[FFmpeg-devel] [PATCH] avformat/aviobuf: fix broken logic in ffio_ensure_seekback()

[Marton Balint]

On Thu, 17 Sep 2020, Paul B Mahol wrote:

> This removes big CPU overhead for demuxing chained ogg streams.
>
> Signed-off-by: Paul B Mahol <onemda at gmail.com>
> ---
> libavformat/aviobuf.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
> index a77517d712..ce9b7d59c9 100644
> --- a/libavformat/aviobuf.c
> +++ b/libavformat/aviobuf.c
> @@ -996,20 +996,20 @@ int ffio_ensure_seekback(AVIOContext *s, int64_t buf_size)
>     uint8_t *buffer;
>     int max_buffer_size = s->max_packet_size ?
>                           s->max_packet_size : IO_BUFFER_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 < filled || s->seekable || !s->read_packet)
> +    if (buf_size <= s->buffer_size || s->seekable || !s->read_packet)
>         return 0;

This is still not correct. You can only return if
buf_size <= s->buf_end - s->buf_ptr
OR
buf_size <= s->buffer + s->buffer_size - s->buf_ptr - max_buffer_size + 1

And the new minimum buffer size is what is calculated now, so that should 
not be changed.

I am not sure if this fixes you original issue. It might make sense to 
allocate max_buffer_size*2 length buffers by default, but the buffer size 
revert logic must be fixed to support that...

Regards,
Marton



> +    buf_size += s->buffer_size;
> +    buf_size = FFMAX(buf_size, max_buffer_size);
> +
>     av_assert0(!s->write_flag);
>
>     buffer = av_malloc(buf_size);
>     if (!buffer)
>         return AVERROR(ENOMEM);
> 
> -    memcpy(buffer, s->buffer, filled);
> +    memcpy(buffer, s->buffer, s->buffer_size);
>     av_free(s->buffer);
>     s->buf_ptr = buffer + (s->buf_ptr - s->buffer);
>     s->buf_end = buffer + (s->buf_end - s->buffer);
> -- 
> 2.17.1
>
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