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

[Marton Balint]

On Wed, 20 May 2020, Limin Wang wrote:

> On Wed, May 20, 2020 at 08:26:37AM +0200, Marton Balint wrote:
>> 
>> 
>> On Wed, 20 May 2020, lance.lmwang at gmail.com 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.
>> >
>> >Before modifying and returning directly, I considered whether pos
>> >is a multiple of raw_packet_size, by the debug information, it's
>> >not true.
>> 
>> Not always, because sometimes there are 0x47 sync bytes in the
>> packet payload as well. But we siply ignore that case because of the
>> return later originated from your patch. Checking for modulo allows
>> faster detection and it also allows detection when UDP packets
>> contain a single MPEGTS packet.
>> 
>> >Also it's possible
>> >for the pos of two sync byte > 188/192/204 * n. If that's true, it's not correct
>> >to look it as stats hit by the modulo logic.
>> 
>> I don't understand what you mean here.
>
> Let me give one example, the size is 192*2, but the second 192 isn't start by 0x47
> sync byte, checking with modulo will not correct for the case

The mpegts demuxer can only detect 188, 192, or 204 byte packet sizes. 
Arbitrary sizes (like 2*192) is simply not supported, so we don't have to 
detect it.

Regards,
Marton

>
>
>> 
>> Regards,
>> Marton
>> 
>> 
>> >
>> >>
>> >>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;
>> >>     }
>> >>
>> >>     ts->size_stat_count ++;
>> >>-- 
>> >>2.26.1
>> >>
>> >>_______________________________________________
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>> >>
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>> >
>> >-- 
>> >Thanks,
>> >Limin Wang
>> >_______________________________________________
>> >ffmpeg-devel mailing list
>> >ffmpeg-devel at ffmpeg.org
>> >https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>> >
>> >To unsubscribe, visit link above, or email
>> >ffmpeg-devel-request at ffmpeg.org with subject "unsubscribe".
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>
> -- 
> Thanks,
> Limin Wang
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