[FFmpeg-devel] [PATCH 1/2] mpeg4_unpack_bframes: Avoid allocations and copies of packet structures

[Andreas Rheinhardt]

Andreas Rheinhardt:
> 1. Since bd90a2ec, mpeg4_unpack_bframes caches whole packets instead of
> just the pointer to the buffer and the buffer's size in order to be able
> to make use of refcounting to avoid copying of data; this unfortunately
> introduced copies of packet structures and side data (if existing),
> although the only fields that are needed are the buffer-related ones
> (data, size and buf). This can be changed without compromising the
> advantages of refcounting by storing a reference to the buffer.
> 
> 2. This change also makes it easy to use only one packet throughout
> so that an allocation and free of an AVPacket structure per filtered
> packet can be saved by switching to ff_bsf_get_packet_ref.
> 
> 3. Furthermore, this commit also fixes a memleak introduced in bd90a2ec:
> If a stored b_frame with side data was used for a later frame, the side
> data would leak when the input frame's properties were copied into the
> output frame.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> ---
>  libavcodec/mpeg4_unpack_bframes_bsf.c | 77 ++++++++++++---------------
>  1 file changed, 35 insertions(+), 42 deletions(-)
> 
> diff --git a/libavcodec/mpeg4_unpack_bframes_bsf.c b/libavcodec/mpeg4_unpack_bframes_bsf.c
> index 1daf133ce5..382070b423 100644
> --- a/libavcodec/mpeg4_unpack_bframes_bsf.c
> +++ b/libavcodec/mpeg4_unpack_bframes_bsf.c
> @@ -25,7 +25,7 @@
>  #include "mpeg4video.h"
>  
>  typedef struct UnpackBFramesBSFContext {
> -    AVPacket *b_frame;
> +    AVBufferRef *b_frame_ref;
>  } UnpackBFramesBSFContext;
>  
>  /* determine the position of the packed marker in the userdata,
> @@ -56,32 +56,32 @@ static void scan_buffer(const uint8_t *buf, int buf_size,
>      }
>  }
>  
> -static int mpeg4_unpack_bframes_filter(AVBSFContext *ctx, AVPacket *out)
> +static int mpeg4_unpack_bframes_filter(AVBSFContext *ctx, AVPacket *pkt)
>  {
>      UnpackBFramesBSFContext *s = ctx->priv_data;
>      int pos_p = -1, nb_vop = 0, pos_vop2 = -1, ret = 0;
> -    AVPacket *in;
>  
> -    ret = ff_bsf_get_packet(ctx, &in);
> +    ret = ff_bsf_get_packet_ref(ctx, pkt);
>      if (ret < 0)
>          return ret;
>  
> -    scan_buffer(in->data, in->size, &pos_p, &nb_vop, &pos_vop2);
> +    scan_buffer(pkt->data, pkt->size, &pos_p, &nb_vop, &pos_vop2);
>      av_log(ctx, AV_LOG_DEBUG, "Found %d VOP startcode(s) in this packet.\n", nb_vop);
>  
>      if (pos_vop2 >= 0) {
> -        if (s->b_frame->data) {
> +        if (s->b_frame_ref) {
>              av_log(ctx, AV_LOG_WARNING,
>                     "Missing one N-VOP packet, discarding one B-frame.\n");
> -            av_packet_unref(s->b_frame);
> +            av_buffer_unref(&s->b_frame_ref);
>          }
> -        /* store the packed B-frame in the BSFContext */
> -        ret = av_packet_ref(s->b_frame, in);
> -        if (ret < 0) {
> +        /* store a reference to the packed B-frame's data in the BSFContext */
> +        s->b_frame_ref = av_buffer_ref(pkt->buf);
> +        if (!s->b_frame_ref) {
> +            ret = AVERROR(ENOMEM);
>              goto fail;
>          }
> -        s->b_frame->size -= pos_vop2;
> -        s->b_frame->data += pos_vop2;
> +        s->b_frame_ref->data = pkt->data + pos_vop2;
> +        s->b_frame_ref->size = pkt->size - pos_vop2;
>      }
>  
>      if (nb_vop > 2) {
> @@ -89,56 +89,49 @@ static int mpeg4_unpack_bframes_filter(AVBSFContext *ctx, AVPacket *out)
>         "Found %d VOP headers in one packet, only unpacking one.\n", nb_vop);
>      }
>  
> -    if (nb_vop == 1 && s->b_frame->data) {
> -        /* use frame from BSFContext */
> -        av_packet_move_ref(out, s->b_frame);
> +    if (nb_vop == 1 && s->b_frame_ref) {
> +        AVBufferRef *tmp = pkt->buf;
>  
> -        /* use properties from current input packet */
> -        ret = av_packet_copy_props(out, in);
> -        if (ret < 0) {
> -            goto fail;
> -        }
> +        /* make tmp accurately reflect the packet's data */
> +        tmp->data = pkt->data;
> +        tmp->size = pkt->size;
> +
> +        /* replace data in packet with stored data */
> +        pkt->buf  = s->b_frame_ref;
> +        pkt->data = s->b_frame_ref->data;
> +        pkt->size = s->b_frame_ref->size;
> +
> +        /* store reference to data into BSFContext */
> +        s->b_frame_ref = tmp;
>  
> -        if (in->size <= MAX_NVOP_SIZE) {
> -            /* N-VOP */
> +        if (s->b_frame_ref->size <= MAX_NVOP_SIZE) {
> +            /* N-VOP - discard stored data */
>              av_log(ctx, AV_LOG_DEBUG, "Skipping N-VOP.\n");
> -        } else {
> -            /* copy packet into BSFContext */
> -            av_packet_move_ref(s->b_frame, in);
> +            av_buffer_unref(&s->b_frame_ref);
>          }
>      } else if (nb_vop >= 2) {
>          /* use first frame of the packet */
> -        av_packet_move_ref(out, in);
> -        out->size = pos_vop2;
> +        pkt->size = pos_vop2;
>      } else if (pos_p >= 0) {
> -        ret = av_packet_make_writable(in);
> +        ret = av_packet_make_writable(pkt);
>          if (ret < 0)
>              goto fail;
>          av_log(ctx, AV_LOG_DEBUG, "Updating DivX userdata (remove trailing 'p').\n");
> -        av_packet_move_ref(out, in);
>          /* remove 'p' (packed) from the end of the (DivX) userdata string */
> -        out->data[pos_p] = '\0';
> +        pkt->data[pos_p] = '\0';
>      } else {
> -        /* copy packet */
> -        av_packet_move_ref(out, in);
> +        /* use packet as is */
>      }
>  
>  fail:
>      if (ret < 0)
> -        av_packet_unref(out);
> -    av_packet_free(&in);
> +        av_packet_unref(pkt);
>  
>      return ret;
>  }
>  
>  static int mpeg4_unpack_bframes_init(AVBSFContext *ctx)
>  {
> -    UnpackBFramesBSFContext *s = ctx->priv_data;
> -
> -    s->b_frame = av_packet_alloc();
> -    if (!s->b_frame)
> -        return AVERROR(ENOMEM);
> -
>      if (ctx->par_in->extradata) {
>          int pos_p_ext = -1;
>          scan_buffer(ctx->par_in->extradata, ctx->par_in->extradata_size, &pos_p_ext, NULL, NULL);
> @@ -155,13 +148,13 @@ static int mpeg4_unpack_bframes_init(AVBSFContext *ctx)
>  static void mpeg4_unpack_bframes_flush(AVBSFContext *bsfc)
>  {
>      UnpackBFramesBSFContext *ctx = bsfc->priv_data;
> -    av_packet_unref(ctx->b_frame);
> +    av_buffer_unref(&ctx->b_frame_ref);
>  }
>  
>  static void mpeg4_unpack_bframes_close(AVBSFContext *bsfc)
>  {
>      UnpackBFramesBSFContext *ctx = bsfc->priv_data;
> -    av_packet_free(&ctx->b_frame);
> +    av_buffer_unref(&ctx->b_frame_ref);
>  }
>  
>  static const enum AVCodecID codec_ids[] = {
> 
Ping.

- Andreas