[FFmpeg-devel] [RFC PATCH] avformat/utils: always seek back after avformat_find_stream_info()

[Andreas Rheinhardt]

Aman Gupta:
> On Wed, Apr 10, 2019 at 1:00 AM Hendrik Leppkes <h.leppkes at gmail.com> wrote:
> 
>> On Wed, Apr 10, 2019 at 9:55 AM Aman Gupta <ffmpeg at tmm1.net> wrote:
>>>
>>> On Tue, Apr 9, 2019 at 9:49 PM Hendrik Leppkes <h.leppkes at gmail.com>
>> wrote:
>>>
>>>> On Wed, Apr 10, 2019 at 2:21 AM Aman Gupta <ffmpeg at tmm1.net> wrote:
>>>>>
>>>>> From: Aman Gupta <aman at tmm1.net>
>>>>>
>>>>> Previously, the initial seek position was recorded into
>>>>> old_offset at the beginning of avformat_find_stream_info(),
>>>>> and passed into estimate_timings(). In the case of mpegts
>>>>> with a known filesize, it was further passed into
>>>>> estimate_timings_from_pts() which called avio_seek(SEEK_SET)
>>>>> after doing its timing related seeks. (Interestingly, this
>>>>> seeked all the way back to the initial position before
>>>>> the probe, rather than only back before the timing
>>>>> related seeks to the end of the file).
>>>>>
>>>>> With this commit, we pull the avio_seek() out of the mpegts
>>>>> specific code-path and unconditionally seek all streams back
>>>>> to their original position after probing is complete.
>>>>>
>>>>> This effectively prevents data that is consumed during
>>>>> avformat_find_stream_info() from being discarded, so packets
>>>>> contained at the beginning of a file are still passed back
>>>>> to the user for playback.
>>>>>
>>>>
>>>> I don't think I ever had a case where data was apparently lost from
>>>> the beginning of the stream. Can you give examples of what this fixes?
>>>
>>>
>>> Perhaps I'm missing something.
>>>
>>> If I see the debug log message "After avformat_find_stream_info() pos:
>>> 1000000 bytes  seeks:0", are the packets from the first megabyte of the
>>> file still returned from API? Maybe they are being cached somewhere and I
>>> did not notice.
>>>
>>
>> As far as I know, the packets read during stream info probing are
>> being cached in a buffer and returned to the caller on av_read_frame.
> 
> 
> Okay that seems to be the missing piece. If someone can point me to where
> this buffer is in the code, that would be helpful.
> 
The packet queues are in the AVFormatInternal struct and the packets
are put there via ff_packet_list_put. The most interesting thing for
you might be ff_read_packet.

- Andreas