From e2586eed9b04d501cd5805711cb6ad5553c1816b Mon Sep 17 00:00:00 2001
From: gnattu <gnattuoc@me.com>
Date: Wed, 5 Aug 2026 00:33:42 +0800
Subject: Fix concurrent ffmpeg segment racing
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This is a nasty one. The failure mode is:

1. Request A started FFmpeg and waited for a segment.
2. Request B requested an earlier or far away segment.
3. Jellyfin thought FFmpeg should to restart at a different position.
4. Request B killed the existing transcoding job.
5. Killing that job cancelled the same token request A was using.
6. The cancellation produced http 500 to request A.

To fix this:

we lock transcoding job state changes and segment handling per playlist, and use a thread safe counter to track how many http responses are still using each job’s segments. A job is only stopped or replaced once that counter reaches zero.
---
 Jellyfin.Api/Controllers/DynamicHlsController.cs | 31 ++++++++++++++----------
 1 file changed, 18 insertions(+), 13 deletions(-)

(limited to 'Jellyfin.Api/Controllers/DynamicHlsController.cs')

diff --git a/Jellyfin.Api/Controllers/DynamicHlsController.cs b/Jellyfin.Api/Controllers/DynamicHlsController.cs
index 4aa728b5bf..a6555a2beb 100644
--- a/Jellyfin.Api/Controllers/DynamicHlsController.cs
+++ b/Jellyfin.Api/Controllers/DynamicHlsController.cs
@@ -1456,22 +1456,16 @@ public class DynamicHlsController : BaseJellyfinApiController
 
         var segmentExtension = EncodingHelper.GetSegmentFileExtension(state.Request.SegmentContainer);
 
-        TranscodingJob? job;
-
-        if (System.IO.File.Exists(segmentPath))
-        {
-            job = _transcodeManager.OnTranscodeBeginRequest(playlistPath, TranscodingJobType);
-            _logger.LogDebug("returning {0} [it exists, try 1]", segmentPath);
-            return await GetSegmentResult(state, playlistPath, segmentPath, segmentExtension, segmentId, job, cancellationToken).ConfigureAwait(false);
-        }
-
+        // Keep segment selection and transcoding replacement under the same playlist lock.
+        // An out-of-order request must not replace a job while another request is using its output.
         using (await _transcodeManager.LockAsync(playlistPath, cancellationToken).ConfigureAwait(false))
         {
+            TranscodingJob? job;
             var startTranscoding = false;
             if (System.IO.File.Exists(segmentPath))
             {
                 job = _transcodeManager.OnTranscodeBeginRequest(playlistPath, TranscodingJobType);
-                _logger.LogDebug("returning {0} [it exists, try 2]", segmentPath);
+                _logger.LogDebug("returning {0} [it exists]", segmentPath);
                 return await GetSegmentResult(state, playlistPath, segmentPath, segmentExtension, segmentId, job, cancellationToken).ConfigureAwait(false);
             }
 
@@ -1505,6 +1499,9 @@ public class DynamicHlsController : BaseJellyfinApiController
                 // If the playlist doesn't already exist, startup ffmpeg
                 try
                 {
+                    var currentJob = _transcodeManager.GetTranscodingJob(playlistPath, TranscodingJobType);
+                    await WaitForActiveTranscodingRequests(currentJob, cancellationToken).ConfigureAwait(false);
+
                     await _transcodeManager.KillTranscodingJobs(streamingRequest.DeviceId, streamingRequest.PlaySessionId, p => false)
                         .ConfigureAwait(false);
 
@@ -1540,11 +1537,19 @@ public class DynamicHlsController : BaseJellyfinApiController
                     await job.TranscodingThrottler.UnpauseTranscoding().ConfigureAwait(false);
                 }
             }
+
+            _logger.LogDebug("returning {0} [general case]", segmentPath);
+            job ??= _transcodeManager.OnTranscodeBeginRequest(playlistPath, TranscodingJobType);
+            return await GetSegmentResult(state, playlistPath, segmentPath, segmentExtension, segmentId, job, cancellationToken).ConfigureAwait(false);
         }
+    }
 
-        _logger.LogDebug("returning {0} [general case]", segmentPath);
-        job ??= _transcodeManager.OnTranscodeBeginRequest(playlistPath, TranscodingJobType);
-        return await GetSegmentResult(state, playlistPath, segmentPath, segmentExtension, segmentId, job, cancellationToken).ConfigureAwait(false);
+    internal static async Task WaitForActiveTranscodingRequests(TranscodingJob? job, CancellationToken cancellationToken)
+    {
+        while (job?.ActiveRequestCount > 0)
+        {
+            await Task.Delay(100, cancellationToken).ConfigureAwait(false);
+        }
     }
 
     private static double[] GetSegmentLengths(StreamState state)
-- 
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