[FFmpeg-devel,07/13] avformat/matroskaenc: Don't segfault when seekability changes

If the Matroska muxer's AVIOContext was unseekable when writing the
header, but is seekable when writing the trailer, the code for writing
the trailer presumes that a dynamic buffer exists and tries to update
its content in order to overwrite data that has already been
preliminarily written when writing the header, yet said buffer doesn't
exist as it has been written finally and not preliminarily when writing
the header (because of the unseekability it was presumed that one won't
be able to update the data anyway).

This commit adds a check for this and also for a similar situation
involving updating extradata with new data from packet side-data.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
---
It seems that the whole code was based around the presumption that the
seekability does not change mid-stream. E.g. before add68dcca there were
the way level 1 elements were written depended upon the seekability and
the output would be corrupt if it changed between opening and closing. 

Given the lack of tickets about this it seems that this assumption holds
true. Therefore this patch uses the existence of the Tracks buffer as
proxy for seekability.

 libavformat/matroskaenc.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

Andreas Rheinhardt:
> If the Matroska muxer's AVIOContext was unseekable when writing the
> header, but is seekable when writing the trailer, the code for writing
> the trailer presumes that a dynamic buffer exists and tries to update
> its content in order to overwrite data that has already been
> preliminarily written when writing the header, yet said buffer doesn't
> exist as it has been written finally and not preliminarily when writing
> the header (because of the unseekability it was presumed that one won't
> be able to update the data anyway).
> 
> This commit adds a check for this and also for a similar situation
> involving updating extradata with new data from packet side-data.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
> ---
> It seems that the whole code was based around the presumption that the
> seekability does not change mid-stream. E.g. before add68dcca there were
> the way level 1 elements were written depended upon the seekability and
> the output would be corrupt if it changed between opening and closing. 
> 
> Given the lack of tickets about this it seems that this assumption holds
> true. Therefore this patch uses the existence of the Tracks buffer as
> proxy for seekability.
> 
>  libavformat/matroskaenc.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
> index 99b549ecc4..1a142403fa 100644
> --- a/libavformat/matroskaenc.c
> +++ b/libavformat/matroskaenc.c
> @@ -2186,7 +2186,7 @@ static int mkv_check_new_extra_data(AVFormatContext *s, const AVPacket *pkt)
>  
>      switch (par->codec_id) {
>      case AV_CODEC_ID_AAC:
> -        if (side_data_size && (s->pb->seekable & AVIO_SEEKABLE_NORMAL) && !mkv->is_live) {
> +        if (side_data_size && mkv->track.bc) {
>              int filler, output_sample_rate = 0;
>              ret = get_aac_sample_rates(s, side_data, side_data_size, &track->sample_rate,
>                                         &output_sample_rate);
> @@ -2213,7 +2213,7 @@ static int mkv_check_new_extra_data(AVFormatContext *s, const AVPacket *pkt)
>          }
>          break;
>      case AV_CODEC_ID_FLAC:
> -        if (side_data_size && (s->pb->seekable & AVIO_SEEKABLE_NORMAL) && !mkv->is_live) {
> +        if (side_data_size && mkv->track.bc) {
>              uint8_t *old_extradata = par->extradata;
>              if (side_data_size != par->extradata_size) {
>                  av_log(s, AV_LOG_ERROR, "Invalid FLAC STREAMINFO metadata for output stream %d\n",
> @@ -2229,8 +2229,7 @@ static int mkv_check_new_extra_data(AVFormatContext *s, const AVPacket *pkt)
>      // FIXME: Remove the following once libaom starts propagating extradata during init()
>      //        See https://bugs.chromium.org/p/aomedia/issues/detail?id=2012
>      case AV_CODEC_ID_AV1:
> -        if (side_data_size && (s->pb->seekable & AVIO_SEEKABLE_NORMAL) && !mkv->is_live &&
> -            !par->extradata_size) {
> +        if (side_data_size && mkv->track.bc && !par->extradata_size) {
>              AVIOContext *dyn_cp;
>              uint8_t *codecpriv;
>              int codecpriv_size;
> @@ -2541,6 +2540,7 @@ static int mkv_write_trailer(AVFormatContext *s)
>          if (ret < 0)
>              return ret;
>  
> +        if (mkv->info.bc) {
>          // update the duration
>          av_log(s, AV_LOG_DEBUG, "end duration = %" PRIu64 "\n", mkv->duration);
>          avio_seek(mkv->info.bc, mkv->duration_offset, SEEK_SET);
> @@ -2549,6 +2549,7 @@ static int mkv_write_trailer(AVFormatContext *s)
>                                      MATROSKA_ID_INFO, 0, 0, 0);
>          if (ret < 0)
>              return ret;
> +        }
>  
>          if (mkv->track.bc) {
>              // write Tracks master
> 
Will apply this patchset tomorrow unless there are objections.

- Andreas