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

[Marton Balint]

On Wed, 20 May 2020, Marton Balint wrote:

> 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...

Ping, will apply soon.

Regards,
Marton

> ---
> 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
>
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