avformat/matroskaenc: Remove unnecessary avio_tell(), avio_seek()

avio_close_dyn_buf() has a bug: When the write pointer does not point to
the end of the written data when calling it (i.e. when one has performed
a seek back to update already written data), it would not add padding to
the end of the buffer, but to the current position, overwriting other
data; furthermore the reported size would be wrong (off by the amount of
data it has overwritten with padding).

In order not to run into this when updating already written elements or
elements for which size has only been reserved, the Matroska muxer would
first record the current position of the dynamic buffer, then seek to
the desired position, perform the update and seek back to the earlier
position.

But now that end_ebml_master_crc32() does not make use of
avio_close_dyn_buf() any more, this is no longer necessary.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskaenc.c

@@ -2182,7 +2182,6 @@ static int mkv_check_new_extra_data(AVFormatContext *s, AVPacket *pkt)
     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)
@@ -2193,7 +2192,6 @@ static int mkv_check_new_extra_data(AVFormatContext *s, AVPacket *pkt)
             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);
@@ -2202,7 +2200,6 @@ static int mkv_check_new_extra_data(AVFormatContext *s, AVPacket *pkt)
             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");
@@ -2212,7 +2209,6 @@ static int mkv_check_new_extra_data(AVFormatContext *s, AVPacket *pkt)
     case AV_CODEC_ID_FLAC:
         if (side_data_size && (s->pb->seekable & AVIO_SEEKABLE_NORMAL) && !mkv->is_live) {
             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);
@@ -2227,10 +2223,8 @@ static int mkv_check_new_extra_data(AVFormatContext *s, AVPacket *pkt)
                 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;
@@ -2242,7 +2236,6 @@ static int mkv_check_new_extra_data(AVFormatContext *s, AVPacket *pkt)
             AVIOContext *dyn_cp;
             uint8_t *codecpriv;
             int codecpriv_size;
-            int64_t curpos;
             ret = avio_open_dyn_buf(&dyn_cp);
             if (ret < 0)
                 return ret;
@@ -2252,12 +2245,10 @@ static int mkv_check_new_extra_data(AVFormatContext *s, AVPacket *pkt)
                 av_free(codecpriv);
                 return AVERROR_INVALIDDATA;
             }
-            curpos = avio_tell(mkv->tracks_bc);
             avio_seek(mkv->tracks_bc, track->codecpriv_offset, SEEK_SET);
             // Do not write the OBUs as we don't have space saved for them
             put_ebml_binary(mkv->tracks_bc, MATROSKA_ID_CODECPRIVATE, codecpriv, 4);
             av_free(codecpriv);
-            avio_seek(mkv->tracks_bc, curpos, SEEK_SET);
             ret = ff_alloc_extradata(par, side_data_size);
             if (ret < 0)
                 return ret;
@@ -2556,7 +2547,6 @@ static int mkv_write_trailer(AVFormatContext *s)
         // update stream durations
         if (!mkv->is_live) {
             int i;
-            int64_t curr = avio_tell(mkv->tags_bc);
             for (i = 0; i < s->nb_streams; ++i) {
                 AVStream *st = s->streams[i];
                 mkv_track *track = &mkv->tracks[i];
@@ -2577,7 +2567,6 @@ static int mkv_write_trailer(AVFormatContext *s)
                     put_ebml_binary(mkv->tags_bc, MATROSKA_ID_TAGSTRING, duration_string, 20);
                 }
             }
-            avio_seek(mkv->tags_bc, curr, SEEK_SET);
         }
         if (mkv->tags_bc && !mkv->is_live) {
             avio_seek(pb, mkv->tags_pos, SEEK_SET);