[FFmpeg-devel] [PATCH 1/2] mov: fix decode of fragments that overlap in time

John Stebbins stebbins at jetheaddev.com
Fri Oct 6 00:38:48 EEST 2017

On 10/05/2017 09:45 AM, John Stebbins wrote:
> On 10/04/2017 03:21 PM, Michael Niedermayer wrote:
>> On Wed, Oct 04, 2017 at 10:58:19AM -0700, John Stebbins wrote:
>>> On 10/04/2017 10:13 AM, Michael Niedermayer wrote:
>>>> On Wed, Oct 04, 2017 at 08:18:59AM -0700, John Stebbins wrote:
>>>>> On 10/04/2017 03:50 AM, Michael Niedermayer wrote:
>>>>>> On Fri, Sep 29, 2017 at 08:54:08AM -0700, John Stebbins wrote:
>>>>>>> When keyframe intervals of dash segments are not perfectly aligned,
>>>>>>> fragments in the stream can overlap in time. Append new "trun" index
>>>>>>> entries to the end of the index instead of sorting by timestamp.
>>>>>>> Sorting by timestamp causes packets to be read out of decode order and
>>>>>>> results in decode errors.
>>>>>>> ---
>>>>>>>  libavformat/mov.c | 4 ++--
>>>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>>> diff --git a/libavformat/mov.c b/libavformat/mov.c
>>>>>>> index 899690d920..c7422cd9ed 100644
>>>>>>> --- a/libavformat/mov.c
>>>>>>> +++ b/libavformat/mov.c
>>>>>>> @@ -4340,8 +4340,8 @@ static int mov_read_trun(MOVContext *c, AVIOContext *pb, MOVAtom atom)
>>>>>>>                                    MOV_FRAG_SAMPLE_FLAG_DEPENDS_YES));
>>>>>>>          if (keyframe)
>>>>>>>              distance = 0;
>>>>>>> -        ctts_index = av_add_index_entry(st, offset, dts, sample_size, distance,
>>>>>>> -                                        keyframe ? AVINDEX_KEYFRAME : 0);
>>>>>>> +        ctts_index = add_index_entry(st, offset, dts, sample_size, distance,
>>>>>>> +                                     keyframe ? AVINDEX_KEYFRAME : 0);
>>>>>> can this lead to timestamps being out of order not just changing
>>>>>> from strictly monotone to monotone ?
>>>>>> Maybe iam missing somehing but out of order could/would cause problems
>>>>>> with av_index_search_timestamp() and possibly others
>>>>> I'm not sure I understand the question.  But I think I can answer.  The new fragment can start before the last fragment
>>>>> ends. I'll make a concrete example.  Lets say the new fragment's first DTS is 10 frames before the end of the previous
>>>>> fragment. So the first DTS of the new fragment is before the timestamp of 10 entries in the index from the previous
>>>>> fragment.  av_add_index_entry searches the existing index and inserts the first sample of the new fragment in position
>>>>> nb_index_entries - 10 (and shifts the existing entries).  The next 9 samples of the new fragment get intermixed with the
>>>>> remaining 9 samples of the previous fragment, sorted by DTS. When the samples are read out, you get samples from the
>>>>> last fragment and the new fragment interleaved together causing decoding errors.
>>>>> Using add_index_entry will result in the timestamps in the index going backwards by 10 frames at the fragment boundary
>>>>> in this example.  In the other patch that accompanied this one, I've marked the samples from the new fragment that
>>>>> overlap previous samples with AVINDEX_DISCARD. ff_index_search_timestamp appears to be AVINDEX_DISCARD aware.  So I
>>>>> think av_index_search_timestamp will do the right thing.
>>>> yes, that makes sense now.
>>>> Please correct me if iam wrong but then patch 1 would introduce a
>>>> issue that the 2nd fixes. So both patches should be merged to avoid
>>>> this
>>>> But theres another problem, trun can be read out of order, when one
>>>> seeks around, so the next might have to be put elsewhere than after the
>>>> previous
>>>> thanks
>>> Hmm, can you describe the circumstances where this would happen.  I looked at the seek code and can't see any way for it
>>> to seek to the middle somewhere without first reading previous trun.  It looks to me like if avformat_seek_file or
>>> av_seek_frame fails to find the desired timestamp in the index it falls back to seek_frame_generic which seeks to the
>>> position of the last sample in the index and performs av_read_frame until it gets to the timestamp it wants.  Is there a
>>> path I've missed where it can skip to the middle of the file somehow?
>> I used
>> -rw-r----- 1 michael michael 66908195 Dec 11  2015 buck480p30_na.mp4
>> ./ffplay buck480p30_na.mp4
>> (i can upload this if needed, i dont know where its from exactly)
>> and when seeking around by using the right mouse buttonq it sometimes read
>> trun chunks with lower times than previous (seen from the av_logs in
>> there)
>> I hope i made no mistake and would assume this happens with any file
>> with these chunks
>> ...
>> [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f3884000940] AVIndex stream 0, sample 151, offset 60134, dts 450000, size 194, distance 25, keyframe 0
>> ...
>> Seek to 68% ( 0:07:11) of total duration ( 0:10:34)
>> ...
>> [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f3884000940] AVIndex stream 0, sample 152, offset 2b74fd6, dts 38757000, size 8284, distance 0, keyframe 1
>> ...
>> Seek to 14% ( 0:01:29) of total duration ( 0:10:34)
>> ...
>> [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7f3884000940] AVIndex stream 0, sample 152, offset 959164, dts 7749000, size 55027, distance 0, keyframe 1
> When seeking mov_read_trun is getting called repeatedly for the same fragment which has a number of undesirable side
> effects, even without my patch.  The following things get updated to incorrect values when seeking backward and the trun
> is re-read:
> sc->data_size
> sc->duration_for_fps
> sc->nb_frames_for_fps
> sc->track_end
> The trun is getting re-read in mov_switch_root because headers_read in MOVFragmentIndex has not yet been set for the
> fragment.  I think a solution to this is to set headers_read for the appropriate entry in MOVFragmentIndex when the trun
> is read the first time.  Does this sound like the right approach?

I got the analysis wrong above.  It's not re-reading the trun.  What's happening is that while seeking forward it can
skip one or more trun.  Then seeking back, it will read that trun.  So, as you said, re-ordering of the index will be
necessary to handle seeking past a trun. 

It can seek forward past a trun because the sidx has the offset to each moof and is used by mov_seek_stream.  I missed
this earlier.

Since the trun can overlap, reordering shouldn't be done by simply sorting by the index_enties timestamps though.  I'm
thinking a good way would be to add a index_entry member to MOVFragmentIndexItem that records where in index_entries the
samples for the trun were written.  The position in index_entries of a *new* trun would be determined by looking at the
position of the MOVFragmentIndexItem that corresponds to that trun and  finding for the next MOVFragmentIndexItem that
has a valid index_entry set (which means it's trun was read and samples inserted into the index). If no next valid
index_entry is found, the samples of the new trun get appended.  If a valid index_entry is found, open a hole in
index_entries before that entry and populate the samples from the new trun in the hole. Then fix up the index_entry
members of MOVFragmentIndexItems to account for the hole.

Looking again at sc->* most of these are accurate I think.  I question sc->track_end though.  It seem like is should not
be going backwards when seeking backwards.

Am I on the right track now?

John      GnuPG fingerprint: D0EC B3DB C372 D1F1 0B01  83F0 49F1 D7B2 60D4 D0F7

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