avformat/matroskaenc: write updated STREAMINFO metadata for FLAC streams if available

FLAC streams originating from the FLAC encoder send updated and more
complete STREAMINFO metadata as part of the last packet, so write that
to CodecPrivate instead of the incomplete one available in extradata
during init.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskaenc.c

@@ -93,6 +93,7 @@ typedef struct mkv_cues {
 typedef struct mkv_track {
     int             write_dts;
     int             has_cue;
+    int64_t         codecpriv_offset;
     int64_t         ts_offset;
 } mkv_track;
 
@@ -1272,6 +1273,7 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
     }
 
     if (mkv->mode != MODE_WEBM || par->codec_id != AV_CODEC_ID_WEBVTT) {
+        mkv->tracks[i].codecpriv_offset = avio_tell(pb);
         ret = mkv_write_codecprivate(s, pb, par, native_id, qt_id);
         if (ret < 0)
             return ret;
@@ -2097,6 +2099,52 @@ static void mkv_start_new_cluster(AVFormatContext *s, AVPacket *pkt)
     avio_flush(s->pb);
 }
 
+static int mkv_check_new_extra_data(AVFormatContext *s, AVPacket *pkt)
+{
+    MatroskaMuxContext *mkv = s->priv_data;
+    mkv_track *track        = &mkv->tracks[pkt->stream_index];
+    AVCodecParameters *par  = s->streams[pkt->stream_index]->codecpar;
+    uint8_t *side_data;
+    int side_data_size = 0, ret;
+
+    side_data = av_packet_get_side_data(pkt, AV_PKT_DATA_NEW_EXTRADATA,
+                                        &side_data_size);
+
+    switch (par->codec_id) {
+    case AV_CODEC_ID_FLAC:
+        if (side_data_size && s->pb->seekable) {
+            AVCodecParameters *codecpriv_par;
+            int64_t curpos;
+            if (side_data_size != par->extradata_size) {
+                av_log(s, AV_LOG_ERROR, "Invalid FLAC STREAMINFO metadata for output stream %d\n",
+                       pkt->stream_index);
+                return AVERROR(EINVAL);
+            }
+            codecpriv_par = avcodec_parameters_alloc();
+            if (!codecpriv_par)
+                return AVERROR(ENOMEM);
+            ret = avcodec_parameters_copy(codecpriv_par, par);
+            if (ret < 0) {
+                avcodec_parameters_free(&codecpriv_par);
+                return ret;
+            }
+            memcpy(codecpriv_par->extradata, side_data, side_data_size);
+            curpos = avio_tell(mkv->tracks_bc);
+            avio_seek(mkv->tracks_bc, track->codecpriv_offset, SEEK_SET);
+            mkv_write_codecprivate(s, mkv->tracks_bc, codecpriv_par, 1, 0);
+            avio_seek(mkv->tracks_bc, curpos, SEEK_SET);
+            avcodec_parameters_free(&codecpriv_par);
+        }
+        break;
+    default:
+        if (side_data_size)
+            av_log(s, AV_LOG_DEBUG, "Ignoring new extradata in a packet for stream %d.\n", pkt->stream_index);
+        break;
+    }
+
+    return 0;
+}
+
 static int mkv_write_packet_internal(AVFormatContext *s, AVPacket *pkt, int add_cue)
 {
     MatroskaMuxContext *mkv = s->priv_data;
@@ -2189,6 +2237,10 @@ static int mkv_write_packet(AVFormatContext *s, AVPacket *pkt)
     int ret;
     int start_new_cluster;
 
+    ret = mkv_check_new_extra_data(s, pkt);
+    if (ret < 0)
+        return ret;
+
     if (mkv->tracks[pkt->stream_index].write_dts)
         cluster_time = pkt->dts - mkv->cluster_pts;
     else