[FFmpeg-devel] [PATCH 2/9] avformat/aviobuf: check if requested seekback buffer is already read

[Marton Balint]

On Sun, 4 Oct 2020, Andriy Gelman wrote:

> Hi Marton,
>
> On Tue, 29. Sep 23:10, Marton Balint wrote:
>> Existing code did not check if the requested seekback buffer is
>> already read entirely. In this case, nothing has to be done to guarantee
>> seekback.
>> 
>> Signed-off-by: Marton Balint <cus at passwd.hu>
>> ---
>>  libavformat/aviobuf.c | 3 +++
>>  1 file changed, 3 insertions(+)
>> 
>> diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
>> index 9675425349..d94be478ac 100644
>> --- a/libavformat/aviobuf.c
>> +++ b/libavformat/aviobuf.c
>> @@ -999,6 +999,9 @@ int ffio_ensure_seekback(AVIOContext *s, int64_t buf_size)
>>      int filled = s->buf_end - s->buffer;
>>      ptrdiff_t checksum_ptr_offset = s->checksum_ptr ? s->checksum_ptr - s->buffer : -1;
>> 
>> +    if (buf_size <= s->buf_end - s->buf_ptr)
>> +        return 0;
>> +
>>      buf_size += s->buf_ptr - s->buffer + max_buffer_size;
>>
>>      if (buf_size < filled || s->seekable || !s->read_packet)
>
> I was testing this set, and saw commit 53c25ee0736. It discards buffered data by
> resetting s->{buf_ptr,buf_end} when we need to read more data. 
>
> Maybe I missed something, but wouldn't this mean that seekback no longer works if
> fill_buffer() is called in avio_read_partial().. so defeating the purpose of
> ffio_ensure_seekback()?

I think you are right. It looks like that commit is not needed after
2ca48e466675a8a3630061cd2c15325eab8eda97, so probably it should be 
reverted now to unbreak ffio_ensure_seekback().

Regards,
Marton