[FFmpeg-devel] [PATCH] avformat/mux: Check pkt->stream_index before using it

[Andreas Rheinhardt]

Anton Khirnov:
> Quoting Andreas Rheinhardt (2020-05-10 21:35:54)
>> Anton Khirnov:
>>> Quoting Marton Balint (2020-05-10 19:45:04)
>>>>
>>>>
>>>> On Sun, 10 May 2020, Anton Khirnov wrote:
>>>>
>>>>> Quoting Andreas Rheinhardt (2020-05-08 00:55:00)
>>>>>> This commit fixes two recent regressions both of which are about using
>>>>>> pkt->stream_index as index in an AVFormatContext's streams array before
>>>>>> actually comparing the value with the count of streams in said array.
>>>>>> 96e5e6abb9851d7a26ba21703955d5826ac857c0 did this in
>>>>>> prepare_input_packet() and 64063512227c4c87a7d16a1076481dc6baf19841 did
>>>>>> likewise in write_packets_common().
>>>>>>
>>>>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>>>>>> ---
>>>>>> The same error in the same file applied on the same day by two different
>>>>>> people. How unlikely.
>>>>>
>>>>> How is it a regression? Isn't it rather invalid API use?
>>>>
>>>> Fun fact: 7b03b65bf0d02519c86750d2da33f413e11cf0c6
>>>>
>>>> Yes, it is kind of invalid API use, but since the check is already there, 
>>>> we should make it actually worthwile.
>>>
>>> lol
>>>
>>> I agree that checking for it is a good idea, obviously, but I wouldn't
>>> call it a regression.
>>>
>> How about rephrasing the first sentence to: "This commit stops using
>> pkt->stream_index as index in an AVFormatContext's streams array before
>> actually comparing the value with the count of streams in said array."
> 
> Sure, sounds good.
> 
>>>>
>>>>>
>>>>> Not that I object to having a check. But then why is check_packet()
>>>>> called so deep and not immediately on entry to the muxer?
>>>>
>>>> I guess it is not that deep, but recent factorization efforts hidden it a 
>>>> bit.
>>>
>>> You can see in my original commit it is the very first thing done after
>>> entering the muxer. Right now it's several function calls deep.
>>>
>> I could make it the very first thing called in write_packets_common().
> 
> Why not move the check_packet() call out of prepare_packet() into
> av_[interleaved_]write_frame() instead?
> 
This would add code duplication and IMO it is nicer to only check a
packet for validity if there is actually a packet to check.

- Andreas