[mythtv] Call for testing branch devel/ffmpeg-resync (mpegts-harmonize)

[John Pilkington]

On 27/07/2022 10:59, John Pilkington wrote:
> On 27/07/2022 02:54, Scott Theisen wrote:
>> On 7/26/22 18:41, John Pilkington wrote:
>>> On 26/07/2022 20:09, Scott Theisen wrote:
>>>> On 7/26/22 14:10, Scott Theisen wrote:
>>>>> Initial impression: probable double free 
>>>>
>>>> I would like mythcommflag logs with `-v libav:debug` from both 
>>>> master and ffmpeg-resync.
>>>>
>>>> It appears that since add_section_stream() reuses the SectionContext 
>>>> if it already exists, this leads to two DSMCC streams with the same 
>>>> value of the pointer priv_data which then is double freed since it 
>>>> is freed with each stream.
>>>>
>>>> This should fix the double free, but I need you to test if the 
>>>> DSMCC/MHEG streams still work correctly since I can't:
>>>> ```
>>>> diff --git a/mythtv/external/FFmpeg/libavformat/mpegts-mythtv.c 
>>>> b/mythtv/external/FFmpeg/libavformat/mpegts-mythtv.c
>>>> index 4a74c5c3ac..90229cf6ae 100644
>>>> --- a/mythtv/external/FFmpeg/libavformat/mpegts-mythtv.c
>>>> +++ b/mythtv/external/FFmpeg/libavformat/mpegts-mythtv.c
>>>> @@ -2494,12 +2494,17 @@ static SectionContext 
>>>> *add_section_stream(MpegTSContext *ts, int pid, int stream
>>>>       MpegTSFilter *tss = ts->pids[pid];
>>>>       SectionContext *sect = 0;
>>>>       if (tss) { /* filter already exists */
>>>> +#if 0
>>>> +/*
>>>> +returning the preexisting SectionContext causes it to be double 
>>>> freed when its
>>>> +owning AVStream is freed.  (It gets freed via AVStream.priv_data.)
>>>> +*/
>>>>           if (tss->type == MPEGTS_SECTION)
>>>>               sect = (SectionContext*) tss->u.section_filter.opaque;
>>>>
>>>>           if (sect && (sect->stream_type == stream_type))
>>>>               return sect; /* if it's the same stream type, just 
>>>> return ok */
>>>> -
>>>> +#endif
>>>>           /* otherwise, kill it, and start a new stream */
>>>>           mpegts_close_filter(ts, tss);
>>>>       }
>>>>
>>>> ```
>>>>
>>>> Regards,
>>>>
>>>> Scott
>>>
>>> Attachment is from the unpatched build of ffmpeg-resync.  Will try 
>>> the rest tomorrow.
>>
>>  From that log I noticed a minor bug in the codec descriptors location 
>> and I have added a new commit to my pull requests.  (This only effects 
>> the logging.  Instead of `unknown_codec` it should say `dsmcc_b` for 
>> codec 0x17813, AV_CODEC_ID_DSMCC_B.)
>>
>> I also noticed that it appears to create 5 streams for each of the two 
>> PIDs for DSMCC streams, but I await the log from master for comparison.
>>
>> Regards,
>>
>> Scott
> 
> Attachment using master from the same short recording of BBC ONE evening 
> news.
> 
> mythcommflag --rebuild --file <filename> -v libav:debug --logpath 
> ~/SGs/Recs/
> 
> Quitting that recording in ffmpeg-resync causes a segfault.  That 
> doesn't happen with eg SkyArts, which doesn't have the extra streams.
> 
> TBH my first action is usually to strip off non-AV streams.  That has 
> seemed a historically effective way of avoiding problems.  I have rarely 
> used MythTV with what is described in the UK as Interactive TV (although 
> I use the Normal recording profile) so am probably not a good judge of 
> how DSMCC/MHEG is handled.  I did come across this document which might 
> be of interest - or just TL;DR
> 
> https://www.freeview.co.uk/sites/default/files/2020-08/freeview-play-technical-specification-2021.pdf 
> 
> 
My apologies, but I'm having git problems again and ought to do 
something else.  I created a local sub-branch for the earlier work on 
frame counting, and it's getting in the way.

 > John