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.

7 files changed, 145 insertions, 67 deletions

diff --git a/MediaBrowser.MediaEncoding/Subtitles/ISubtitleParser.cs b/MediaBrowser.MediaEncoding/Subtitles/ISubtitleParser.cs
index bd13437fb6..7566616f70 100644
--- a/MediaBrowser.MediaEncoding/Subtitles/ISubtitleParser.cs
+++ b/MediaBrowser.MediaEncoding/Subtitles/ISubtitleParser.cs
@@ -1,7 +1,7 @@
 #pragma warning disable CS1591
 
 using System.IO;
-using MediaBrowser.Model.MediaInfo;
+using Nikse.SubtitleEdit.Core.Common;
 
 namespace MediaBrowser.MediaEncoding.Subtitles
 {
@@ -12,8 +12,8 @@ namespace MediaBrowser.MediaEncoding.Subtitles
         /// </summary>
         /// <param name="stream">The stream.</param>
         /// <param name="fileExtension">The file extension.</param>
-        /// <returns>SubtitleTrackInfo.</returns>
-        SubtitleTrackInfo Parse(Stream stream, string fileExtension);
+        /// <returns>The parsed subtitle.</returns>
+        Subtitle Parse(Stream stream, string fileExtension);
 
         /// <summary>
         /// Determines whether the file extension is supported by the parser.
diff --git a/MediaBrowser.MediaEncoding/Subtitles/SubtitleEditParser.cs b/MediaBrowser.MediaEncoding/Subtitles/SubtitleEditParser.cs
index d060b247da..d75eea5904 100644
--- a/MediaBrowser.MediaEncoding/Subtitles/SubtitleEditParser.cs
+++ b/MediaBrowser.MediaEncoding/Subtitles/SubtitleEditParser.cs
@@ -1,10 +1,8 @@
 using System;
 using System.Collections.Generic;
-using System.Globalization;
 using System.IO;
 using System.Linq;
 using Jellyfin.Extensions;
-using MediaBrowser.Model.MediaInfo;
 using Microsoft.Extensions.Logging;
 using Nikse.SubtitleEdit.Core.Common;
 using SubtitleFormat = Nikse.SubtitleEdit.Core.SubtitleFormats.SubtitleFormat;
@@ -30,7 +28,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
         }
 
         /// <inheritdoc />
-        public SubtitleTrackInfo Parse(Stream stream, string fileExtension)
+        public Subtitle Parse(Stream stream, string fileExtension)
         {
             var subtitle = new Subtitle();
             var lines = stream.ReadAllLines().ToList();
@@ -76,21 +74,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
                 throw new ArgumentException("Unsupported format: " + fileExtension);
             }
 
-            var trackInfo = new SubtitleTrackInfo();
-            int len = subtitle.Paragraphs.Count;
-            var trackEvents = new SubtitleTrackEvent[len];
-            for (int i = 0; i < len; i++)
-            {
-                var p = subtitle.Paragraphs[i];
-                trackEvents[i] = new SubtitleTrackEvent(p.Number.ToString(CultureInfo.InvariantCulture), p.Text)
-                {
-                    StartPositionTicks = p.StartTime.TimeSpan.Ticks,
-                    EndPositionTicks = p.EndTime.TimeSpan.Ticks
-                };
-            }
-
-            trackInfo.TrackEvents = trackEvents;
-            return trackInfo;
+            return subtitle;
         }
 
         /// <inheritdoc />
diff --git a/MediaBrowser.MediaEncoding/Subtitles/SubtitleEncoder.cs b/MediaBrowser.MediaEncoding/Subtitles/SubtitleEncoder.cs
index 5301f52e01..974dded9ea 100644
--- a/MediaBrowser.MediaEncoding/Subtitles/SubtitleEncoder.cs
+++ b/MediaBrowser.MediaEncoding/Subtitles/SubtitleEncoder.cs
@@ -73,7 +73,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
             _serverConfigurationManager = serverConfigurationManager;
         }
 
-        private MemoryStream ConvertSubtitles(
+        internal MemoryStream ConvertSubtitles(
             Stream stream,
             SubtitleInfo inputInfo,
             string outputFormat,
@@ -81,7 +81,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
             long endTimeTicks,
             bool preserveOriginalTimestamps)
         {
-            var subtitle = Subtitle.Parse(stream, Path.GetExtension(inputInfo.Path));
+            var subtitle = _subtitleParser.Parse(stream, inputInfo.Format);
 
             FilterEvents(subtitle, startTimeTicks, endTimeTicks, preserveOriginalTimestamps);
 
diff --git a/tests/Jellyfin.MediaEncoding.Tests/Subtitles/AssParserTests.cs b/tests/Jellyfin.MediaEncoding.Tests/Subtitles/AssParserTests.cs
index 1f908d7e0e..b03651e5e9 100644
--- a/tests/Jellyfin.MediaEncoding.Tests/Subtitles/AssParserTests.cs
+++ b/tests/Jellyfin.MediaEncoding.Tests/Subtitles/AssParserTests.cs
@@ -15,13 +15,13 @@ namespace Jellyfin.MediaEncoding.Subtitles.Tests
             using var stream = File.OpenRead("Test Data/example.ass");
 
             var parsed = new SubtitleEditParser(new NullLogger<SubtitleEditParser>()).Parse(stream, "ass");
-            Assert.Single(parsed.TrackEvents);
-            var trackEvent = parsed.TrackEvents[0];
+            Assert.Single(parsed.Paragraphs);
+            var paragraph = parsed.Paragraphs[0];
 
-            Assert.Equal("1", trackEvent.Id);
-            Assert.Equal(TimeSpan.Parse("00:00:01.18", CultureInfo.InvariantCulture).Ticks, trackEvent.StartPositionTicks);
-            Assert.Equal(TimeSpan.Parse("00:00:06.85", CultureInfo.InvariantCulture).Ticks, trackEvent.EndPositionTicks);
-            Assert.Equal("{\\pos(400,570)}Like an Angel with pity on nobody" + Environment.NewLine + "The second line in subtitle", trackEvent.Text);
+            Assert.Equal(1, paragraph.Number);
+            Assert.Equal(TimeSpan.Parse("00:00:01.18", CultureInfo.InvariantCulture).Ticks, paragraph.StartTime.TimeSpan.Ticks);
+            Assert.Equal(TimeSpan.Parse("00:00:06.85", CultureInfo.InvariantCulture).Ticks, paragraph.EndTime.TimeSpan.Ticks);
+            Assert.Equal("{\\pos(400,570)}Like an Angel with pity on nobody" + Environment.NewLine + "The second line in subtitle", paragraph.Text);
         }
     }
 }
diff --git a/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SrtParserTests.cs b/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SrtParserTests.cs
index b7152961cd..01a35e6cb0 100644
--- a/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SrtParserTests.cs
+++ b/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SrtParserTests.cs
@@ -15,19 +15,19 @@ namespace Jellyfin.MediaEncoding.Subtitles.Tests
             using var stream = File.OpenRead("Test Data/example.srt");
 
             var parsed = new SubtitleEditParser(new NullLogger<SubtitleEditParser>()).Parse(stream, "srt");
-            Assert.Equal(2, parsed.TrackEvents.Count);
-
-            var trackEvent1 = parsed.TrackEvents[0];
-            Assert.Equal("1", trackEvent1.Id);
-            Assert.Equal(TimeSpan.Parse("00:02:17.440", CultureInfo.InvariantCulture).Ticks, trackEvent1.StartPositionTicks);
-            Assert.Equal(TimeSpan.Parse("00:02:20.375", CultureInfo.InvariantCulture).Ticks, trackEvent1.EndPositionTicks);
-            Assert.Equal("Senator, we're making" + Environment.NewLine + "our final approach into Coruscant.", trackEvent1.Text);
-
-            var trackEvent2 = parsed.TrackEvents[1];
-            Assert.Equal("2", trackEvent2.Id);
-            Assert.Equal(TimeSpan.Parse("00:02:20.476", CultureInfo.InvariantCulture).Ticks, trackEvent2.StartPositionTicks);
-            Assert.Equal(TimeSpan.Parse("00:02:22.501", CultureInfo.InvariantCulture).Ticks, trackEvent2.EndPositionTicks);
-            Assert.Equal("Very good, Lieutenant.", trackEvent2.Text);
+            Assert.Equal(2, parsed.Paragraphs.Count);
+
+            var paragraph1 = parsed.Paragraphs[0];
+            Assert.Equal(1, paragraph1.Number);
+            Assert.Equal(TimeSpan.Parse("00:02:17.440", CultureInfo.InvariantCulture).Ticks, paragraph1.StartTime.TimeSpan.Ticks);
+            Assert.Equal(TimeSpan.Parse("00:02:20.375", CultureInfo.InvariantCulture).Ticks, paragraph1.EndTime.TimeSpan.Ticks);
+            Assert.Equal("Senator, we're making" + Environment.NewLine + "our final approach into Coruscant.", paragraph1.Text);
+
+            var paragraph2 = parsed.Paragraphs[1];
+            Assert.Equal(2, paragraph2.Number);
+            Assert.Equal(TimeSpan.Parse("00:02:20.476", CultureInfo.InvariantCulture).Ticks, paragraph2.StartTime.TimeSpan.Ticks);
+            Assert.Equal(TimeSpan.Parse("00:02:22.501", CultureInfo.InvariantCulture).Ticks, paragraph2.EndTime.TimeSpan.Ticks);
+            Assert.Equal("Very good, Lieutenant.", paragraph2.Text);
         }
 
         [Fact]
@@ -36,19 +36,19 @@ namespace Jellyfin.MediaEncoding.Subtitles.Tests
             using var stream = File.OpenRead("Test Data/example2.srt");
 
             var parsed = new SubtitleEditParser(new NullLogger<SubtitleEditParser>()).Parse(stream, "srt");
-            Assert.Equal(2, parsed.TrackEvents.Count);
-
-            var trackEvent1 = parsed.TrackEvents[0];
-            Assert.Equal("311", trackEvent1.Id);
-            Assert.Equal(TimeSpan.Parse("00:16:46.465", CultureInfo.InvariantCulture).Ticks, trackEvent1.StartPositionTicks);
-            Assert.Equal(TimeSpan.Parse("00:16:49.009", CultureInfo.InvariantCulture).Ticks, trackEvent1.EndPositionTicks);
-            Assert.Equal("Una vez que la gente se entere" + Environment.NewLine + Environment.NewLine + "de que ustedes están aquí,", trackEvent1.Text);
-
-            var trackEvent2 = parsed.TrackEvents[1];
-            Assert.Equal("312", trackEvent2.Id);
-            Assert.Equal(TimeSpan.Parse("00:16:49.092", CultureInfo.InvariantCulture).Ticks, trackEvent2.StartPositionTicks);
-            Assert.Equal(TimeSpan.Parse("00:16:51.470", CultureInfo.InvariantCulture).Ticks, trackEvent2.EndPositionTicks);
-            Assert.Equal("este lugar se convertirá" + Environment.NewLine + Environment.NewLine + "en un maldito zoológico.", trackEvent2.Text);
+            Assert.Equal(2, parsed.Paragraphs.Count);
+
+            var paragraph1 = parsed.Paragraphs[0];
+            Assert.Equal(311, paragraph1.Number);
+            Assert.Equal(TimeSpan.Parse("00:16:46.465", CultureInfo.InvariantCulture).Ticks, paragraph1.StartTime.TimeSpan.Ticks);
+            Assert.Equal(TimeSpan.Parse("00:16:49.009", CultureInfo.InvariantCulture).Ticks, paragraph1.EndTime.TimeSpan.Ticks);
+            Assert.Equal("Una vez que la gente se entere" + Environment.NewLine + Environment.NewLine + "de que ustedes están aquí,", paragraph1.Text);
+
+            var paragraph2 = parsed.Paragraphs[1];
+            Assert.Equal(312, paragraph2.Number);
+            Assert.Equal(TimeSpan.Parse("00:16:49.092", CultureInfo.InvariantCulture).Ticks, paragraph2.StartTime.TimeSpan.Ticks);
+            Assert.Equal(TimeSpan.Parse("00:16:51.470", CultureInfo.InvariantCulture).Ticks, paragraph2.EndTime.TimeSpan.Ticks);
+            Assert.Equal("este lugar se convertirá" + Environment.NewLine + Environment.NewLine + "en un maldito zoológico.", paragraph2.Text);
         }
     }
 }
diff --git a/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SsaParserTests.cs b/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SsaParserTests.cs
index 5b7aa7eaa9..d814088593 100644
--- a/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SsaParserTests.cs
+++ b/tests/Jellyfin.MediaEncoding.Tests/Subtitles/SsaParserTests.cs
@@ -20,19 +20,19 @@ namespace Jellyfin.MediaEncoding.Subtitles.Tests
         {
             using Stream stream = new MemoryStream(Encoding.UTF8.GetBytes(ssa));
 
-            SubtitleTrackInfo subtitleTrackInfo = _parser.Parse(stream, "ssa");
+            var subtitle = _parser.Parse(stream, "ssa");
 
-            Assert.Equal(expectedSubtitleTrackEvents.Count, subtitleTrackInfo.TrackEvents.Count);
+            Assert.Equal(expectedSubtitleTrackEvents.Count, subtitle.Paragraphs.Count);
 
             for (int i = 0; i < expectedSubtitleTrackEvents.Count; ++i)
             {
                 SubtitleTrackEvent expected = expectedSubtitleTrackEvents[i];
-                SubtitleTrackEvent actual = subtitleTrackInfo.TrackEvents[i];
+                var actual = subtitle.Paragraphs[i];
 
-                Assert.Equal(expected.Id, actual.Id);
+                Assert.Equal(expected.Id, actual.Number.ToString(CultureInfo.InvariantCulture));
                 Assert.Equal(expected.Text, actual.Text);
-                Assert.Equal(expected.StartPositionTicks, actual.StartPositionTicks);
-                Assert.Equal(expected.EndPositionTicks, actual.EndPositionTicks);
+                Assert.Equal(expected.StartPositionTicks, actual.StartTime.TimeSpan.Ticks);
+                Assert.Equal(expected.EndPositionTicks, actual.EndTime.TimeSpan.Ticks);
             }
         }
 
@@ -75,13 +75,13 @@ namespace Jellyfin.MediaEncoding.Subtitles.Tests
             using var stream = File.OpenRead("Test Data/example.ssa");
 
             var parsed = _parser.Parse(stream, "ssa");
-            Assert.Single(parsed.TrackEvents);
-            var trackEvent = parsed.TrackEvents[0];
+            Assert.Single(parsed.Paragraphs);
+            var paragraph = parsed.Paragraphs[0];
 
-            Assert.Equal("1", trackEvent.Id);
-            Assert.Equal(TimeSpan.Parse("00:00:01.18", CultureInfo.InvariantCulture).Ticks, trackEvent.StartPositionTicks);
-            Assert.Equal(TimeSpan.Parse("00:00:06.85", CultureInfo.InvariantCulture).Ticks, trackEvent.EndPositionTicks);
-            Assert.Equal("{\\pos(400,570)}Like an angel with pity on nobody", trackEvent.Text);
+            Assert.Equal(1, paragraph.Number);
+            Assert.Equal(TimeSpan.Parse("00:00:01.18", CultureInfo.InvariantCulture).Ticks, paragraph.StartTime.TimeSpan.Ticks);
+            Assert.Equal(TimeSpan.Parse("00:00:06.85", CultureInfo.InvariantCulture).Ticks, paragraph.EndTime.TimeSpan.Ticks);
+            Assert.Equal("{\\pos(400,570)}Like an angel with pity on nobody", paragraph.Text);
         }
     }
 }
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();
+        }
     }
 }