[FFmpeg-devel] [PATCH 1/2] avcodec/h264_ps: Check for truncation at fixed_frame_rate_flag

[James Almer]

On 12/15/2019 8:50 PM, Carl Eugen Hoyos wrote:
> Am So., 15. Dez. 2019 um 23:25 Uhr schrieb James Almer <jamrial at gmail.com>:
>>
>> On 12/15/2019 7:00 PM, Michael Niedermayer wrote:
>>> Fixes: Ticket7249 (No longer displaying a scary red message)
>>>
>>> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>>> ---
>>>  libavcodec/h264_ps.c | 4 ++++
>>>  1 file changed, 4 insertions(+)
>>>
>>> diff --git a/libavcodec/h264_ps.c b/libavcodec/h264_ps.c
>>> index e8738d8502..74f12f8979 100644
>>> --- a/libavcodec/h264_ps.c
>>> +++ b/libavcodec/h264_ps.c
>>> @@ -203,6 +203,10 @@ static inline int decode_vui_parameters(GetBitContext *gb, AVCodecContext *avctx
>>>              sps->num_units_in_tick = num_units_in_tick;
>>>              sps->time_scale = time_scale;
>>>          }
>>> +        if (get_bits_left(gb) == 0) {
>>
>> Doing this check here sounds like it's tailored specifically for the
>> sample in the ticket. What if another file is truncated one bit after
>> this point? Or one bit before? You will get the scary red message for
>> it. Will you also add a check for it?
>> We can't clutter this file with new get_bits_left() checks every time a
>> user shares their broken samples with us just to get a less scary output
>> in their terminals.
> 
> Do you know of another proprietary or open-source encoder that encodes
> streams with such a truncation?

There was an old hevc encoder that did something like that. We ended up
adding a workaround for it as well. And i can imagine capturing a
transport stream with bad network conditions could result in all kinds
randomly placed truncation.

This patch is not fixing anything, it's just removing an error level log
message with a warning one so it's "less scary", since ultimately the
file is decodeable with or without it because truncation is ignored by
the decoder.
I gave a suggestion to properly remove the scary messages and prevent
littering the decoder with custom checks in arbitrary places.