Fix concurrent ffmpeg segment racing

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.

MediaBrowser.Controller/MediaEncoding/TranscodingJob.cs

@@ -15,6 +15,7 @@ public sealed class TranscodingJob : IDisposable
     private readonly Lock _processLock = new();
     private readonly Lock _timerLock = new();
 
+    private int _activeRequestCount;
     private Timer? _killTimer;
 
     /// <summary>
@@ -64,7 +65,11 @@ public sealed class TranscodingJob : IDisposable
     /// <summary>
     /// Gets or sets the active request count.
     /// </summary>
-    public int ActiveRequestCount { get; set; }
+    public int ActiveRequestCount
+    {
+        get => Volatile.Read(ref _activeRequestCount);
+        set => Volatile.Write(ref _activeRequestCount, value);
+    }
 
     /// <summary>
     /// Gets or sets device id.
@@ -151,6 +156,20 @@ public sealed class TranscodingJob : IDisposable
     /// </summary>
     public int PingTimeout { get; set; }
 
+    /// <summary>
+    /// Increments the active request count.
+    /// </summary>
+    /// <returns>The incremented count.</returns>
+    public int IncrementActiveRequestCount()
+        => Interlocked.Increment(ref _activeRequestCount);
+
+    /// <summary>
+    /// Decrements the active request count.
+    /// </summary>
+    /// <returns>The decremented count.</returns>
+    public int DecrementActiveRequestCount()
+        => Interlocked.Decrement(ref _activeRequestCount);
+
     /// <summary>
     /// Stop kill timer.
     /// </summary>