libavformat/matroskadec: set fixed duration for subtitles

The matroska specification states the start time and duration of
subtitle entries (of any format) are always encoded in the block TimeStamp
and BlockDuration. The duration of those packets should be considered
absolute/fixed and should not ever change throughout the pipeline, even
if the duration is 0. Currently the demuxer corrects the initial
subtitle packets duration with the pts of the first subtitle
which the duration is not 0. While this mechanism might be useful for
other use-cases (e.g. for sub formats without end time) it is not valid
for matroska. Hence, it introduces AV_PKT_FLAG_FIXED_DURATION flag which
decoders might use to flag the duration of a given packet should not be
changed.

Signed-off-by: Miguel Borges de Freitas <enen92@kodi.tv>

libavcodec/packet.h

@@ -446,6 +446,11 @@ typedef struct AVPacketList {
  * be discarded by the decoder.  I.e. Non-reference frames.
  */
 #define AV_PKT_FLAG_DISPOSABLE 0x0010
+/**
+ * Flag is used to indicate packets in which the duration is absolute
+ * and should not be changed.
+ */
+#define AV_PKT_FLAG_FIXED_DURATION 0x0020
 
 enum AVSideDataParamChangeFlags {
 #if FF_API_OLD_CHANNEL_LAYOUT

libavformat/demux.c

@@ -914,7 +914,8 @@ static void update_initial_durations(AVFormatContext *s, AVStream *st,
             pktl->pkt.dts = cur_dts;
             if (!sti->avctx->has_b_frames)
                 pktl->pkt.pts = cur_dts;
-            pktl->pkt.duration = duration;
+            if ((pktl->pkt.flags & AV_PKT_FLAG_FIXED_DURATION) != AV_PKT_FLAG_FIXED_DURATION)
+                pktl->pkt.duration = duration;
         } else
             break;
         cur_dts = pktl->pkt.dts + pktl->pkt.duration;

libavformat/matroskadec.c

@@ -3673,6 +3673,9 @@ static int matroska_parse_frame(MatroskaDemuxContext *matroska,
     pkt->pos = pos;
     pkt->duration = lace_duration;
 
+    if (st->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE)
+        pkt->flags |= AV_PKT_FLAG_FIXED_DURATION;
+
     res = avpriv_packet_list_put(&matroska->queue, pkt, NULL, 0);
     if (res < 0) {
         av_packet_unref(pkt);