[FFmpeg-devel] [PATCH v2] avcodec/bsf: restructure the internal implementation of the bistream filter API

[Andreas Rheinhardt]

James Almer:
> Process input data as soon as it's fed to av_bsf_send_packet(), instead of
> storing a single packet and expecting the user to call av_bsf_receive_packet()
> in order to trigger the decoding process before they are allowed to feed more
> data.
> 
> This puts the bsf API more in line with the decoupled decode API, without
> breaking existing using it.
> 
> Signed-off-by: James Almer <jamrial at gmail.com>
> ---
> Now without creating a new reference for the input packet, and behaving the
> exact same as before for current users that were following the doxy right down
> to the letter.
> 
> Also didn't rename buffer_pkt to reduce the size of the diff and easily find
> the actual changes and additions.
> 
>  libavcodec/avcodec.h |  6 +-----
>  libavcodec/bsf.c     | 42 +++++++++++++++++++++++++++++++-----------
>  2 files changed, 32 insertions(+), 16 deletions(-)
> 
> diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> index b79b025e53..af2a1b0e90 100644
> --- a/libavcodec/avcodec.h
> +++ b/libavcodec/avcodec.h
> @@ -4735,10 +4735,6 @@ int av_bsf_init(AVBSFContext *ctx);
>  /**
>   * Submit a packet for filtering.
>   *
> - * After sending each packet, the filter must be completely drained by calling
> - * av_bsf_receive_packet() repeatedly until it returns AVERROR(EAGAIN) or
> - * AVERROR_EOF.
> - *
>   * @param pkt the packet to filter. The bitstream filter will take ownership of
>   * the packet and reset the contents of pkt. pkt is not touched if an error occurs.
>   * If pkt is empty (i.e. NULL, or pkt->data is NULL and pkt->side_data_elems zero),
> @@ -4770,7 +4766,7 @@ int av_bsf_send_packet(AVBSFContext *ctx, AVPacket *pkt);
>   * an error occurs.
>   *
>   * @note one input packet may result in several output packets, so after sending
> - * a packet with av_bsf_send_packet(), this function needs to be called
> + * a packet with av_bsf_send_packet(), this function may need to be called
>   * repeatedly until it stops returning 0. It is also possible for a filter to
>   * output fewer packets than were sent to it, so this function may return
>   * AVERROR(EAGAIN) immediately after a successful av_bsf_send_packet() call.
> diff --git a/libavcodec/bsf.c b/libavcodec/bsf.c
> index b9fc771a88..c79a8ebbdf 100644
> --- a/libavcodec/bsf.c
> +++ b/libavcodec/bsf.c
> @@ -30,6 +30,7 @@
>  
>  struct AVBSFInternal {
>      AVPacket *buffer_pkt;
> +    AVPacket *out_pkt;
>      int eof;
>  };
>  
> @@ -46,8 +47,10 @@ void av_bsf_free(AVBSFContext **pctx)
>      if (ctx->filter->priv_class && ctx->priv_data)
>          av_opt_free(ctx->priv_data);
>  
> -    if (ctx->internal)
> +    if (ctx->internal) {
>          av_packet_free(&ctx->internal->buffer_pkt);
> +        av_packet_free(&ctx->internal->out_pkt);
> +    }
>      av_freep(&ctx->internal);
>      av_freep(&ctx->priv_data);
>  
> @@ -112,7 +115,8 @@ int av_bsf_alloc(const AVBitStreamFilter *filter, AVBSFContext **pctx)
>      ctx->internal = bsfi;
>  
>      bsfi->buffer_pkt = av_packet_alloc();
> -    if (!bsfi->buffer_pkt) {
> +    bsfi->out_pkt = av_packet_alloc();
> +    if (!bsfi->buffer_pkt || !bsfi->out_pkt) {
>          ret = AVERROR(ENOMEM);
>          goto fail;
>      }
> @@ -185,6 +189,7 @@ void av_bsf_flush(AVBSFContext *ctx)
>      bsfi->eof = 0;
>  
>      av_packet_unref(bsfi->buffer_pkt);
> +    av_packet_unref(bsfi->out_pkt);
>  
>      if (ctx->filter->flush)
>          ctx->filter->flush(ctx);
> @@ -205,10 +210,21 @@ int av_bsf_send_packet(AVBSFContext *ctx, AVPacket *pkt)
>          return AVERROR(EINVAL);
>      }
>  
> +    if (!bsfi->buffer_pkt->data &&
> +        !bsfi->buffer_pkt->side_data_elems)

I think that this file needs a macro for "packet is empty", especially
with a patch like yours, so I have just sent [1].

> +        goto end;
> +
> +    if (!bsfi->out_pkt->data && !bsfi->out_pkt->side_data_elems) {
> +        ret = ctx->filter->filter(ctx, bsfi->out_pkt);
> +        if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)
> +            return ret;
> +    }
> +
>      if (bsfi->buffer_pkt->data ||
>          bsfi->buffer_pkt->side_data_elems)
>          return AVERROR(EAGAIN);
>  
> +end:
>      ret = av_packet_make_refcounted(pkt);
>      if (ret < 0)
>          return ret;
> @@ -219,7 +235,17 @@ int av_bsf_send_packet(AVBSFContext *ctx, AVPacket *pkt)
>  
>  int av_bsf_receive_packet(AVBSFContext *ctx, AVPacket *pkt)
>  {
> -    return ctx->filter->filter(ctx, pkt);
> +    AVBSFInternal *bsfi = ctx->internal;
> +    int ret;
> +
> +    if (!bsfi->out_pkt->data && !bsfi->out_pkt->side_data_elems) {
> +        ret = ctx->filter->filter(ctx, bsfi->out_pkt);

The way you are doing this leads to an unnecessary av_packet_move_ref()
and an unnecessary check (you can just "return ctx->filter->filter(ctx,
pkt);"). Given that the doc states that the supplied pkt has to be clean
and given that the filters unref it on error if they have modified it,
the packet will be clean on error. If you worry that some padding bytes
might have been modified, then change the doc to "On failure, pkt will
be clean/blank on return" instead of adding an av_packet_move_ref().

> +        if (ret < 0)
> +            return ret;
> +    }
> +    av_packet_move_ref(pkt, bsfi->out_pkt);
> +
> +    return 0;
>  }
>  
>  int ff_bsf_get_packet(AVBSFContext *ctx, AVPacket **pkt)
> @@ -227,12 +253,9 @@ int ff_bsf_get_packet(AVBSFContext *ctx, AVPacket **pkt)
>      AVBSFInternal *bsfi = ctx->internal;
>      AVPacket *tmp_pkt;
>  
> -    if (bsfi->eof)
> -        return AVERROR_EOF;
> -
>      if (!bsfi->buffer_pkt->data &&
>          !bsfi->buffer_pkt->side_data_elems)
> -        return AVERROR(EAGAIN);
> +        return bsfi->eof ? AVERROR_EOF : AVERROR(EAGAIN);
>  
>      tmp_pkt = av_packet_alloc();
>      if (!tmp_pkt)
> @@ -248,12 +271,9 @@ int ff_bsf_get_packet_ref(AVBSFContext *ctx, AVPacket *pkt)
>  {
>      AVBSFInternal *bsfi = ctx->internal;
>  
> -    if (bsfi->eof)
> -        return AVERROR_EOF;
> -
>      if (!bsfi->buffer_pkt->data &&
>          !bsfi->buffer_pkt->side_data_elems)
> -        return AVERROR(EAGAIN);
> +        return bsfi->eof ? AVERROR_EOF : AVERROR(EAGAIN);
>  
>      av_packet_move_ref(pkt, bsfi->buffer_pkt);
>  
> 

- Andreas

[1]: https://ffmpeg.org/pipermail/ffmpeg-devel/2020-April/261013.html