From 7efdc9c7f5e5f10b6644732b80769804c018ebe8 Mon Sep 17 00:00:00 2001
From: Jan Schmidt <jan@centricular.com>
Date: Sun, 27 Feb 2022 02:39:28 +1100
Subject: [PATCH] matroskademux: Don't parse Tracks element twice

If the tracks element was parsed from the SeekEntry, don't
parse it a second time and recreate tracks, as this
loses any tags that were read using the seek table.

If a genuinely new Tracks element is found, do read that
as it is needed for MSE support.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1798>
---
 .../gst/matroska/matroska-demux.c                 | 15 +++++++++------
 .../gst/matroska/matroska-demux.h                 |  4 +++-
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/subprojects/gst-plugins-good/gst/matroska/matroska-demux.c b/subprojects/gst-plugins-good/gst/matroska/matroska-demux.c
index f4da496521..ef51dca298 100644
--- a/subprojects/gst-plugins-good/gst/matroska/matroska-demux.c
+++ b/subprojects/gst-plugins-good/gst/matroska/matroska-demux.c
@@ -325,7 +325,7 @@ gst_matroska_demux_reset (GstElement * element)
   demux->group_id = G_MAXUINT;
 
   demux->clock = NULL;
-  demux->tracks_parsed = FALSE;
+  demux->tracks_ebml_offset = G_MAXUINT64;
 
   if (demux->clusters) {
     g_array_unref (demux->clusters);
@@ -3400,6 +3400,7 @@ gst_matroska_demux_parse_tracks (GstMatroskaDemux * demux, GstEbmlRead * ebml)
 {
   GstFlowReturn ret = GST_FLOW_OK;
   guint32 id;
+  guint64 ebml_offset = ebml->offset;
 
   DEBUG_ELEMENT_START (demux, ebml, "Tracks");
 
@@ -3440,7 +3441,7 @@ gst_matroska_demux_parse_tracks (GstMatroskaDemux * demux, GstEbmlRead * ebml)
   }
   DEBUG_ELEMENT_STOP (demux, ebml, "Tracks", ret);
 
-  demux->tracks_parsed = TRUE;
+  demux->tracks_ebml_offset = ebml_offset;
   GST_DEBUG_OBJECT (demux, "signaling no more pads");
   gst_element_no_more_pads (GST_ELEMENT (demux));
 
@@ -5633,20 +5634,22 @@ gst_matroska_demux_parse_id (GstMatroskaDemux * demux, guint32 id,
           break;
         case GST_MATROSKA_ID_TRACKS:
           GST_READ_CHECK (gst_matroska_demux_take (demux, read, &ebml));
-          if (!demux->tracks_parsed) {
+          if (demux->tracks_ebml_offset == G_MAXUINT64) {
             ret = gst_matroska_demux_parse_tracks (demux, &ebml);
-          } else {
+          } else if (demux->tracks_ebml_offset != ebml.offset) {
+            /* This is a new Tracks entry, as can happen in MSE
+             * playback */
             ret = gst_matroska_demux_update_tracks (demux, &ebml);
           }
           break;
         case GST_MATROSKA_ID_CLUSTER:
-          if (G_UNLIKELY (!demux->tracks_parsed)) {
+          if (G_UNLIKELY (demux->tracks_ebml_offset == G_MAXUINT64)) {
             if (demux->streaming) {
               GST_DEBUG_OBJECT (demux, "Cluster before Track");
               goto not_streamable;
             } else {
               ret = gst_matroska_demux_find_tracks (demux);
-              if (!demux->tracks_parsed)
+              if (demux->tracks_ebml_offset == G_MAXUINT64)
                 goto no_tracks;
             }
           }
diff --git a/subprojects/gst-plugins-good/gst/matroska/matroska-demux.h b/subprojects/gst-plugins-good/gst/matroska/matroska-demux.h
index a0a27947e5..dda76692fa 100644
--- a/subprojects/gst-plugins-good/gst/matroska/matroska-demux.h
+++ b/subprojects/gst-plugins-good/gst/matroska/matroska-demux.h
@@ -68,8 +68,10 @@ typedef struct _GstMatroskaDemux {
   gboolean                 seek_first;
 
   /* did we parse cues/tracks/segmentinfo already? */
-  gboolean                 tracks_parsed;
   GList                   *seek_parsed;
+  /* Offset of the last Tracks element parsed,
+   * to avoid reparsing or recreating tracks unecessarily */
+  guint64                 tracks_ebml_offset;
 
   /* cluster positions (optional) */
   GArray                  *clusters;