Probe .idx instead of .sub for external VobSub subtitle language detection

External VobSub subtitle pairs (.idx and .sub) were only probed via the
bare .sub file. In cases where multiple languages are present, this
results in missing language metadata.

Fix by detecting the matching .idx file during media info resolution
to run ffprobe on that file and skip processing the .sub entirely.
ffprobe will automatically find the matching (same directory,
case-sensitive base) .sub file and process both.

Added regression tests covering idx/sub pairing, unpaired files,
cross-directory pairs, and language-flagged filenames.

Fixes #17745

1 files changed, 140 insertions, 0 deletions

diff --git a/tests/Jellyfin.Providers.Tests/MediaInfo/MediaInfoResolverTests.cs b/tests/Jellyfin.Providers.Tests/MediaInfo/MediaInfoResolverTests.cs
index 876f18741f..ce451861ef 100644
--- a/tests/Jellyfin.Providers.Tests/MediaInfo/MediaInfoResolverTests.cs
+++ b/tests/Jellyfin.Providers.Tests/MediaInfo/MediaInfoResolverTests.cs
@@ -179,6 +179,146 @@ public class MediaInfoResolverTests
         Assert.Empty(streams);
     }
 
+    [Fact]
+    public void GetExternalFiles_VobSubIdxAndSubPair_OnlyReturnsIdxFile()
+    {
+        // VobSub (.sub) payloads only carry per-track language metadata when read
+        // alongside their paired .idx index file. When both are present, only the
+        // .idx file should be returned so it (not the raw .sub) gets probed.
+        BaseItem.MediaSourceManager = Mock.Of<IMediaSourceManager>();
+
+        var video = new Movie
+        {
+            Path = VideoDirectoryPath + "/My.Video.mkv"
+        };
+
+        var directoryService = new Mock<IDirectoryService>(MockBehavior.Strict);
+        directoryService.Setup(ds => ds.GetFilePaths(It.IsRegex(VideoDirectoryRegex), It.IsAny<bool>()))
+            .Returns(new[] { VideoDirectoryPath + "/My.Video.idx", VideoDirectoryPath + "/My.Video.sub" });
+        directoryService.Setup(ds => ds.GetFilePaths(It.IsRegex(MetadataDirectoryRegex), It.IsAny<bool>()))
+            .Returns(Array.Empty<string>());
+
+        var streams = _subtitleResolver.GetExternalFiles(video, directoryService.Object, false).ToList();
+
+        var stream = Assert.Single(streams);
+        Assert.EndsWith(".idx", stream.Path, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void GetExternalFiles_VobSubIdxWithoutMatchingSub_DoesNotReturnIdxFile()
+    {
+        // An .idx file with no paired .sub cannot be probed for anything, so it must be
+        // left out entirely rather than surfaced as a doomed-to-fail probe candidate.
+        // Surfacing it anyway would also make it "exist" from Jellyfin's perspective
+        // even after the real .sub is deleted, preventing stale subtitle stream data
+        // from ever being cleared on a rescan.
+        BaseItem.MediaSourceManager = Mock.Of<IMediaSourceManager>();
+
+        var video = new Movie
+        {
+            Path = VideoDirectoryPath + "/My.Video.mkv"
+        };
+
+        var directoryService = GetDirectoryServiceForExternalFile("My.Video.idx");
+        var streams = _subtitleResolver.GetExternalFiles(video, directoryService, false).ToList();
+
+        Assert.Empty(streams);
+    }
+
+    [Fact]
+    public void GetExternalFiles_StandaloneSubWithoutIdx_StillReturnsSubFile()
+    {
+        // Guards against the .idx/.sub pairing suppression firing when there is no
+        // .idx sidecar at all - a lone .sub file must still be returned.
+        BaseItem.MediaSourceManager = Mock.Of<IMediaSourceManager>();
+
+        var video = new Movie
+        {
+            Path = VideoDirectoryPath + "/My.Video.mkv"
+        };
+
+        var directoryService = GetDirectoryServiceForExternalFile("My.Video.sub");
+        var streams = _subtitleResolver.GetExternalFiles(video, directoryService, false).ToList();
+
+        var stream = Assert.Single(streams);
+        Assert.EndsWith(".sub", stream.Path, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void GetExternalFiles_VobSubIdxAndSubInDifferentDirectories_DoesNotPair()
+    {
+        // A same-named .idx and .sub split across the video folder and the internal
+        // metadata folder cannot be paired by ffprobe (it only looks next to the .idx),
+        // so the .sub must still be returned, but the orphaned .idx (no sibling .sub in
+        // its own directory) must be left out since it cannot be probed.
+        BaseItem.MediaSourceManager = Mock.Of<IMediaSourceManager>();
+
+        var video = new Movie
+        {
+            Path = VideoDirectoryPath + "/My.Video.mkv"
+        };
+
+        var directoryService = new Mock<IDirectoryService>(MockBehavior.Strict);
+        directoryService.Setup(ds => ds.GetFilePaths(It.IsRegex(VideoDirectoryRegex), It.IsAny<bool>()))
+            .Returns(new[] { VideoDirectoryPath + "/My.Video.sub" });
+        directoryService.Setup(ds => ds.GetFilePaths(It.IsRegex(MetadataDirectoryRegex), It.IsAny<bool>()))
+            .Returns(new[] { MetadataDirectoryPath + "/My.Video.idx" });
+
+        var streams = _subtitleResolver.GetExternalFiles(video, directoryService.Object, false).ToList();
+
+        var stream = Assert.Single(streams);
+        Assert.EndsWith(".sub", stream.Path, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void GetExternalFiles_VobSubIdxAndSubWithMatchingLanguageFlag_SuppressesSub()
+    {
+        // A .idx/.sub pair sharing the same filename flags (e.g. a language token) should
+        // still pair and suppress the .sub, just like an unflagged pair.
+        BaseItem.MediaSourceManager = Mock.Of<IMediaSourceManager>();
+
+        var video = new Movie
+        {
+            Path = VideoDirectoryPath + "/My.Video.mkv"
+        };
+
+        var directoryService = new Mock<IDirectoryService>(MockBehavior.Strict);
+        directoryService.Setup(ds => ds.GetFilePaths(It.IsRegex(VideoDirectoryRegex), It.IsAny<bool>()))
+            .Returns(new[] { VideoDirectoryPath + "/My.Video.en.idx", VideoDirectoryPath + "/My.Video.en.sub" });
+        directoryService.Setup(ds => ds.GetFilePaths(It.IsRegex(MetadataDirectoryRegex), It.IsAny<bool>()))
+            .Returns(Array.Empty<string>());
+
+        var streams = _subtitleResolver.GetExternalFiles(video, directoryService.Object, false).ToList();
+
+        var stream = Assert.Single(streams);
+        Assert.EndsWith(".idx", stream.Path, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void GetExternalFiles_VobSubIdxAndSubWithMismatchedNames_DoesNotPair()
+    {
+        // An .idx and .sub with different basenames (e.g. differing filename flags) are not
+        // a pair ffprobe would resolve. The .sub must still be returned, but the orphaned
+        // .idx (no same-named sibling .sub) must be left out since it cannot be probed.
+        BaseItem.MediaSourceManager = Mock.Of<IMediaSourceManager>();
+
+        var video = new Movie
+        {
+            Path = VideoDirectoryPath + "/My.Video.mkv"
+        };
+
+        var directoryService = new Mock<IDirectoryService>(MockBehavior.Strict);
+        directoryService.Setup(ds => ds.GetFilePaths(It.IsRegex(VideoDirectoryRegex), It.IsAny<bool>()))
+            .Returns(new[] { VideoDirectoryPath + "/My.Video.idx", VideoDirectoryPath + "/My.Video.en.sub" });
+        directoryService.Setup(ds => ds.GetFilePaths(It.IsRegex(MetadataDirectoryRegex), It.IsAny<bool>()))
+            .Returns(Array.Empty<string>());
+
+        var streams = _subtitleResolver.GetExternalFiles(video, directoryService.Object, false).ToList();
+
+        var stream = Assert.Single(streams);
+        Assert.EndsWith(".sub", stream.Path, StringComparison.OrdinalIgnoreCase);
+    }
+
     [Theory]
     [InlineData("https://url.com/My.Video.mkv")]
     [InlineData(VideoDirectoryPath)] // valid but no files found for this test