From 6e3c1874936b767754cd9267749f35b27f2ae65f Mon Sep 17 00:00:00 2001
From: Piotr Niełacny <piotr.nielacny@gmail.com>
Date: Wed, 15 Jul 2026 14:55:55 +0200
Subject: Fix race condition in concurrent subtitle conversion

SubtitleEncoder.ConvertSubtitles parsed subtitles with libse's static
Subtitle.Parse, which iterates a statically cached list of shared
SubtitleFormat instances. Format parsers keep mutable per-parse state on
the instance, so concurrent subtitle requests corrupted each other's
output (cues mixed across streams and languages, truncated files) or
failed with NullReferenceException when format detection broke down and
Subtitle.Parse returned null.

Parse through the injected ISubtitleParser instead. SubtitleEditParser
instantiates a fresh format parser per call, so requests no longer share
state. Its Parse method now returns the libse Subtitle directly (the
SubtitleTrackInfo flattening was unused since the SubtitleEdit writer
rework) so the writers keep full fidelity such as ASS styling.
---
 .../Subtitles/SubtitleEncoderTests.cs              | 94 ++++++++++++++++++++++
 1 file changed, 94 insertions(+)

(limited to 'tests/Jellyfin.MediaEncoding.Tests/Subtitles/SubtitleEncoderTests.cs')

diff --git a/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SubtitleEncoderTests.cs b/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SubtitleEncoderTests.cs
index ce1f005f40..2d0fa29c9a 100644
--- a/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SubtitleEncoderTests.cs
+++ b/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SubtitleEncoderTests.cs
@@ -1,3 +1,8 @@
+using System;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
 using AutoFixture;
@@ -6,12 +11,16 @@ using MediaBrowser.MediaEncoding.Subtitles;
 using MediaBrowser.Model.Dto;
 using MediaBrowser.Model.Entities;
 using MediaBrowser.Model.MediaInfo;
+using Microsoft.Extensions.Logging.Abstractions;
 using Xunit;
 
 namespace Jellyfin.MediaEncoding.Subtitles.Tests
 {
     public class SubtitleEncoderTests
     {
+        private const int StreamCount = 8;
+        private const int CueCount = 500;
+
         public static TheoryData<MediaSourceInfo, MediaStream, SubtitleEncoder.SubtitleInfo> GetReadableFile_Valid_TestData()
         {
             var data = new TheoryData<MediaSourceInfo, MediaStream, SubtitleEncoder.SubtitleInfo>();
@@ -103,5 +112,90 @@ namespace Jellyfin.MediaEncoding.Subtitles.Tests
             Assert.Equal(subtitleInfo.Format, result.Format);
             Assert.Equal(subtitleInfo.IsExternal, result.IsExternal);
         }
+
+        [Fact]
+        public void ConvertSubtitles_SequentialCalls_AreDeterministic()
+        {
+            using var encoder = CreateEncoder();
+            var sources = GenerateSources();
+
+            var first = ConvertAllSequential(encoder, sources);
+            var second = ConvertAllSequential(encoder, sources);
+
+            for (var i = 0; i < StreamCount; i++)
+            {
+                Assert.Contains($"S{i}C{CueCount - 1}", first[i], StringComparison.Ordinal);
+                Assert.Equal(first[i], second[i]);
+            }
+        }
+
+        [Fact]
+        public async Task ConvertSubtitles_ConcurrentCalls_MatchSequentialBaseline()
+        {
+            const int Iterations = 10;
+
+            using var encoder = CreateEncoder();
+            var sources = GenerateSources();
+            var baseline = ConvertAllSequential(encoder, sources);
+
+            for (var iteration = 0; iteration < Iterations; iteration++)
+            {
+                var results = await Task.WhenAll(Enumerable.Range(0, StreamCount)
+                    .Select(i => Task.Run(() => Convert(encoder, sources[i], i)))
+                    .ToArray());
+
+                for (var i = 0; i < StreamCount; i++)
+                {
+                    Assert.True(
+                        string.Equals(baseline[i], results[i], StringComparison.Ordinal),
+                        $"Iteration {iteration}: stream {i} returned corrupted content ({results[i].Length} chars vs {baseline[i].Length} baseline)");
+                }
+            }
+        }
+
+        private static SubtitleEncoder CreateEncoder()
+        {
+            var fixture = new Fixture().Customize(new AutoMoqCustomization { ConfigureMembers = true });
+            fixture.Inject<ISubtitleParser>(new SubtitleEditParser(NullLogger<SubtitleEditParser>.Instance));
+            return fixture.Create<SubtitleEncoder>();
+        }
+
+        private static byte[][] GenerateSources()
+        {
+            return Enumerable.Range(0, StreamCount)
+                .Select(i => Encoding.UTF8.GetBytes(GenerateSrt(i, CueCount)))
+                .ToArray();
+        }
+
+        private static string Convert(SubtitleEncoder encoder, byte[] source, int streamIndex)
+        {
+            using var input = new MemoryStream(source);
+            var info = new SubtitleEncoder.SubtitleInfo { Path = $"track{streamIndex}.srt", Format = "srt" };
+            using var output = encoder.ConvertSubtitles(input, info, "vtt", 0, 0, false);
+            return Encoding.UTF8.GetString(output.ToArray());
+        }
+
+        private static string[] ConvertAllSequential(SubtitleEncoder encoder, byte[][] sources)
+        {
+            return sources.Select((source, i) => Convert(encoder, source, i)).ToArray();
+        }
+
+        private static string GenerateSrt(int streamIndex, int cueCount)
+        {
+            var builder = new StringBuilder();
+            for (var i = 0; i < cueCount; i++)
+            {
+                var start = TimeSpan.FromSeconds(i * 4);
+                var end = start + TimeSpan.FromSeconds(2);
+                builder.Append(i + 1).AppendLine()
+                    .Append(start.ToString(@"hh\:mm\:ss\,fff", CultureInfo.InvariantCulture))
+                    .Append(" --> ")
+                    .AppendLine(end.ToString(@"hh\:mm\:ss\,fff", CultureInfo.InvariantCulture))
+                    .Append('S').Append(streamIndex).Append('C').Append(i).AppendLine()
+                    .AppendLine();
+            }
+
+            return builder.ToString();
+        }
     }
 }
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