+
+    [Fact]
+    public async Task NothingListeningIsANetworkError()
+    {
+        using var unreachable = new CepisClient(new CepisClientOptions
+        {
+            BaseUrl = "http://127.0.0.1:1",
+            MaxAttempts = 2,
+            BackoffBase = TimeSpan.FromMilliseconds(1),
+            BackoffMax = TimeSpan.FromMilliseconds(5),
+            Timeout = TimeSpan.FromSeconds(30),
+        });
+
+        await Assert.ThrowsAsync<NetworkException>(() => unreachable.LookupAsync("01001000"));
+    }
+
+    [Fact]
+    public async Task TheTotalTimeoutStopsTheCallShortOfItsAttempts()
+    {
+        const int budgetedAttempts = 200;
+        using var bounded = new CepisClient(new CepisClientOptions
+        {
+            BaseUrl = Server.Url,
+            Timeout = TimeSpan.FromSeconds(30),
+            MaxAttempts = budgetedAttempts,
+            BackoffBase = TimeSpan.FromMilliseconds(100),
+            BackoffMax = TimeSpan.FromMilliseconds(100),
+            TotalTimeout = TimeSpan.FromSeconds(3),
+        });
+        Server.Respond("/01001000", Reply.Of(500, Fixtures.InternalError));
+
+        var failure = await Assert.ThrowsAsync<ServerErrorException>(
+            () => bounded.LookupAsync("01001000"));
+
+        var attempts = Server.Count("/01001000");
+        Assert.Equal(500, failure.Status);
+        Assert.InRange(attempts, 1, budgetedAttempts - 1);
+    }
+
+    [Fact]
+    public async Task TheTotalTimeoutEndsACallThatNeverGetsAnAnswer()
+    {
+        using var bounded = new CepisClient(new CepisClientOptions
+        {
+            BaseUrl = Server.Url,
+            Timeout = TimeSpan.FromMinutes(1),
+            MaxAttempts = 3,
+            TotalTimeout = TimeSpan.FromMilliseconds(500),
+        });
+        Server.Handle(_ =>
+        {
+            Thread.Sleep(TimeSpan.FromSeconds(10));
+            return Reply.Of(200, Fixtures.SeAddress);
+        });
+
+        await Assert.ThrowsAsync<NetworkException>(
+            () => bounded.LookupAsync("01001000").WaitAsync(TimeSpan.FromSeconds(60)));
+
+        Assert.Equal(1, Server.Count("/01001000"));
+    }
+}