avformat/matroskaenc: check packet side data for AAC extradata updates

This adapts and merges commit f4bf2363
from libav, originally skipped in 13a211e6
as it was not necessary back then.

Is's applied now in preparation for the following patches, where the
aac_adtstoasc bitstream filter will start to correctly propagate the new
extradata through packet side data.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskaenc.c

@@ -93,6 +93,8 @@ typedef struct mkv_cues {
 typedef struct mkv_track {
     int             write_dts;
     int             has_cue;
+    int             sample_rate;
+    int64_t         sample_rate_offset;
     int64_t         codecpriv_offset;
     int64_t         ts_offset;
 } mkv_track;
@@ -715,24 +717,40 @@ static int put_flac_codecpriv(AVFormatContext *s,
     return 0;
 }
 
-static int get_aac_sample_rates(AVFormatContext *s, AVCodecParameters *par,
+static int get_aac_sample_rates(AVFormatContext *s, uint8_t *extradata, int extradata_size,
                                 int *sample_rate, int *output_sample_rate)
 {
     MPEG4AudioConfig mp4ac;
+    int ret;
 
-    if (avpriv_mpeg4audio_get_config(&mp4ac, par->extradata,
-                                     par->extradata_size * 8, 1) < 0) {
+    ret = avpriv_mpeg4audio_get_config(&mp4ac, extradata,
+                                       extradata_size * 8, 1);
+    /* Don't abort if the failure is because of missing extradata. Assume in that
+     * case a bitstream filter will provide the muxer with the extradata in the
+     * first packet.
+     * Abort however if s->pb is not seekable, as we would not be able to seek back
+     * to write the sample rate elements once the extradata shows up, anyway. */
+    if (ret < 0 && (extradata_size || !(s->pb->seekable & AVIO_SEEKABLE_NORMAL))) {
         av_log(s, AV_LOG_ERROR,
                "Error parsing AAC extradata, unable to determine samplerate.\n");
         return AVERROR(EINVAL);
     }
 
+    if (ret < 0) {
+        /* This will only happen when this function is called while writing the
+         * header and no extradata is available. The space for this element has
+         * to be reserved for when this function is called again after the
+         * extradata shows up in the first packet, as there's no way to know if
+         * output_sample_rate will be different than sample_rate or not. */
+        *output_sample_rate = *sample_rate;
+    } else {
         *sample_rate        = mp4ac.sample_rate;
         *output_sample_rate = mp4ac.ext_sample_rate;
+    }
     return 0;
 }
 
-static int mkv_write_native_codecprivate(AVFormatContext *s,
+static int mkv_write_native_codecprivate(AVFormatContext *s, AVIOContext *pb,
                                          AVCodecParameters *par,
                                          AVIOContext *dyn_cp)
 {
@@ -761,6 +779,12 @@ static int mkv_write_native_codecprivate(AVFormatContext *s,
             avio_write(dyn_cp, par->extradata + 12,
                        par->extradata_size - 12);
         break;
+    case AV_CODEC_ID_AAC:
+        if (par->extradata_size)
+            avio_write(dyn_cp, par->extradata, par->extradata_size);
+        else
+            put_ebml_void(pb, MAX_PCE_SIZE + 2 + 4);
+        break;
     default:
         if (par->codec_id == AV_CODEC_ID_PRORES &&
             ff_codec_get_id(ff_codec_movvideo_tags, par->codec_tag) == AV_CODEC_ID_PRORES) {
@@ -785,7 +809,7 @@ static int mkv_write_codecprivate(AVFormatContext *s, AVIOContext *pb,
         return ret;
 
     if (native_id) {
-        ret = mkv_write_native_codecprivate(s, par, dyn_cp);
+        ret = mkv_write_native_codecprivate(s, pb, par, dyn_cp);
     } else if (par->codec_type == AVMEDIA_TYPE_VIDEO) {
         if (qt_id) {
             if (!par->codec_tag)
@@ -1175,7 +1199,8 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
     }
 
     if (par->codec_id == AV_CODEC_ID_AAC) {
-        ret = get_aac_sample_rates(s, par, &sample_rate, &output_sample_rate);
+        ret = get_aac_sample_rates(s, par->extradata, par->extradata_size, &sample_rate,
+                                   &output_sample_rate);
         if (ret < 0)
             return ret;
     }
@@ -1370,6 +1395,8 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
 
         subinfo = start_ebml_master(pb, MATROSKA_ID_TRACKAUDIO, 0);
         put_ebml_uint  (pb, MATROSKA_ID_AUDIOCHANNELS    , par->channels);
+
+        mkv->tracks[i].sample_rate_offset = avio_tell(pb);
         put_ebml_float (pb, MATROSKA_ID_AUDIOSAMPLINGFREQ, sample_rate);
         if (output_sample_rate)
             put_ebml_float(pb, MATROSKA_ID_AUDIOOUTSAMPLINGFREQ, output_sample_rate);
@@ -2233,6 +2260,37 @@ static int mkv_check_new_extra_data(AVFormatContext *s, AVPacket *pkt)
                                         &side_data_size);
 
     switch (par->codec_id) {
+    case AV_CODEC_ID_AAC:
+        if (side_data_size && (s->pb->seekable & AVIO_SEEKABLE_NORMAL) && !mkv->is_live) {
+            int filler, output_sample_rate = 0;
+            int64_t curpos;
+            ret = get_aac_sample_rates(s, side_data, side_data_size, &track->sample_rate,
+                                       &output_sample_rate);
+            if (ret < 0)
+                return ret;
+            if (!output_sample_rate)
+                output_sample_rate = track->sample_rate; // Space is already reserved, so it's this or a void element.
+            av_freep(&par->extradata);
+            ret = ff_alloc_extradata(par, side_data_size);
+            if (ret < 0)
+                return ret;
+            memcpy(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, par, 1, 0);
+            filler = MAX_PCE_SIZE + 2 + 4 - (avio_tell(mkv->tracks_bc) - track->codecpriv_offset);
+            if (filler)
+                put_ebml_void(mkv->tracks_bc, filler);
+            avio_seek(mkv->tracks_bc, track->sample_rate_offset, SEEK_SET);
+            put_ebml_float(mkv->tracks_bc, MATROSKA_ID_AUDIOSAMPLINGFREQ, track->sample_rate);
+            put_ebml_float(mkv->tracks_bc, MATROSKA_ID_AUDIOOUTSAMPLINGFREQ, output_sample_rate);
+            avio_seek(mkv->tracks_bc, curpos, SEEK_SET);
+        } else if (!par->extradata_size && !track->sample_rate) {
+            // No extradata (codecpar or packet side data).
+            av_log(s, AV_LOG_ERROR, "Error parsing AAC extradata, unable to determine samplerate.\n");
+            return AVERROR(EINVAL);
+        }
+        break;
     case AV_CODEC_ID_FLAC:
         if (side_data_size && (s->pb->seekable & AVIO_SEEKABLE_NORMAL) && !mkv->is_live) {
             AVCodecParameters *codecpriv_par;