1 files changed, 101 insertions, 0 deletions
diff --git a/tests/Jellyfin.Server.Implementations.Tests/Library/LibraryManagerSortTests.cs b/tests/Jellyfin.Server.Implementations.Tests/Library/LibraryManagerSortTests.cs
new file mode 100644
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+++ b/tests/Jellyfin.Server.Implementations.Tests/Library/LibraryManagerSortTests.cs
@@ -0,0 +1,101 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using AutoFixture;
+using AutoFixture.AutoMoq;
+using Emby.Naming.Common;
+using Emby.Server.Implementations.Library;
+using Emby.Server.Implementations.Sorting;
+using Jellyfin.Data.Enums;
+using Jellyfin.Database.Implementations.Enums;
+using MediaBrowser.Controller.Configuration;
+using MediaBrowser.Controller.Entities;
+using MediaBrowser.Controller.Entities.Audio;
+using MediaBrowser.Controller.Library;
+using MediaBrowser.Controller.Persistence;
+using MediaBrowser.Controller.Resolvers;
+using MediaBrowser.Controller.Sorting;
+using MediaBrowser.Model.IO;
+using Moq;
+using Xunit;
+using BaseItem = MediaBrowser.Controller.Entities.BaseItem;
+
+namespace Jellyfin.Server.Implementations.Tests.Library;
+
+public class LibraryManagerSortTests
+{
+    [Fact]
+    public void Sort_UserDependentKey_NullUser_ThrowsArgumentException()
+    {
+        var libraryManager = CreateLibraryManager(
+            new IBaseItemComparer[] { new PlayCountComparer(), new SortNameComparer() });
+
+        BaseItem[] items =
+        {
+            new Audio { Name = "Zulu", SortName = "Zulu", Id = Guid.NewGuid() },
+            new Audio { Name = "Alpha", SortName = "Alpha", Id = Guid.NewGuid() },
+        };
+
+        // A user-dependent sort key with no user is a caller contract violation — throw,
+        // don't silently fall back to a different key (review feedback on #17395).
+        Assert.Throws<ArgumentException>(() => libraryManager.Sort(
+            items,
+            user: null,
+            new[] { (ItemSortBy.PlayCount, SortOrder.Descending) }).ToArray());
+    }
+
+    [Fact]
+    public void Sort_DateLastContentAdded_NullUser_OrdersByDateNotSortName()
+    {
+        // DateLastMediaAddedComparer does not use User (its GetDate is static), so it must NOT be
+        // treated as a user-dependent comparer: with no user it should still sort by date, not fall
+        // back to SortName.
+        var libraryManager = CreateLibraryManager(
+            new IBaseItemComparer[] { new DateLastMediaAddedComparer(), new SortNameComparer() });
+
+        // Names are chosen so date-descending and SortName-descending DISAGREE: Alpha is newest
+        // (date-desc rank 1), but Zulu sorts last alphabetically (SortName-desc rank 1). If the
+        // comparer were still tagged IUserBaseItemComparer, the null-user SortName fallback would
+        // return [Zulu, Mike, Alpha] and this assertion would fail.
+        BaseItem[] items =
+        {
+            MakeFolder("Alpha", new DateTime(2026, 1, 1)),
+            MakeFolder("Mike", new DateTime(2025, 1, 1)),
+            MakeFolder("Zulu", new DateTime(2024, 1, 1))
+        };
+
+        var sorted = libraryManager.Sort(
+            items,
+            user: null,
+            new[] { (ItemSortBy.DateLastContentAdded, SortOrder.Descending) }).ToArray();
+
+        // Descending by date => newest first: Alpha, Mike, Zulu. (SortName-desc would be Zulu, Mike, Alpha.)
+        Assert.Equal(new[] { "Alpha", "Mike", "Zulu" }, sorted.Select(i => i.Name));
+    }
+
+    private static Folder MakeFolder(string name, DateTime dateLastMediaAdded)
+        => new() { Name = name, Id = Guid.NewGuid(), DateLastMediaAdded = dateLastMediaAdded };
+
+    private static Emby.Server.Implementations.Library.LibraryManager CreateLibraryManager(IReadOnlyCollection<IBaseItemComparer> comparers)
+    {
+        var fixture = new Fixture().Customize(new AutoMoqCustomization());
+        fixture.Register(() => new NamingOptions());
+        var configMock = fixture.Freeze<Mock<IServerConfigurationManager>>();
+        configMock.Setup(c => c.ApplicationPaths.ProgramDataPath).Returns("/data");
+        // BaseItem.SortName/CreateSortName dereference this static; set it so SortName-fallback
+        // paths don't NRE in-process (mirrors AudioResolverTests in the sibling test project).
+        BaseItem.ConfigurationManager ??= configMock.Object;
+        var itemRepository = fixture.Freeze<Mock<IItemRepository>>();
+        itemRepository.Setup(i => i.RetrieveItem(It.IsAny<Guid>())).Returns<BaseItem>(null);
+        var fileSystemMock = fixture.Freeze<Mock<IFileSystem>>();
+        fileSystemMock.Setup(f => f.GetFileInfo(It.IsAny<string>())).Returns<string>(path => new FileSystemMetadata { FullName = path });
+
+        return fixture.Build<Emby.Server.Implementations.Library.LibraryManager>().Do(s => s.AddParts(
+                fixture.Create<IEnumerable<IResolverIgnoreRule>>(),
+                fixture.Create<IEnumerable<IItemResolver>>(),
+                fixture.Create<IEnumerable<IIntroProvider>>(),
+                comparers,
+                fixture.Create<IEnumerable<ILibraryPostScanTask>>()))
+            .Create();
+    }
+}