Tried to write some unit tests, but unable due to dependency on DirectoryService.

Finally fixed the issue around memory mapped files preventing deleting. (Thought I did, but didn't) - Going to commit to try a full rewrite of the image work with NetVips instead.

API.Tests/Services/CoverDbServiceTests.cs

@@ -1,19 +1,36 @@
-using System.Threading.Tasks;
+using System.IO;
+using System.IO.Abstractions;
+using System.Reflection;
+using System.Threading.Tasks;
+using API.Constants;
+using API.Entities.Enums;
+using API.Extensions;
 using API.Services;
 using API.Services.Tasks.Metadata;
 using API.SignalR;
 using EasyCaching.Core;
+using Kavita.Common;
 using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Logging;
 using NSubstitute;
+using Xunit;
 
 namespace API.Tests.Services;
 
 public class CoverDbServiceTests : AbstractDbTest
 {
-    private readonly IDirectoryService _directoryService;
+    private readonly DirectoryService _directoryService;
     private readonly IEasyCachingProviderFactory _cacheFactory = Substitute.For<IEasyCachingProviderFactory>();
     private readonly ICoverDbService _coverDbService;
+
+    private static readonly string FaviconPath = Path.Join(Directory.GetCurrentDirectory(),
+        "../../../Services/Test Data/CoverDbService/Favicons");
+    /// <summary>
+    /// Path to download files temp to. Should be empty after each test.
+    /// </summary>
+    private static readonly string TempPath = Path.Join(Directory.GetCurrentDirectory(),
+        "../../../Services/Test Data/CoverDbService/Temp");
+
     public CoverDbServiceTests()
     {
         _directoryService = new DirectoryService(Substitute.For<ILogger<DirectoryService>>(), CreateFileSystem());
@@ -27,4 +44,74 @@ public class CoverDbServiceTests : AbstractDbTest
     {
         throw new System.NotImplementedException();
     }
+
+
+    #region Download Favicon
+
+    /// <summary>
+    /// I cannot figure out how to test this code due to the reliance on the _directoryService.FaviconDirectory and not being
+    /// able to redirect it to the real filesystem.
+    /// </summary>
+    public async Task DownloadFaviconAsync_ShouldDownloadAndMatchExpectedFavicon()
+    {
+        // Arrange
+        var testUrl = "https://anilist.co/anime/6205/Kmpfer/";
+        var encodeFormat = EncodeFormat.WEBP;
+        var expectedFaviconPath = Path.Combine(FaviconPath, "anilist.co.webp");
+
+        // Ensure TempPath exists
+        _directoryService.ExistOrCreate(TempPath);
+
+        var baseUrl = "https://anilist.co";
+
+        // Ensure there is no cache result for this URL
+        var provider = Substitute.For<IEasyCachingProvider>();
+        provider.GetAsync<string>(baseUrl).Returns(new CacheValue<string>(null, false));
+        _cacheFactory.GetCachingProvider(EasyCacheProfiles.Favicon).Returns(provider);
+
+
+        // // Replace favicon directory with TempPath
+        // var directoryService = (DirectoryService)_directoryService;
+        // directoryService.FaviconDirectory = TempPath;
+
+        // Hack: Swap FaviconDirectory with TempPath for ability to download real files
+        typeof(DirectoryService)
+            .GetField("FaviconDirectory", BindingFlags.NonPublic | BindingFlags.Instance)
+            ?.SetValue(_directoryService, TempPath);
+
+
+        // Act
+        var resultFilename = await _coverDbService.DownloadFaviconAsync(testUrl, encodeFormat);
+        var actualFaviconPath = Path.Combine(TempPath, resultFilename);
+
+        // Assert file exists
+        Assert.True(File.Exists(actualFaviconPath), "Downloaded favicon does not exist in temp path");
+
+        // Load and compare similarity
+
+        var similarity = expectedFaviconPath.CalculateSimilarity(actualFaviconPath); // Assuming you have this extension
+        Assert.True(similarity > 0.9f, $"Image similarity too low: {similarity}");
+    }
+
+    [Fact]
+    public async Task DownloadFaviconAsync_ShouldThrowKavitaException_WhenPreviouslyFailedUrlExistsInCache()
+    {
+        // Arrange
+        var testUrl = "https://example.com";
+        var encodeFormat = EncodeFormat.WEBP;
+
+        var provider = Substitute.For<IEasyCachingProvider>();
+        provider.GetAsync<string>(Arg.Any<string>())
+            .Returns(new CacheValue<string>(string.Empty, true)); // Simulate previous failure
+
+        _cacheFactory.GetCachingProvider(EasyCacheProfiles.Favicon).Returns(provider);
+
+        // Act & Assert
+        await Assert.ThrowsAsync<KavitaException>(() =>
+            _coverDbService.DownloadFaviconAsync(testUrl, encodeFormat));
+    }
+
+    #endregion
+
+
 }