Re: [FFmpeg-devel] [PATCH] avcodec/decode: Return EAGAIN instead of overwriting unused packet

[Marton Balint <cus@passwd.hu>]

On Mon, 10 Jul 2023, Anton Khirnov wrote:

> Quoting Andreas Rheinhardt (2023-07-10 01:36:41)
>> Should fix #10457, a regression caused by
>> 69516ab3e917a6e91d26e38d04183c60fd71cbab.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
>> ---
>> I am not sure about this one. The problem is that avcodec_send_packet()
>> and avcodec_receive_frame() must not return EAGAIN at the same time.
>> If buffer_frame contains a frame when entering avcodec_send_packet(),
>> the next call to avcodec_receive_frame() will not return EAGAIN.
>> But is this actually guaranteed if buffer_pkt is not empty?
>
> It's not guaranteed, but I believe the correct interpretation of the
> "must not return EAGAIN at the same time" line is that
> send->receive->send must make progress and not be 3x EAGAIN.

I don't think that 2x EAGAIN is allowed, the docs says:

... the only guarantee is that an AVERROR(EAGAIN) return value on a 
send/receive call on one end implies that a receive/send call on the other 
end will succeed, or at least will not fail with AVERROR(EAGAIN). In 
general, no codec will permit unlimited buffering of input or output.

Regards,
Marton


>
>
>>
>>  libavcodec/decode.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/libavcodec/decode.c b/libavcodec/decode.c
>> index 269633ce10..6595c3ca34 100644
>> --- a/libavcodec/decode.c
>> +++ b/libavcodec/decode.c
>> @@ -666,6 +666,9 @@ int attribute_align_arg avcodec_send_packet(AVCodecContext *avctx, const AVPacke
>>      if (avpkt && !avpkt->size && avpkt->data)
>>          return AVERROR(EINVAL);
>>
>> +    if (!AVPACKET_IS_EMPTY(avci->buffer_pkt))
>> +        return AVERROR(EAGAIN);
>> +
>>      av_packet_unref(avci->buffer_pkt);
>
> This unref here becomes redundant.
>
> Otherwise looks good, thanks.
>
> -- 
> Anton Khirnov
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