[FFmpeg-devel] [PATCH 2/2] avcodec/bsf: add av_bsf_join() to chain bitstream filters

[Andreas Rheinhardt]

Marton Balint:
> Signed-off-by: Marton Balint <cus at passwd.hu>
> ---
>  doc/APIchanges       |  3 +++
>  libavcodec/avcodec.h | 19 ++++++++++++++++
>  libavcodec/bsf.c     | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  libavcodec/version.h |  2 +-
>  4 files changed, 85 insertions(+), 1 deletion(-)
> 
> diff --git a/doc/APIchanges b/doc/APIchanges
> index f1d7eac2ee..1473742139 100644
> --- a/doc/APIchanges
> +++ b/doc/APIchanges
> @@ -15,6 +15,9 @@ libavutil:     2017-10-21
>  
>  API changes, most recent first:
>  
> +2020-04-xx - xxxxxxxxxx - lavc 58.77.102 - avcodec.h
> +  Add av_bsf_join() to chain bitstream filters.
> +
>  2020-03-29 - xxxxxxxxxx - lavf 58.42.100 - avformat.h
>    av_read_frame() now guarantees to handle uninitialized input packets
>    and to return refcounted packets on success.
> diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> index 8fc0ad92c9..2812055e8a 100644
> --- a/libavcodec/avcodec.h
> +++ b/libavcodec/avcodec.h
> @@ -6056,6 +6056,25 @@ int av_bsf_receive_packet(AVBSFContext *ctx, AVPacket *pkt);
>   */
>  void av_bsf_flush(AVBSFContext *ctx);
>  
> +/**
> + * Join a new bitstream filter to an already operating one.
> + *
> + * @param[in,out] out_bsf *out_bsf contains the existing, already initialized bitstream
> + *                        filter, the new filter will be joined to the output of this.
> + *                        It can be NULL, in which case an empty filter is assumed.
> + *                        Upon successful return *out_bsf will contain the new, combined
> + *                        bitstream filter which will be initialized.
> + * @param[in]     bsf     the bitstream filter to be joined to the output of *out_bsf.
> + *                        This filter must be uninitialized but it must be ready to be
> + *                        initalized, so input codec parameters and time base must be
> + *                        set if *out_bsf is NULL, otherwise the function will set these
> + *                        automatically based on the output parameters of *out_bsf.
> + *                        Upon successful return the bsf will be initialized.
> + *
> + * @return 0 on success, negative on error.

One needs to be more explicit about what happens on error: bsf may be
in a partially initialized state and is essentially only good for
freeing (maybe the bsf parameter should be AVBSFContext **, too, and
automatically free bsf on error?). But IMO we should aim to not cripple
*out_bsf and document this.

> + */
> +int av_bsf_join(AVBSFContext **out_bsf, AVBSFContext *bsf);
> +
>  /**
>   * Free a bitstream filter context and everything associated with it; write NULL
>   * into the supplied pointer.
> diff --git a/libavcodec/bsf.c b/libavcodec/bsf.c
> index b9fc771a88..1bca28d1ae 100644
> --- a/libavcodec/bsf.c
> +++ b/libavcodec/bsf.c
> @@ -487,6 +487,68 @@ end:
>      return ret;
>  }
>  
> +int av_bsf_join(AVBSFContext **out_bsf, AVBSFContext *bsf)
> +{
> +    BSFListContext *lst;
> +    AVBSFContext *obsf = *out_bsf;
> +    AVBSFContext *new_list_bsf = NULL;
> +    int ret;
> +
> +    if (!obsf) {
> +        ret = av_bsf_init(bsf);
> +        if (ret < 0)
> +            return ret;
> +        *out_bsf = bsf;
> +        return 0;
> +    }
> +
> +    if (obsf->filter != &ff_list_bsf) {
> +        ret = av_bsf_alloc(&ff_list_bsf, &new_list_bsf);
> +        if (ret < 0)
> +            return ret;
> +        lst = new_list_bsf->priv_data;
> +        ret = av_dynarray_add_nofree(&lst->bsfs, &lst->nb_bsfs, obsf);
> +        if (ret < 0)
> +            goto fail;
> +        ret = avcodec_parameters_copy(new_list_bsf->par_in, obsf->par_in);
> +        if (ret < 0)
> +            goto fail;
> +        new_list_bsf->time_base_in = obsf->time_base_in;
> +        obsf = new_list_bsf;
> +    } else {
> +        lst = obsf->priv_data;
> +    }
> +
> +    ret = avcodec_parameters_copy(bsf->par_in, lst->bsfs[lst->nb_bsfs-1]->par_out);
> +    if (ret < 0)
> +        goto fail;
> +    bsf->time_base_in = lst->bsfs[lst->nb_bsfs-1]->time_base_out;
> +
> +    ret = av_bsf_init(&bsf);
> +    if (ret < 0)
> +        goto fail;
> +
> +    ret = avcodec_parameters_copy(obsf->par_out, bsf->par_out);

This will change obsf even on failure (when *out_bsf was already of type
ff_list_bsf). This is something that ff_stream_add_bitstream_filter()
currently doesn't do. I think we should leave obsf in a mostly unchanged
state even if it implies an unnecessary allocation (it is really only a
single allocation). This can be achieved by taking obsf's par_out in the
else branch above and replacing it with fresh AVCodecParameters. On
success, the old par_out is freed; on failure it is restored.

> +    if (ret < 0)
> +        goto fail;
> +    obsf->time_base_out = bsf->time_base_out;
> +
> +    ret = av_dynarray_add_nofree(&lst->bsfs, &lst->nb_bsfs, bsf);
> +    if (ret < 0)
> +        goto fail;
> +
> +    *out_bsf = obsf;
> +    return 0;
> +
> +fail:
> +    if (new_list_bsf) {
> +        if (lst->nb_bsfs)
> +            lst->bsfs[0] = NULL;
> +        av_bsf_free(&new_list_bsf);
> +    }
> +    return ret;
> +}
> +
>  static int bsf_parse_single(const char *str, AVBSFList *bsf_lst)
>  {
>      char *bsf_name, *bsf_options_str, *buf;
> diff --git a/libavcodec/version.h b/libavcodec/version.h
> index f4d1d4de21..dadca75430 100644
> --- a/libavcodec/version.h
> +++ b/libavcodec/version.h
> @@ -29,7 +29,7 @@
>  
>  #define LIBAVCODEC_VERSION_MAJOR  58
>  #define LIBAVCODEC_VERSION_MINOR  77
> -#define LIBAVCODEC_VERSION_MICRO 101
> +#define LIBAVCODEC_VERSION_MICRO 102

Adding a new public function always requires a minor bump.
>  
>  #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \
>                                                 LIBAVCODEC_VERSION_MINOR, \
> 

Finally, (I feel bad having to tell you this given that you have clearly
invested a lot of time in this) we currently have no case where we need
more than one automatically inserted bitstream filter per stream. We do
not know whether the current code does the right thing in case more than
one packet needs to be analyzed to determine whether to insert bitstream
filters (maybe the packets should instead be cached so that no bsf
misses out on the initial packets?); of course what is the right thing
might be format and codec-dependent. So I don't know whether we should
add this function now (given that public API is hard to remove).

This does not mean that we should not use a bsf_list bsf to simplify
automatic bitstream filtering in mux.c (it really simplifies this alot
and gets rid of code duplication). But I think we should leave the topic
of adding a bsf to an already existing bsf open so that it can be
determined when the need indeed arises.

- Andreas