[FFmpeg-devel] [PATCH] avformat/mpegts: use get_packet_size in mpegts_resync for determining raw_packet_size

[Marton Balint]

The old resync logic had some bugs, for example the packet size could stuck
into 192 bytes, because pos47_full was not updated for every packet, and for
unseekable inputs the resync logic simply skipped some 0x47 sync bytes,
therefore the calculated distance between sync bytes was a multiple of 188
bytes.

AVIO only buffers a single packet (for UDP/mpegts, that usually means 1316
bytes), so for every ten consecutive 188-byte MPEGTS packets there was always a
seek failure, and that caused the old code to not find the 188 byte pattern
across 10 consecutive packets.

This patch changes the custom logic to the one which is used when probing to
determine the packet size. This was already proposed as a FIXME for a long
time...
---
 libavformat/mpegts.c | 51 ++++++++++----------------------------------
 1 file changed, 11 insertions(+), 40 deletions(-)

diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
index a065c61c40..c6fd3e1cef 100644
--- a/libavformat/mpegts.c
+++ b/libavformat/mpegts.c
@@ -123,10 +123,6 @@ struct MpegTSContext {
     /** raw packet size, including FEC if present */
     int raw_packet_size;
 
-    int size_stat[3];
-    int size_stat_count;
-#define SIZE_STAT_THRESHOLD 10
-
     int64_t pos47_full;
 
     /** if true, all pids are analyzed to find streams */
@@ -2840,41 +2836,6 @@ static int handle_packet(MpegTSContext *ts, const uint8_t *packet, int64_t pos)
     return 0;
 }
 
-static void reanalyze(MpegTSContext *ts) {
-    AVIOContext *pb = ts->stream->pb;
-    int64_t pos = avio_tell(pb);
-    if (pos < 0)
-        return;
-    pos -= ts->pos47_full;
-    if (pos == TS_PACKET_SIZE) {
-        ts->size_stat[0] ++;
-    } else if (pos == TS_DVHS_PACKET_SIZE) {
-        ts->size_stat[1] ++;
-    } else if (pos == TS_FEC_PACKET_SIZE) {
-        ts->size_stat[2] ++;
-    }
-
-    ts->size_stat_count ++;
-    if (ts->size_stat_count > SIZE_STAT_THRESHOLD) {
-        int newsize = 0;
-        if (ts->size_stat[0] > SIZE_STAT_THRESHOLD) {
-            newsize = TS_PACKET_SIZE;
-        } else if (ts->size_stat[1] > SIZE_STAT_THRESHOLD) {
-            newsize = TS_DVHS_PACKET_SIZE;
-        } else if (ts->size_stat[2] > SIZE_STAT_THRESHOLD) {
-            newsize = TS_FEC_PACKET_SIZE;
-        }
-        if (newsize && newsize != ts->raw_packet_size) {
-            av_log(ts->stream, AV_LOG_WARNING, "changing packet size to %d\n", newsize);
-            ts->raw_packet_size = newsize;
-        }
-        ts->size_stat_count = 0;
-        memset(ts->size_stat, 0, sizeof(ts->size_stat));
-    }
-}
-
-/* XXX: try to find a better synchro over several packets (use
- * get_packet_size() ?) */
 static int mpegts_resync(AVFormatContext *s, int seekback, const uint8_t *current_packet)
 {
     MpegTSContext *ts = s->priv_data;
@@ -2896,8 +2857,18 @@ static int mpegts_resync(AVFormatContext *s, int seekback, const uint8_t *curren
         if (avio_feof(pb))
             return AVERROR_EOF;
         if (c == 0x47) {
+            int new_packet_size, ret;
             avio_seek(pb, -1, SEEK_CUR);
-            reanalyze(s->priv_data);
+            pos = avio_tell(pb);
+            ret = ffio_ensure_seekback(pb, PROBE_PACKET_MAX_BUF);
+            if (ret < 0)
+                return ret;
+            new_packet_size = get_packet_size(s);
+            if (new_packet_size > 0 && new_packet_size != ts->raw_packet_size) {
+                av_log(ts->stream, AV_LOG_WARNING, "changing packet size to %d\n", new_packet_size);
+                ts->raw_packet_size = new_packet_size;
+            }
+            avio_seek(pb, pos, SEEK_SET);
             return 0;
         }
     }
-- 
2.26.1