avformat/dvdvideodec: discard duplicate or partial AC3 samples

Some DVD muxers signal segments to start with duplicate audio samples
when starting extraction from a chapter marker or terminate seamless PGs
on partial audio samples (causing corrupt AC3 frames). Clean up after
these muxers by tracking frames with duplicate PTS and eliminating
partial AC3 frames.

This results in smoother chapter extraction and overall seeking experience,
with linear PTS and AC3 delay within 32ms (1 frame) away from the video.

The issue was not apparent until the flushing pattern was replaced with
a full subdemux reset, as the flushing dropped the frames prematurely,
along side others, as such they were never present to begin with.

libavformat/dvdvideodec.c

@@ -38,6 +38,7 @@
 #include <dvdread/ifo_types.h>
 #include <dvdread/nav_read.h>
 
+#include "libavcodec/ac3_parser.h"
 #include "libavutil/avstring.h"
 #include "libavutil/avutil.h"
 #include "libavutil/intreadwrite.h"
@@ -166,6 +167,7 @@ typedef struct DVDVideoDemuxContext {
     int64_t                     first_pts;          /* the PTS of the first video keyframe */
     int                         play_started;       /* signal that playback has started */
     DVDVideoPlaybackState       play_state;         /* the active playback state */
+    int64_t                     *prev_pts;          /* track the previous PTS emitted per stream */
     int64_t                     pts_offset;         /* PTS discontinuity offset (ex. VOB change) */
     int                         seek_warned;        /* signal that we warned about seeking limits */
     int                         subdemux_reset;     /* signal that subdemuxer should be reset */
@@ -1566,7 +1568,7 @@ static int dvdvideo_read_header(AVFormatContext *s)
             (ret = dvdvideo_subdemux_open(s)) < 0)
         return ret;
 
-        return 0;
+        goto end_ready;
     }
 
     if (c->opt_pgc && (c->opt_chapter_start > 1 || c->opt_chapter_end > 0 || c->opt_preindex)) {
@@ -1603,6 +1605,14 @@ static int dvdvideo_read_header(AVFormatContext *s)
         (ret = dvdvideo_subdemux_open(s)) < 0)
         return ret;
 
+end_ready:
+    c->prev_pts = av_malloc(s->nb_streams * sizeof(int64_t));
+    if (!c->prev_pts)
+        return AVERROR(ENOMEM);
+
+    for (int i = 0; i < s->nb_streams; i++)
+        c->prev_pts[i] = AV_NOPTS_VALUE;
+
     return 0;
 }
 
@@ -1614,6 +1624,8 @@ static int dvdvideo_read_packet(AVFormatContext *s, AVPacket *pkt)
     int is_key     = 0;
     int st_mapped  = 0;
     AVStream *st_subdemux;
+    uint8_t  ac3_bitstream_id;
+    uint16_t ac3_frame_size;
 
     ret = av_read_frame(c->mpeg_ctx, pkt);
     if (ret < 0) {
@@ -1661,11 +1673,27 @@ static int dvdvideo_read_packet(AVFormatContext *s, AVPacket *pkt)
     if (pkt->pts < 0)
         goto discard;
 
+    /* clean up after DVD muxers which end seamless PGs on duplicate or partial AC3 samples */
+    if (st_subdemux->codecpar->codec_type == AVMEDIA_TYPE_AUDIO &&
+        st_subdemux->codecpar->codec_id == AV_CODEC_ID_AC3) {
+
+        if (pkt->pts <= c->prev_pts[pkt->stream_index])
+            goto discard;
+
+        ret = av_ac3_parse_header(pkt->buf->data, pkt->size,
+                                  &ac3_bitstream_id, &ac3_frame_size);
+
+        if (ret < 0 || pkt->size != ac3_frame_size)
+            goto discard;
+    }
+
     av_log(s, AV_LOG_TRACE, "st=%d pts=%" PRId64 " dts=%" PRId64 " "
                             "pts_offset=%" PRId64 " first_pts=%" PRId64 "\n",
                             pkt->stream_index, pkt->pts, pkt->dts,
                             c->pts_offset, c->first_pts);
 
+    c->prev_pts[pkt->stream_index] = pkt->pts;
+
     return 0;
 
 discard:
@@ -1673,6 +1701,9 @@ discard:
            "Discarding frame @ st=%d pts=%" PRId64 " dts=%" PRId64 " is_key=%d st_mapped=%d\n",
            st_mapped ? pkt->stream_index : -1, pkt->pts, pkt->dts, is_key, st_mapped);
 
+    if (st_mapped)
+        c->prev_pts[pkt->stream_index] = pkt->pts;
+
     return FFERROR_REDO;
 }
 
@@ -1689,6 +1720,9 @@ static int dvdvideo_close(AVFormatContext *s)
 
     dvdvideo_ifo_close(s);
 
+    if (c->prev_pts)
+        av_freep(&c->prev_pts);
+
     return 0;
 }