Re: [FFmpeg-devel] [PATCH] fftools/ffmpeg: store a separate copy of input codec parameters

[James Almer <jamrial@gmail.com>]

On 8/13/2022 12:36 PM, Anton Khirnov wrote:
> Use it instead of AVStream.codecpar in the main thread. While
> AVStream.codecpar is documented to only be updated when the stream is
> added or avformat_find_stream_info(), it is actually updated during
> demuxing. Accessing it from a different thread then constitutes a race.

Should we consider a bug that some demuxers update stream's codepars 
post init? Or is the documentation not reflecting the actual behavior 
what's wrong?

> 
> Ideally, some mechanism should eventually be provided for signalling
> parameter updates to the user. Then the demuxing thread could pick up
> the changes and propagate them to the decoder.
> ---
>   fftools/ffmpeg.c     | 39 ++++++++++++++++++++-------------------
>   fftools/ffmpeg.h     |  6 ++++++
>   fftools/ffmpeg_opt.c |  6 +++++-
>   3 files changed, 31 insertions(+), 20 deletions(-)
> 
> diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
> index 8eb7759392..ef7177fc33 100644
> --- a/fftools/ffmpeg.c
> +++ b/fftools/ffmpeg.c
> @@ -608,6 +608,7 @@ static void ffmpeg_cleanup(int ret)
>           av_freep(&ist->dts_buffer);
>   
>           avcodec_free_context(&ist->dec_ctx);
> +        avcodec_parameters_free(&ist->par);
>   
>           av_freep(&input_streams[i]);
>       }
> @@ -1492,7 +1493,7 @@ static void print_final_stats(int64_t total_size)
>   
>           for (j = 0; j < f->nb_streams; j++) {
>               InputStream *ist = input_streams[f->ist_index + j];
> -            enum AVMediaType type = ist->st->codecpar->codec_type;
> +            enum AVMediaType type = ist->par->codec_type;
>   
>               total_size    += ist->data_size;
>               total_packets += ist->nb_packets;
> @@ -1809,7 +1810,7 @@ static void flush_encoders(void)
>                   for (x = 0; x < fg->nb_inputs; x++) {
>                       InputFilter *ifilter = fg->inputs[x];
>                       if (ifilter->format < 0 &&
> -                        ifilter_parameters_from_codecpar(ifilter, ifilter->ist->st->codecpar) < 0) {
> +                        ifilter_parameters_from_codecpar(ifilter, ifilter->ist->par) < 0) {
>                           av_log(NULL, AV_LOG_ERROR, "Error copying paramerets from input stream\n");
>                           exit_program(1);
>                       }
> @@ -1912,11 +1913,11 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
>       if (pkt->dts == AV_NOPTS_VALUE) {
>           opkt->dts = av_rescale_q(ist->dts, AV_TIME_BASE_Q, ost->mux_timebase);
>       } else if (ost->st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
> -        int duration = av_get_audio_frame_duration2(ist->st->codecpar, pkt->size);
> +        int duration = av_get_audio_frame_duration2(ist->par, pkt->size);
>           if(!duration)
> -            duration = ist->st->codecpar->frame_size;
> +            duration = ist->par->frame_size;
>           opkt->dts = av_rescale_delta(ist->st->time_base, pkt->dts,
> -                                    (AVRational){1, ist->st->codecpar->sample_rate}, duration,
> +                                    (AVRational){1, ist->par->sample_rate}, duration,
>                                       &ist->filter_in_rescale_delta_last, ost->mux_timebase);
>           /* dts will be set immediately afterwards to what pts is now */
>           opkt->pts = opkt->dts - ost_tb_start_time;
> @@ -1976,7 +1977,7 @@ static int ifilter_send_frame(InputFilter *ifilter, AVFrame *frame, int keep_ref
>       /* determine if the parameters for this input changed */
>       need_reinit = ifilter->format != frame->format;
>   
> -    switch (ifilter->ist->st->codecpar->codec_type) {
> +    switch (ifilter->ist->par->codec_type) {
>       case AVMEDIA_TYPE_AUDIO:
>           need_reinit |= ifilter->sample_rate    != frame->sample_rate ||
>                          av_channel_layout_compare(&ifilter->ch_layout, &frame->ch_layout);
> @@ -2056,7 +2057,7 @@ static int ifilter_send_eof(InputFilter *ifilter, int64_t pts)
>       } else {
>           // the filtergraph was never configured
>           if (ifilter->format < 0) {
> -            ret = ifilter_parameters_from_codecpar(ifilter, ifilter->ist->st->codecpar);
> +            ret = ifilter_parameters_from_codecpar(ifilter, ifilter->ist->par);
>               if (ret < 0)
>                   return ret;
>           }
> @@ -2212,9 +2213,9 @@ static int decode_video(InputStream *ist, AVPacket *pkt, int *got_output, int64_
>   
>       // The following line may be required in some cases where there is no parser
>       // or the parser does not has_b_frames correctly
> -    if (ist->st->codecpar->video_delay < ist->dec_ctx->has_b_frames) {
> +    if (ist->par->video_delay < ist->dec_ctx->has_b_frames) {
>           if (ist->dec_ctx->codec_id == AV_CODEC_ID_H264) {
> -            ist->st->codecpar->video_delay = ist->dec_ctx->has_b_frames;
> +            ist->par->video_delay = ist->dec_ctx->has_b_frames;
>           } else
>               av_log(ist->dec_ctx, AV_LOG_WARNING,
>                      "video_delay is larger in decoder than demuxer %d > %d.\n"
> @@ -2222,7 +2223,7 @@ static int decode_video(InputStream *ist, AVPacket *pkt, int *got_output, int64_
>                      "of this file to https://streams.videolan.org/upload/ "
>                      "and contact the ffmpeg-devel mailing list. (ffmpeg-devel@ffmpeg.org)\n",
>                      ist->dec_ctx->has_b_frames,
> -                   ist->st->codecpar->video_delay);
> +                   ist->par->video_delay);
>       }
>   
>       if (ret != AVERROR_EOF)
> @@ -2391,7 +2392,7 @@ static int send_filter_eof(InputStream *ist)
>   /* pkt = NULL means EOF (needed to flush decoder buffers) */
>   static int process_input_packet(InputStream *ist, const AVPacket *pkt, int no_eof)
>   {
> -    const AVCodecParameters *par = ist->st->codecpar;
> +    const AVCodecParameters *par = ist->par;
>       int ret = 0, i;
>       int repeating = 0;
>       int eof_reached = 0;
> @@ -2751,7 +2752,7 @@ static int init_output_stream_streamcopy(OutputStream *ost)
>       if (!codec_ctx)
>           return AVERROR(ENOMEM);
>   
> -    ret = avcodec_parameters_to_context(codec_ctx, ist->st->codecpar);
> +    ret = avcodec_parameters_to_context(codec_ctx, ist->par);
>       if (ret >= 0)
>           ret = av_opt_set_dict(codec_ctx, &ost->encoder_opts);
>       if (ret < 0) {
> @@ -3120,8 +3121,8 @@ static int init_output_stream_encode(OutputStream *ost, AVFrame *frame)
>       case AVMEDIA_TYPE_SUBTITLE:
>           enc_ctx->time_base = AV_TIME_BASE_Q;
>           if (!enc_ctx->width) {
> -            enc_ctx->width     = input_streams[ost->source_index]->st->codecpar->width;
> -            enc_ctx->height    = input_streams[ost->source_index]->st->codecpar->height;
> +            enc_ctx->width     = input_streams[ost->source_index]->par->width;
> +            enc_ctx->height    = input_streams[ost->source_index]->par->height;
>           }
>           break;
>       case AVMEDIA_TYPE_DATA:
> @@ -3678,7 +3679,7 @@ static void decode_flush(InputFile *ifile)
>   
>           if (ist->decoding_needed) {
>               /* report last frame duration to the demuxer thread */
> -            if (ist->st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
> +            if (ist->par->codec_type == AVMEDIA_TYPE_AUDIO) {
>                   LastFrameDuration dur;
>   
>                   dur.stream_idx = i;
> @@ -3721,7 +3722,7 @@ static void ts_discontinuity_detect(InputFile *ifile, InputStream *ist,
>                          "timestamp discontinuity for stream #%d:%d "
>                          "(id=%d, type=%s): %"PRId64", new offset= %"PRId64"\n",
>                          ist->file_index, ist->st->index, ist->st->id,
> -                       av_get_media_type_string(ist->st->codecpar->codec_type),
> +                       av_get_media_type_string(ist->par->codec_type),
>                          delta, ifile->ts_offset_discont);
>                   pkt->dts -= av_rescale_q(delta, AV_TIME_BASE_Q, ist->st->time_base);
>                   if (pkt->pts != AV_NOPTS_VALUE)
> @@ -3772,8 +3773,8 @@ static void ts_discontinuity_process(InputFile *ifile, InputStream *ist,
>           pkt->pts += offset;
>   
>       // detect timestamp discontinuities for audio/video
> -    if ((ist->st->codecpar->codec_type == AVMEDIA_TYPE_VIDEO ||
> -         ist->st->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) &&
> +    if ((ist->par->codec_type == AVMEDIA_TYPE_VIDEO ||
> +         ist->par->codec_type == AVMEDIA_TYPE_AUDIO) &&
>           pkt->dts != AV_NOPTS_VALUE)
>           ts_discontinuity_detect(ifile, ist, pkt);
>   }
> @@ -3872,7 +3873,7 @@ static int process_input(int file_index)
>       if (debug_ts) {
>           av_log(NULL, AV_LOG_INFO, "demuxer+ffmpeg -> ist_index:%d type:%s pkt_pts:%s pkt_pts_time:%s pkt_dts:%s pkt_dts_time:%s duration:%s duration_time:%s off:%s off_time:%s\n",
>                  ifile->ist_index + pkt->stream_index,
> -               av_get_media_type_string(ist->st->codecpar->codec_type),
> +               av_get_media_type_string(ist->par->codec_type),
>                  av_ts2str(pkt->pts), av_ts2timestr(pkt->pts, &ist->st->time_base),
>                  av_ts2str(pkt->dts), av_ts2timestr(pkt->dts, &ist->st->time_base),
>                  av_ts2str(pkt->duration), av_ts2timestr(pkt->duration, &ist->st->time_base),
> diff --git a/fftools/ffmpeg.h b/fftools/ffmpeg.h
> index 6991ba7632..44cc23fa84 100644
> --- a/fftools/ffmpeg.h
> +++ b/fftools/ffmpeg.h
> @@ -323,6 +323,12 @@ typedef struct InputStream {
>   #define DECODING_FOR_FILTER 2
>       int processing_needed;   /* non zero if the packets must be processed */
>   
> +    /**
> +     * Codec parameters - to be used by the decoding/streamcopy code.
> +     * st->codecpar should not be accessed, because it may be modified
> +     * concurrently by the demuxing thread.
> +     */
> +    AVCodecParameters *par;
>       AVCodecContext *dec_ctx;
>       const AVCodec *dec;
>       AVFrame *decoded_frame;
> diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
> index 97f14b2a5b..30ca5cd609 100644
> --- a/fftools/ffmpeg_opt.c
> +++ b/fftools/ffmpeg_opt.c
> @@ -1090,7 +1090,11 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
>               abort();
>           }
>   
> -        ret = avcodec_parameters_from_context(par, ist->dec_ctx);
> +        ist->par = avcodec_parameters_alloc();
> +        if (!ist->par)
> +            exit_program(1);
> +
> +        ret = avcodec_parameters_from_context(ist->par, ist->dec_ctx);
>           if (ret < 0) {
>               av_log(NULL, AV_LOG_ERROR, "Error initializing the decoder context.\n");
>               exit_program(1);
_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
https://ffmpeg.org/mailman/listinfo/ffmpeg-devel

To unsubscribe, visit link above, or email
ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".