Chained Ogg stream with FLAC audio never sets AVSTREAM_EVENT_FLAG_METADATA_UPDATED flag

[John Regan]

Summary of the bug:

If you use libavformat to decode a stream of chained Ogg files with FLAC audio, each file with their own metadata - libavformat only returns metadata for the first file in the chain.

If my reading of the specs is correct, this should function similarly to chained Ogg with Opus or Vorbis audio, with the effect of updating metadata as each Ogg file in the chain is read.

Attached is a small C program to demo the issue - when a stream metadata updates, it prints all tags to stdout.

Here's the the source code of the demo program. I made sure to renounce all ownership of the code, just to make sure that doesn't cause any kind of copyright issue. It's pretty trivial.

/*
This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or
distribute this software, either in source code form or as a compiled
binary, for any purpose, commercial or non-commercial, and by any
means.

In jurisdictions that recognize copyright laws, the author or authors
of this software dedicate any and all copyright interest in the
software to the public domain. We make this dedication for the benefit
of the public at large and to the detriment of our heirs and
successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org/>
*/

#include <stdio.h>

#include <libavformat/avformat.h>
#include <libavutil/dict.h>

/* program that read frames and prints out metadata
 * on a metadata update */

static void dump_and_clear_tags(AVStream *stream, int frameNumber) {
    const AVDictionaryEntry *tag = NULL;

    printf("Frame %d: tags found\n", frameNumber);
    while((tag = av_dict_get(stream->metadata, "", tag, AV_DICT_IGNORE_SUFFIX))) {
        printf("tag found: %s=%s\n",tag->key,tag->value);
    }
    stream->event_flags = 0;
    av_dict_free(&stream->metadata);
}


int main(int argc, const char *argv[]) {
    AVFormatContext *formatContext = NULL;
    AVPacket *packet = NULL;
    int ret = 1;
    int i = 0;

    if(argc < 2) {
        fprintf(stderr,"Usage: %s input.ogg\n",argv[0]);
        goto cleanup;
    }

    if(avformat_open_input(&formatContext, argv[1], NULL, NULL) < 0) {
        fprintf(stderr,"Error opening path %s\n",argv[1]);
        goto cleanup;
    }

    if(avformat_find_stream_info(formatContext, NULL) < 0) {
        fprintf(stderr,"Unable to find stream info\n");
        goto cleanup;
    }

    /* assuming stream 0 is the stream we want, typical in chained ogg */

    if(formatContext->streams[0]->event_flags & AVSTREAM_EVENT_FLAG_METADATA_UPDATED) {
        dump_and_clear_tags(formatContext->streams[0],i);
    }

    packet = av_packet_alloc();
    while(av_read_frame(formatContext,packet) == 0) {
        i++;

        if(formatContext->streams[0]->event_flags & AVSTREAM_EVENT_FLAG_METADATA_UPDATED) {
            dump_and_clear_tags(formatContext->streams[0],i);
        }

        av_packet_unref(packet);
    }
    printf("Found %d frames\n",i);

    ret = 0;
    cleanup:

    if(formatContext != NULL)
        avformat_close_input(&formatContext);

    if(packet != NULL)
        av_packet_free(&packet);

    return ret;
}

Example usage, assuming the program has been compiled and linked as "demo":

First generate some Ogg files with FLAC audio, and concatenate:

% ffmpeg -f lavfi -i anullsrc -c:a flac -to 10 -metadata title="First" -f ogg 01-flac.ogg
% ffmpeg -f lavfi -i anullsrc -c:a flac -to 10 -metadata title="Second" -f ogg 02-flac.ogg
% cat 01-flac.ogg 02-flac.ogg > chained-flac.ogg

Then for reference, Ogg files with Opus audio:

% ffmpeg -f lavfi -i anullsrc -c:a libopus -to 10 -metadata title="First" -f ogg 01-opus.ogg
% ffmpeg -f lavfi -i anullsrc -c:a libopus -to 10 -metadata title="Second" -f ogg 02-opus.ogg
% cat 01-opus.ogg 02-opus.ogg > chained-opus.ogg

Execute the demo program with the Ogg-with-Opus file to verify all metadata is printed:

% ./demo chained-opus.ogg

This prints:

Frame 0: tags found
tag found: encoder=Lavc59.18.100 libopus
tag found: title=First
Frame 502: tags found
tag found: encoder=Lavc59.18.100 libopus
tag found: title=Second
Found 1002 frames

Now with the Ogg-with-FLAC file - only metadata from the first audio file is printed:

% ./demo chained-flac.ogg

This prints:

Frame 0: tags found
tag found: encoder=Lavc59.18.100 flac
tag found: title=First
Found 194 frames

Lastly, the output of ffmpeg -version:

ffmpeg version n5.0 Copyright (c) 2000-2022 the FFmpeg developers
built with gcc 11.2.0 (GCC)
configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librav1e --enable-librsvg --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-libzimg --enable-nvdec --enable-nvenc --enable-shared --enable-version3
libavutil      57. 17.100 / 57. 17.100
libavcodec     59. 18.100 / 59. 18.100
libavformat    59. 16.100 / 59. 16.100
libavdevice    59.  4.100 / 59.  4.100
libavfilter     8. 24.100 /  8. 24.100
libswscale      6.  4.100 /  6.  4.100
libswresample   4.  3.100 /  4.  3.100
libpostproc    56.  3.100 / 56.  3.100