From e8fc4e674b9ecbbbad8e743caf50000b4e39d0d9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?C=C3=A9dric=20Verstraeten?= <cedric@verstraeten.io>
Date: Mon, 4 May 2026 21:07:53 +0200
Subject: [PATCH] Force fragment flush on close keyframes

Track LastKeyframeRawPTS and force a fragment flush when two consecutive keyframes on the video track arrive unexpectedly close (<500ms). This detects upstream loop/restart discontinuities (e.g. ffmpeg stream_loop seams) where a fresh IDR would otherwise become a mid-fragment sync sample and cause MSE players to reject the fragment. Emits a warning when triggered and updates LastKeyframeRawPTS for video samples. Also add a sample MP4 file to machinery/data.
---
 machinery/src/video/mp4.go | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/machinery/src/video/mp4.go b/machinery/src/video/mp4.go
index 626269f..bc7643d 100644
--- a/machinery/src/video/mp4.go
+++ b/machinery/src/video/mp4.go
@@ -56,6 +56,7 @@ type MP4 struct {
 	FreeBoxSize             int64
 	FragmentStartRawPTS     uint64            // Raw PTS for timing when to flush fragments
 	FragmentStartDTS        uint64            // Accumulated VideoTotalDuration at fragment start (matches tfdt)
+	LastKeyframeRawPTS      uint64            // Raw PTS of the most recently seen keyframe (in any fragment)
 	MoofBoxes               int64             // Number of moof boxes in the file
 	MoofBoxSizes            []int64           // Sizes of each moof box
 	SegmentDurations        []uint64          // Duration of each segment in timescale units
@@ -307,6 +308,27 @@ func (mp4 *MP4) AddSampleToTrack(trackID uint32, isKeyframe bool, data []byte, p
 		}
 		shouldFlush := !mp4.Start || elapsed >= FragmentDurationMs
 
+		// Detect upstream source-loop / restart discontinuity. When an MP4 is
+		// looped through virtual-rtsp (ffmpeg `-stream_loop -1 -re`) the loop
+		// seam emits a fresh IDR much sooner than a normal GOP would. PTS keeps
+		// growing monotonically, so the timing-only `elapsed` check above does
+		// not catch it and the seam IDR ends up as a mid-fragment sync sample.
+		// MSE-based players (Video.js / Chromium / Firefox) reject the resulting
+		// fragment with a "media corruption" error because the inner IDR resets
+		// frame_num/POC inside what they expect to be a single GOP. Force a
+		// fragment boundary whenever two consecutive keyframes arrive much
+		// closer than a normal GOP (here: < 500 ms apart). This isolates the
+		// seam IDR into its own fragment so each fragment stays a clean GOP.
+		if !shouldFlush && trackID == uint32(mp4.VideoTrack) && mp4.Start &&
+			mp4.LastKeyframeRawPTS > 0 && pts > mp4.LastKeyframeRawPTS &&
+			pts-mp4.LastKeyframeRawPTS < 500 {
+			log.Log.Warning(fmt.Sprintf("mp4.AddSampleToTrack(): forcing fragment flush at unexpectedly close keyframe (gap=%d ms, fragment elapsed=%d ms) - likely upstream loop/restart discontinuity", pts-mp4.LastKeyframeRawPTS, elapsed))
+			shouldFlush = true
+		}
+		if trackID == uint32(mp4.VideoTrack) {
+			mp4.LastKeyframeRawPTS = pts
+		}
+
 		if shouldFlush {
 			// Write the previous segment to the file
 			if mp4.Start {