Re: [FFmpeg-devel] [PATCH] avformat/mov: correct 0 valued entries in stts

[Gyan Doshi <ffmpeg@gyani.pro>]

On 2021-12-29 05:58 pm, Michael Niedermayer wrote:
> On Tue, Dec 28, 2021 at 10:26:42PM +0530, Gyan Doshi wrote:
>>
>> On 2021-12-28 05:18 am, Michael Niedermayer wrote:
>>> On Tue, Dec 28, 2021 at 01:33:54AM +0530, Gyan Doshi wrote:
>>>> On 2021-12-28 12:38 am, Michael Niedermayer wrote:
>>>>> On Mon, Dec 27, 2021 at 11:27:10AM +0530, Gyan Doshi wrote:
>>>>>> As per ISO 14496-12, sample duration of 0 is invalid except for
>>>>>> the last entry.
>>>>>>
>>>>>> In addition, also catch 0 value for sample count.
>>>>>> ---
>>>>>>     libavformat/mov.c | 12 ++++++++++++
>>>>>>     1 file changed, 12 insertions(+)
>>>>>>
>>>>>> diff --git a/libavformat/mov.c b/libavformat/mov.c
>>>>>> index 2aed6e80ef..fb7406cdd6 100644
>>>>>> --- a/libavformat/mov.c
>>>>>> +++ b/libavformat/mov.c
>>>>>> @@ -2968,6 +2968,18 @@ static int mov_read_stts(MOVContext *c, AVIOContext *pb, MOVAtom atom)
>>>>>>             av_log(c->fc, AV_LOG_TRACE, "sample_count=%d, sample_duration=%d\n",
>>>>>>                     sample_count, sample_duration);
>>>>>> +        if (!sample_count) {
>>>>>> +        av_log(c->fc, AV_LOG_WARNING, "invalid sample count of 0 in stts for st %d at entry %u; changing to 1.\n",
>>>>>> +               c->fc->nb_streams-1, i);
>>>>>> +        sc->stts_data[i].count = sample_count = 1;
>>>>>> +        }
>>>>>> +
>>>>>> +        if (!sample_duration && i != entries-1) {
>>>>>> +        av_log(c->fc, AV_LOG_WARNING, "invalid sample delta of 0 in stts for st %d at entry %u; changing to 1.\n",
>>>>>> +               c->fc->nb_streams-1, i);
>>>>>> +        sc->stts_data[i].duration = sample_duration = 1;
>>>>>> +        }
>>>>>> +
>>>>>>             duration+=(int64_t)sample_duration*(uint64_t)sample_count;
>>>>>>             total_sample_count+=sample_count;
>>>>> This does not produce the same output
>>>>> tickets/2096/m.f4v
>>>>>
>>>>> videos/stretch.mov (2344 matches for "invalid" after this patch)
>>>>>
>>>>> tickets/976/CodecCopyFailing.mp4
>>>>>
>>>>> But there are many more, some maybe even generated by FFmpeg
>>>> Where do I find these files?
>>> https://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket976/CodecCopyFailing.mp4
>>> https://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket2096/m.f4v
>>>
>>> i failed to find the 3rd online
>>>
>>>
>>>>> Taking a step back, the problem started with
>>>>> 203b0e3561dea1ec459be226d805abe73e7535e5
>>>>> which broke a real world file which was outside the specification
>>>> Just to clarify, it did not break that file. That file uses stts in an
>>>> unusual way.
>>>> Before 2015, lavf exported packets with the same desync as the other
>>>> demuxers do so till today.
>>>> Andreas' patch added a hack to make it play in sync. My patch 203b0e356
>>>> broke that hack.
>>>> The patch for max_stts_delta is a way to restore it back.
>>>>
>>>>> you then suggested a fix which crashed with some fuzzed files which
>>>>> where outside the specification
>>>>>
>>>>> and now this fix on top which changes real world files which
>>>>> are outside the specification
>>>>>
>>>>> I think, maybe you should consider the "outside the specification"
>>>>> more. The code above directly and intentionally changes values.
>>>>> So as a reviewer i have to ask the obvious, is that change a
>>>>> bugfix or a bug ?
>>>> Not surprising that the output of out-of-spec files is different - that's
>>>> expected, intended and trivial.
>>>> It would be a bug if in-spec files were treated differently. FATE passes.
>>>>
>>>> If there's a specific / "correct" playback for these files like sync issues,
>>>> I'll see if I can restore it.
>>> First we need to find cases that broke. I certainly will not find every
>>>
>>> If a patch is writen with the goal "dont break any file" it would be easy
>>> But you said that changes are "expected, intended" so then my question
>>> as reviewer would be what about these expected changes ?
>>> Did it change any real files output? did it fix a bug?
>>> What is the idea behind the change ?
>>> please correct me if iam wrong but
>>>
>>> Here it seems you dont care what happens with changed files unless
>>> someone else finds such a file and reports it. (if its not in spec)
>>> And the idea seems that 0 is inconvenient so you change it to 1
>>> Its not that 0 could fundamentally not be intended to mean 0
>>>
>>> We are before a release and id like to fix the regression
>>> ATM objectively the only option i have is reverting
>>> 203b0e3561dea1ec459be226d805abe73e7535e5
>>> can you provide another option ? something that fixes the regression
>>> without breaking something else ?
>> 1) The first priority ought to be to not mishandle compliant files
>> 2) Subject to 1, we should accommodate for out-of-spec files as much as we
>> can.
> Sounds good but that alone doesnt work well
> The codebase is iteratively written, the demuxers move in steps towards
> fewer bugs, more wide file support, better maintainability.
> if you start with a demuxer which supports 500 files and then
> it supports 550 then 700 then 800 and so on eventually it reaches 999
> and now someone finds a spec compliance bug and fixes it so 1 more file
> is supported that developer has to try to not break past efforts of
> supporting other files. Because otherwise alot of past work is thrown
> out and thats just bad. Bad for users, bad for the people who did that work

It depends if all files beyond the first 500 are out-of-spec files. If 
they are it suggests a spec widely ignored.

In this specific case however, I haven't seen any complaints about 
broken MP4 demuxing outside of your samples.
I am on Stack Overflow and other popular support forums where users tend 
to first go to ask or complain.
FATE which is meant to catch undesirable behaviour passes. Both those 
things tell me that funky files whose demuxing
has changed constitute a very tiny set of files at most. And my latest 
set of patches restore the special handling
needed for the set of files you've highlighted.

Regards,
Gyan

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