[FFmpeg-devel] [PATCH] avformat/mpegts: fix resync logic stuck in 192 bytes

[Marton Balint]

On Wed, 20 May 2020, Michael Niedermayer wrote:

> On Tue, May 19, 2020 at 09:06:59PM +0200, Marton Balint wrote:
>> pos47_full is not updated for every packet, and for unseekable inputs the
>> resync logic might simply skip some 0x47 sync bytes. In order to detect these
>> let's check for modulo instead of exact value.
>>
>> Also skip unrecognized sync byte distances instead of considering them as a
>> failure of detection. It only delays the detection of the new packet size.
>>
>> Also note that AVIO only buffers a single packet (for UDP/mpegts, that usually
>> means 1316 bytes), so among every ten consecutive 188-byte MPEGTS packets there
>> will always be a seek failure, and that caused the old code to not find the 188
>> byte pattern across 10 consecutive packets.
>>
>> Signed-off-by: Marton Balint <cus at passwd.hu>
>> ---
>>  libavformat/mpegts.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
>> index a065c61c40..f2b2c05d86 100644
>> --- a/libavformat/mpegts.c
>> +++ b/libavformat/mpegts.c
>> @@ -2846,12 +2846,14 @@ static void reanalyze(MpegTSContext *ts) {
>>      if (pos < 0)
>>          return;
>>      pos -= ts->pos47_full;
>> -    if (pos == TS_PACKET_SIZE) {
>> +    if (pos % TS_PACKET_SIZE == 0) {
>>          ts->size_stat[0] ++;
>> -    } else if (pos == TS_DVHS_PACKET_SIZE) {
>> +    } if (pos % TS_DVHS_PACKET_SIZE == 0) {
>>          ts->size_stat[1] ++;
>> -    } else if (pos == TS_FEC_PACKET_SIZE) {
>> +    } if (pos % TS_FEC_PACKET_SIZE == 0) {
>>          ts->size_stat[2] ++;
>> +    } else {
>> +        return;
>>      }
>
> This patch breaks, or at least changes
>
> ./ffmpeg -i tspacket_size_changeback.ts  -vframes 2 -qscale 2 test.avi
>
> ill mail you the file privatly as i dont remember from where this is
> and google fails to find any public copy

Yeah, it seems worse now. I don't know, this whole resync logic seems 
bogus to me, and it also duplicates functionality of get_packet_size. I 
will send a new patch which uses that instead of re-implementing it.

Regards,
Marton