Refactor

DiscordChatExporter.Domain/Internal/Http.cs

@@ -0,0 +1,61 @@
+using System;
+using System.Globalization;
+using System.IO;
+using System.Net;
+using System.Net.Http;
+using System.Text.RegularExpressions;
+using System.Threading.Tasks;
+using Polly;
+
+namespace DiscordChatExporter.Domain.Internal
+{
+    internal static class Http
+    {
+        public static HttpClient Client { get; } = new HttpClient();
+
+        public static IAsyncPolicy<HttpResponseMessage> ResponsePolicy { get; } =
+            Policy
+                .Handle<IOException>()
+                .Or<HttpRequestException>()
+                .OrResult<HttpResponseMessage>(m => m.StatusCode == HttpStatusCode.TooManyRequests)
+                .OrResult(m => m.StatusCode == HttpStatusCode.RequestTimeout)
+                .OrResult(m => m.StatusCode >= HttpStatusCode.InternalServerError)
+                .WaitAndRetryAsync(8,
+                    (i, result, ctx) =>
+                    {
+                        // If rate-limited, use retry-after as a guide
+                        if (result.Result.StatusCode == HttpStatusCode.TooManyRequests)
+                        {
+                            // Only start respecting retry-after after a few attempts.
+                            // The reason is that Discord often sends unreasonable (20+ minutes) retry-after
+                            // on the very first request.
+                            if (i > 3)
+                            {
+                                var retryAfterDelay = result.Result.Headers.RetryAfter.Delta;
+                                if (retryAfterDelay != null)
+                                    return retryAfterDelay.Value + TimeSpan.FromSeconds(1); // margin just in case
+                            }
+                        }
+
+                        return TimeSpan.FromSeconds(Math.Pow(2, i) + 1);
+                    },
+                    (response, timespan, retryCount, context) => Task.CompletedTask);
+
+        private static HttpStatusCode? TryGetStatusCodeFromException(HttpRequestException ex)
+        {
+            // This is extremely frail, but there's no other way
+            var statusCodeRaw = Regex.Match(ex.Message, @": (\d+) \(").Groups[1].Value;
+            return !string.IsNullOrWhiteSpace(statusCodeRaw)
+                ? (HttpStatusCode) int.Parse(statusCodeRaw, CultureInfo.InvariantCulture)
+                : (HttpStatusCode?) null;
+        }
+
+        public static IAsyncPolicy ExceptionPolicy { get; } =
+            Policy
+                .Handle<IOException>() // dangerous
+                .Or<HttpRequestException>(ex => TryGetStatusCodeFromException(ex) == HttpStatusCode.TooManyRequests)
+                .Or<HttpRequestException>(ex => TryGetStatusCodeFromException(ex) == HttpStatusCode.RequestTimeout)
+                .Or<HttpRequestException>(ex => TryGetStatusCodeFromException(ex) >= HttpStatusCode.InternalServerError)
+                .WaitAndRetryAsync(4, i => TimeSpan.FromSeconds(Math.Pow(2, i) + 1));
+    }
+}