[FFmpeg-devel] [PATCH 1/7] avformat/dvdvideodec: Fix racy PTS calculation and frame drops

[Marth64 <marth64@proxyid.net>]

DVDs naturally consist of segmented MPEG-PS blobs within a VOB
(i.e. VOBs are not linear). NAV packs set the segment boundaries.
When switching between segments, discontinuities occur and thus
the subdemuxer needs to be reset. The current approach to manage
this is by invoking ff_read_frame_flush() on the subdemuxer context,
via a callback function which is invoked during the menu or dvdnav
block functions. The same subdemuxer context is used throughout
the demux, with a stretched PTS wrap bits value (64) + disabled
overflow correction, and then flushed on each segment. Eventually,
a play_end context variable is set to declare EOF.

However, this approach is wrong and racy. The block read flushes the
demuxer before the frame read is complete, causing frames to drop
on discontinuity. The play_end signal likewise ends playback before
the frame read is complete, causing frames to drop at end of the title.
To compound the issue, the PTS wrap bits value of 64 is wrong;
the VOBU limit is actually 32 and the overflow correction should work.

Instead, EOF the MPEG-PS subdemuxer organically when each VOB segment
ends, and re-open it if needed with the offset after the full frame read
is complete. In doing so, correct the PTS wrap behavior to 32 bits,
remove the racy play_end/segment_started signals and the callback pattern.
The behavior is now more similar to the HLS/DASH demuxers.

This commit fixes five intertwined issues, yielding an accurate demux:
(1) Racy segment switching
(2) Racy EOF signaling
(3) Off-by-one leading to missed packets at start of menus
(4) Incorrect PTS wrap behavior
(5) Unnecessary frame discard workarounds removed

Signed-off-by: Marth64 <marth64@proxyid.net>
---
 libavformat/dvdvideodec.c | 198 +++++++++++++++++++-------------------
 1 file changed, 100 insertions(+), 98 deletions(-)

diff --git a/libavformat/dvdvideodec.c b/libavformat/dvdvideodec.c
index 7a859071c3..e745165e00 100644
--- a/libavformat/dvdvideodec.c
+++ b/libavformat/dvdvideodec.c
@@ -58,7 +58,7 @@
 #define DVDVIDEO_MAX_PS_SEARCH_BLOCKS                   128
 #define DVDVIDEO_BLOCK_SIZE                             2048
 #define DVDVIDEO_TIME_BASE_Q                            (AVRational) { 1, 90000 }
-#define DVDVIDEO_PTS_WRAP_BITS                          64 /* VOBUs use 32 (PES allows 33) */
+#define DVDVIDEO_PTS_WRAP_BITS                          32 /* VOBUs use 32 (PES allows 33) */
 #define DVDVIDEO_LIBDVDX_LOG_BUFFER_SIZE                1024
 
 #define PCI_START_BYTE                                  45 /* complement dvdread's DSI_START_BYTE */
@@ -116,8 +116,9 @@ typedef struct DVDVideoPlaybackState {
     int                         pgc_nb_pg_est;      /* number of PGs as reported by IFOs */
     int                         pgcn;               /* ID of the PGC we are playing */
     int                         pgn;                /* ID of the PG we are in now */
+    int                         ptm_discont;        /* signal that a PTM discontinuity occurred */
+    int64_t                     ptm_offset;         /* PTM discontinuity offset (as NAV value) */
     int                         ptt;                /* ID of the chapter we are in now */
-    int64_t                     ts_offset;          /* PTS discontinuity offset (ex. VOB change) */
     uint32_t                    vobu_duration;      /* duration of the current VOBU */
     uint32_t                    vobu_e_ptm;         /* end PTM of the current VOBU */
     int                         vtsn;               /* ID of the active VTS (video title set) */
@@ -164,10 +165,11 @@ typedef struct DVDVideoDemuxContext {
 
     /* playback control */
     int64_t                     first_pts;          /* the PTS of the first video keyframe */
-    int                         play_end;           /* signal EOF to the parent demuxer */
-    DVDVideoPlaybackState       play_state;         /* the active playback state */
     int                         play_started;       /* signal that playback has started */
-    int                         segment_started;    /* signal that subdemuxer is on a segment */
+    DVDVideoPlaybackState       play_state;         /* the active playback state */
+    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 */
 } DVDVideoDemuxContext;
 
 static void dvdvideo_libdvdread_log(void *opaque, dvd_logger_level_t level,
@@ -344,7 +346,7 @@ static int dvdvideo_menu_open(AVFormatContext *s, DVDVideoPlaybackState *state)
     }
 
     /* make sure the PGC is valid */
-    state->pgcn          = c->opt_pgc - 1;
+    state->pgcn          = c->opt_pgc;
     state->pgc           = pgci_ut->lu[c->opt_menu_lu - 1].pgcit->pgci_srp[c->opt_pgc - 1].pgc;
     if (!state->pgc || !state->pgc->program_map || !state->pgc->cell_playback) {
         av_log(s, AV_LOG_ERROR, "Invalid PGC structure for menu [LU %d, PGC %d]\n",
@@ -390,14 +392,16 @@ static int dvdvideo_menu_open(AVFormatContext *s, DVDVideoPlaybackState *state)
 }
 
 static int dvdvideo_menu_next_ps_block(AVFormatContext *s, DVDVideoPlaybackState *state,
-                                       uint8_t *buf, int buf_size,
-                                       void (*flush_cb)(AVFormatContext *s))
+                                       uint8_t *buf, int buf_size, int *p_is_nav_packet)
 {
     int64_t blocks_read                   = 0;
     uint8_t read_buf[DVDVIDEO_BLOCK_SIZE] = {0};
     pci_t pci                             = (pci_t) {0};
     dsi_t dsi                             = (dsi_t) {0};
 
+    (*p_is_nav_packet)  = 0;
+    state->ptm_discont  = 0;
+
     if (buf_size != DVDVIDEO_BLOCK_SIZE) {
         av_log(s, AV_LOG_ERROR, "Invalid buffer size (expected=%d actual=%d)\n",
                                 DVDVIDEO_BLOCK_SIZE, buf_size);
@@ -463,10 +467,8 @@ static int dvdvideo_menu_next_ps_block(AVFormatContext *s, DVDVideoPlaybackState
 
         if (state->in_pgc) {
             if (state->vobu_e_ptm != pci.pci_gi.vobu_s_ptm) {
-                if (flush_cb)
-                    flush_cb(s);
-
-                state->ts_offset += state->vobu_e_ptm - pci.pci_gi.vobu_s_ptm;
+                state->ptm_discont  = 1;
+                state->ptm_offset  += state->vobu_e_ptm - pci.pci_gi.vobu_s_ptm;
             }
         } else {
             state->in_pgc        = 1;
@@ -474,13 +476,17 @@ static int dvdvideo_menu_next_ps_block(AVFormatContext *s, DVDVideoPlaybackState
         }
 
         state->vobu_e_ptm        = pci.pci_gi.vobu_e_ptm;
+        state->vobu_duration     = pci.pci_gi.vobu_e_ptm - pci.pci_gi.vobu_s_ptm;
 
         av_log(s, AV_LOG_DEBUG, "NAV packet: sector=%d "
-                                "vobu_s_ptm=%d vobu_e_ptm=%d ts_offset=%" PRId64 "\n",
+                                "vobu_s_ptm=%d vobu_e_ptm=%d ptm_offset=%" PRId64 "\n",
                                 dsi.dsi_gi.nv_pck_lbn,
-                                pci.pci_gi.vobu_s_ptm, pci.pci_gi.vobu_e_ptm, state->ts_offset);
+                                pci.pci_gi.vobu_s_ptm, pci.pci_gi.vobu_e_ptm, state->ptm_offset);
 
-        return FFERROR_REDO;
+
+        (*p_is_nav_packet) = 1;
+
+        return DVDVIDEO_BLOCK_SIZE;
     }
 
     /* we are in the middle of a VOBU, so pass on the PS packet */
@@ -610,9 +616,7 @@ end_dvdnav_error:
 }
 
 static int dvdvideo_play_next_ps_block(AVFormatContext *s, DVDVideoPlaybackState *state,
-                                       uint8_t *buf, int buf_size,
-                                       int *p_nav_event,
-                                       void (*flush_cb)(AVFormatContext *s))
+                                       uint8_t *buf, int buf_size, int *p_is_nav_packet)
 {
     DVDVideoDemuxContext *c = s->priv_data;
 
@@ -626,6 +630,9 @@ static int dvdvideo_play_next_ps_block(AVFormatContext *s, DVDVideoPlaybackState
     pci_t *e_pci;
     dsi_t *e_dsi;
 
+    (*p_is_nav_packet)  = 0;
+    state->ptm_discont  = 0;
+
     if (buf_size != DVDVIDEO_BLOCK_SIZE) {
         av_log(s, AV_LOG_ERROR, "Invalid buffer size (expected=%d actual=%d)\n",
                                 DVDVIDEO_BLOCK_SIZE, buf_size);
@@ -769,16 +776,14 @@ static int dvdvideo_play_next_ps_block(AVFormatContext *s, DVDVideoPlaybackState
                     state->in_ps = 1;
                 } else {
                     if (state->vobu_e_ptm != e_pci->pci_gi.vobu_s_ptm) {
-                        if (flush_cb)
-                            flush_cb(s);
-
-                        state->ts_offset += state->vobu_e_ptm - e_pci->pci_gi.vobu_s_ptm;
+                        state->ptm_discont = 1;
+                        state->ptm_offset  += state->vobu_e_ptm - e_pci->pci_gi.vobu_s_ptm;
                     }
                 }
 
                 state->vobu_e_ptm = e_pci->pci_gi.vobu_e_ptm;
 
-                (*p_nav_event) = nav_event;
+                (*p_is_nav_packet) = 1;
 
                 return nav_len;
             case DVDNAV_BLOCK_OK:
@@ -809,8 +814,6 @@ static int dvdvideo_play_next_ps_block(AVFormatContext *s, DVDVideoPlaybackState
                                               "this could be due to a missed NAV packet\n",
                                               state->pgn, cur_pgn);
 
-                (*p_nav_event) = nav_event;
-
                 return nav_len;
             case DVDNAV_WAIT:
                 if (dvdnav_wait_skip(state->dvdnav) != DVDNAV_STATUS_OK) {
@@ -908,7 +911,7 @@ static int dvdvideo_chapters_setup_preindex(AVFormatContext *s)
     DVDVideoPlaybackState state = {0};
 
     uint8_t nav_buf[DVDVIDEO_BLOCK_SIZE];
-    int nav_event;
+    int is_nav_packet;
 
     if (c->opt_chapter_start == c->opt_chapter_end)
         return ret;
@@ -924,11 +927,11 @@ static int dvdvideo_chapters_setup_preindex(AVFormatContext *s)
 
     while (!(interrupt = ff_check_interrupt(&s->interrupt_callback))) {
         ret = dvdvideo_play_next_ps_block(s, &state, nav_buf, DVDVIDEO_BLOCK_SIZE,
-                                          &nav_event, NULL);
+                                          &is_nav_packet);
         if (ret < 0 && ret != AVERROR_EOF)
             goto end_close;
 
-        if (nav_event != DVDNAV_NAV_PACKET && ret != AVERROR_EOF)
+        if (!is_nav_packet && ret != AVERROR_EOF)
             continue;
 
         if (state.ptt == last_ptt) {
@@ -1420,46 +1423,40 @@ static int dvdvideo_subp_stream_add_all(AVFormatContext *s)
     return 0;
 }
 
-static void dvdvideo_subdemux_flush(AVFormatContext *s)
-{
-    DVDVideoDemuxContext *c = s->priv_data;
-
-    if (!c->segment_started)
-        return;
-
-    av_log(s, AV_LOG_DEBUG, "flushing sub-demuxer\n");
-    avio_flush(&c->mpeg_pb.pub);
-    ff_read_frame_flush(c->mpeg_ctx);
-    c->segment_started = 0;
-}
-
 static int dvdvideo_subdemux_read_data(void *opaque, uint8_t *buf, int buf_size)
 {
     AVFormatContext *s = opaque;
     DVDVideoDemuxContext *c = s->priv_data;
 
-    int ret = 0;
-    int nav_event;
-
-    if (c->play_end)
-        return AVERROR_EOF;
+    int ret;
+    int is_nav_packet;
 
     if (c->opt_menu)
-        ret = dvdvideo_menu_next_ps_block(s, &c->play_state, buf, buf_size,
-                                          dvdvideo_subdemux_flush);
+        ret = dvdvideo_menu_next_ps_block(s, &c->play_state, buf, buf_size, &is_nav_packet);
     else
-        ret = dvdvideo_play_next_ps_block(opaque, &c->play_state, buf, buf_size,
-                                          &nav_event, dvdvideo_subdemux_flush);
+        ret = dvdvideo_play_next_ps_block(s, &c->play_state, buf, buf_size, &is_nav_packet);
 
-    if (ret == AVERROR_EOF) {
-        c->mpeg_pb.pub.eof_reached = 1;
-        c->play_end = 1;
+    if (ret < 0)
+        goto subdemux_eof;
 
-        return AVERROR_EOF;
-    }
+    if (is_nav_packet) {
+        if (c->play_state.ptm_discont) {
+            c->subdemux_reset = 1;
+
+            ret = AVERROR_EOF;
+            goto subdemux_eof;
+        }
 
-    if (ret >= 0 && nav_event == DVDNAV_NAV_PACKET)
         return FFERROR_REDO;
+    }
+
+    return ret;
+
+subdemux_eof:
+    c->mpeg_pb.pub.eof_reached = 1;
+    c->mpeg_pb.pub.error       = ret;
+    c->mpeg_pb.pub.read_packet = NULL;
+    c->mpeg_pb.pub.buf_end     = c->mpeg_pb.pub.buf_ptr = c->mpeg_pb.pub.buffer;
 
     return ret;
 }
@@ -1469,7 +1466,9 @@ static void dvdvideo_subdemux_close(AVFormatContext *s)
     DVDVideoDemuxContext *c = s->priv_data;
 
     av_freep(&c->mpeg_pb.pub.buffer);
+    memset(&c->mpeg_pb, 0x00, sizeof(c->mpeg_pb));
     avformat_close_input(&c->mpeg_ctx);
+    c->mpeg_ctx = NULL;
 }
 
 static int dvdvideo_subdemux_open(AVFormatContext *s)
@@ -1501,12 +1500,23 @@ static int dvdvideo_subdemux_open(AVFormatContext *s)
     c->mpeg_ctx->max_analyze_duration = 0;
     c->mpeg_ctx->interrupt_callback = s->interrupt_callback;
     c->mpeg_ctx->pb = &c->mpeg_pb.pub;
-    c->mpeg_ctx->correct_ts_overflow = 0;
-    c->mpeg_ctx->io_open = NULL;
 
     return avformat_open_input(&c->mpeg_ctx, "", &ff_mpegps_demuxer.p, NULL);
 }
 
+static int dvdvideo_subdemux_reset(AVFormatContext *s)
+{
+    int ret;
+
+    av_log(s, AV_LOG_DEBUG, "resetting sub-demuxer\n");
+
+    dvdvideo_subdemux_close(s);
+    if ((ret = dvdvideo_subdemux_open(s)) < 0)
+        return ret;
+
+    return 0;
+}
+
 static int dvdvideo_read_header(AVFormatContext *s)
 {
     DVDVideoDemuxContext *c = s->priv_data;
@@ -1604,72 +1614,64 @@ static int dvdvideo_read_packet(AVFormatContext *s, AVPacket *pkt)
     DVDVideoDemuxContext *c = s->priv_data;
 
     int ret;
-    enum AVMediaType st_type;
-    int found_stream = 0;
-
-    if (c->play_end)
-        return AVERROR_EOF;
+    int st_matched = 0;
+    AVStream *st_subdemux;
 
     ret = av_read_frame(c->mpeg_ctx, pkt);
+    if (ret < 0) {
+        if (c->subdemux_reset) {
+            c->subdemux_reset = 0;
+            c->pts_offset     = c->play_state.ptm_offset;
 
-    if (ret < 0)
-        return ret;
+            if ((ret = dvdvideo_subdemux_reset(s)) < 0)
+                return ret;
+
+            return FFERROR_REDO;
+        }
 
-    if (!c->segment_started)
-        c->segment_started = 1;
+        return ret;
+    }
 
-    st_type = c->mpeg_ctx->streams[pkt->stream_index]->codecpar->codec_type;
+    st_subdemux = c->mpeg_ctx->streams[pkt->stream_index];
 
     /* map the subdemuxer stream to the parent demuxer's stream (by startcode) */
     for (int i = 0; i < s->nb_streams; i++) {
-        if (s->streams[i]->id == c->mpeg_ctx->streams[pkt->stream_index]->id) {
+        if (s->streams[i]->id == st_subdemux->id) {
             pkt->stream_index = s->streams[i]->index;
-            found_stream = 1;
+            st_matched = 1;
             break;
         }
     }
 
-    if (!found_stream) {
-        av_log(s, AV_LOG_DEBUG, "discarding frame with stream that was not in IFO headers "
-                                "(stream id=%d)\n", c->mpeg_ctx->streams[pkt->stream_index]->id);
-
-        return FFERROR_REDO;
-    }
+    if (!st_matched)
+        goto discard;
 
     if (pkt->pts != AV_NOPTS_VALUE && pkt->dts != AV_NOPTS_VALUE) {
         if (!c->play_started) {
-            /* try to start at the beginning of a GOP */
-            if (st_type != AVMEDIA_TYPE_VIDEO || !(pkt->flags & AV_PKT_FLAG_KEY)) {
-                av_log(s, AV_LOG_VERBOSE, "Discarding packet which is not a video keyframe or "
-                                          "with unset PTS/DTS at start\n");
-                return FFERROR_REDO;
-            }
-
             c->first_pts = pkt->pts;
             c->play_started = 1;
         }
 
-        pkt->pts += c->play_state.ts_offset - c->first_pts;
-        pkt->dts += c->play_state.ts_offset - c->first_pts;
-
-        if (pkt->pts < 0) {
-            av_log(s, AV_LOG_VERBOSE, "Discarding packet with negative PTS (st=%d pts=%" PRId64 "), "
-                                      "this is OK at start of playback\n",
-                                      pkt->stream_index, pkt->pts);
-
-            return FFERROR_REDO;
-        }
+        pkt->pts += c->pts_offset - c->first_pts;
+        pkt->dts += c->pts_offset - c->first_pts;
     } else {
-        av_log(s, AV_LOG_WARNING, "Unset PTS or DTS @ st=%d pts=%" PRId64 " dts=%" PRId64 "\n",
-                                  pkt->stream_index, pkt->pts, pkt->dts);
+        av_log(s, AV_LOG_WARNING, "Received packet with unset PTS or DTS\n");
     }
 
     av_log(s, AV_LOG_TRACE, "st=%d pts=%" PRId64 " dts=%" PRId64 " "
-                            "ts_offset=%" PRId64 " first_pts=%" PRId64 "\n",
+                            "pts_offset=%" PRId64 " first_pts=%" PRId64 "\n",
                             pkt->stream_index, pkt->pts, pkt->dts,
-                            c->play_state.ts_offset, c->first_pts);
+                            c->pts_offset);
+
+    return 0;
+
+discard:
+    av_log(s, AV_LOG_VERBOSE, "Discarding packet @ st=%d pts=%" PRId64 " dts=%" PRId64 " "
+                              "st_matched=%d\n",
+                              st_matched ? pkt->stream_index : -1, pkt->pts, pkt->dts,
+                              st_matched);
 
-    return c->play_end ? AVERROR_EOF : 0;
+    return FFERROR_REDO;
 }
 
 static int dvdvideo_close(AVFormatContext *s)
-- 
2.34.1

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