Add retry policy to MediaDownloader to accommodate short network outages during download (#404)

DiscordChatExporter.Domain/Exporting/MediaDownloader.cs

@@ -1,11 +1,14 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Net;
 using System.Net.Http;
 using System.Text.RegularExpressions;
 using System.Threading.Tasks;
 using DiscordChatExporter.Domain.Internal;
 using DiscordChatExporter.Domain.Internal.Extensions;
+using Polly;
+using Polly.Retry;
 
 namespace DiscordChatExporter.Domain.Exporting
 {
@@ -13,27 +16,35 @@ namespace DiscordChatExporter.Domain.Exporting
     {
         private readonly HttpClient _httpClient = Singleton.HttpClient;
         private readonly string _workingDirPath;
+        private readonly AsyncRetryPolicy _httpRequestPolicy;
 
         private readonly Dictionary<string, string> _pathMap = new Dictionary<string, string>();
 
         public MediaDownloader(string workingDirPath)
         {
             _workingDirPath = workingDirPath;
+
+            _httpRequestPolicy = Policy
+                .Handle<IOException>()
+                .WaitAndRetryAsync(8, i => TimeSpan.FromSeconds(0.5 * i));
         }
 
         public async ValueTask<string> DownloadAsync(string url)
         {
-            if (_pathMap.TryGetValue(url, out var cachedFilePath))
-                return cachedFilePath;
+            return await _httpRequestPolicy.ExecuteAsync(async () =>
+            {
+                if (_pathMap.TryGetValue(url, out var cachedFilePath))
+                    return cachedFilePath;
 
-            var fileName = GetFileNameFromUrl(url);
-            var filePath = PathEx.MakeUniqueFilePath(Path.Combine(_workingDirPath, fileName));
+                var fileName = GetFileNameFromUrl(url);
+                var filePath = PathEx.MakeUniqueFilePath(Path.Combine(_workingDirPath, fileName));
 
-            Directory.CreateDirectory(_workingDirPath);
+                Directory.CreateDirectory(_workingDirPath);
 
-            await _httpClient.DownloadAsync(url, filePath);
+                await _httpClient.DownloadAsync(url, filePath);
 
-            return _pathMap[url] = filePath;
+                return _pathMap[url] = filePath;
+            });
         }
     }