[FFmpeg-devel] [PATCH 2/3] avformat/aviobuf: fix checks in ffio_ensure_seekback

[Paul B Mahol]

On Sun, Sep 20, 2020 at 10:52:52AM +0200, Marton Balint wrote:
> Signed-off-by: Marton Balint <cus at passwd.hu>
> ---
>  libavformat/aviobuf.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
> index 9675425349..aa1d6c0830 100644
> --- a/libavformat/aviobuf.c
> +++ b/libavformat/aviobuf.c
> @@ -999,9 +999,12 @@ 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;
>  
> -    buf_size += s->buf_ptr - s->buffer + max_buffer_size;
> +    if (buf_size <= s->buf_end - s->buf_ptr)
> +        return 0;
> +
> +    buf_size += s->buf_ptr - s->buffer + max_buffer_size - 1;
>  
> -    if (buf_size < filled || s->seekable || !s->read_packet)
> +    if (buf_size <= s->buffer_size || s->seekable || !s->read_packet)
>          return 0;
>      av_assert0(!s->write_flag);


Not acceptable change.

Your code does this to chained ogg files:

XXX 10
XXX 65307
XXX 65307
102031
106287
110527
114745
119319
[...]

It continues allocating excessive small extra chunks of bytes for no apparent reason in each and every call
which slowly and gradually increases memory usage, but every call causes unnecessary memcpy calls thus causing
almost exponential slowdown of file processing.

Lines with XXX, means that allocation and memcpy was not needed.

I can also post behavior of my solution, which work just fine, and I yet need to find scenario when it does not work.

>  
> -- 
> 2.26.2
> 
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