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

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

James Almer:
> 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?
> 

Some demuxers? It is the generic demuxing code (read_frame_internal(),
demux.c lines 1335-1347) that does this. And some demuxers probably do
it, too. At least the concat demuxer IIRC.

>>
>> 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);
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