[FFmpeg-devel] [PATCH 1/6] avformat/matroskaenc: Move adding SeekEntry into end_ebml_master_crc32()

Andreas Rheinhardt andreas.rheinhardt at gmail.com
Tue May 5 10:01:11 EEST 2020


Andreas Rheinhardt:
> Up until now, SeekEntries were already added before
> start_ebml_master_crc32() was even called and before we were actually
> sure that we really write the element the SeekHead references; after
> all, we might also error out later; and given that the allocations
> implicit in dynamic buffers should be checked, end_ebml_master_crc32()
> will eventually have to return errors itself, so that it is the right
> place to add SeekHead entries.
> 
> The earlier behaviour is of course a remnant of the time in which
> start_ebml_master_crc32() really did output something, so that the
> position before start_ebml_master_crc32() needed to be recorded.
> Erroring out later is also not as dangerous as it seems because in
> this case no SeekHead will be written (if it happened when writing
> the header, the whole muxing process would abort; if it happened
> when writing the trailer (when writing chapters not available initially),
> writing the trailer would be aborted and no SeekHead containing the
> bogus chapter entry would be written).
> 
> This commit does not change the way the SeekEntries are added for those
> elements that are output preliminarily; this is so because the SeekHead
> is written before those elements are finally output and doing it
> otherwise would increase the amount of seeks.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> ---
>  libavformat/matroskaenc.c | 64 ++++++++++++++++++++-------------------
>  1 file changed, 33 insertions(+), 31 deletions(-)
> 
> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
> index a1b613290c..b50fd8dd9b 100644
> --- a/libavformat/matroskaenc.c
> +++ b/libavformat/matroskaenc.c
> @@ -349,6 +349,17 @@ static void end_ebml_master(AVIOContext *pb, ebml_master master)
>      avio_seek(pb, pos, SEEK_SET);
>  }
>  
> +static void mkv_add_seekhead_entry(MatroskaMuxContext *mkv, uint32_t elementid,
> +                                   uint64_t filepos)
> +{
> +    mkv_seekhead *seekhead = &mkv->seekhead;
> +
> +    av_assert1(seekhead->num_entries < MAX_SEEKHEAD_ENTRIES);
> +
> +    seekhead->entries[seekhead->num_entries].elementid    = elementid;
> +    seekhead->entries[seekhead->num_entries++].segmentpos = filepos - mkv->segment_offset;
> +}
> +
>  static int start_ebml_master_crc32(AVIOContext **dyn_cp, MatroskaMuxContext *mkv)
>  {
>      int ret;
> @@ -364,11 +375,15 @@ static int start_ebml_master_crc32(AVIOContext **dyn_cp, MatroskaMuxContext *mkv
>  
>  static void end_ebml_master_crc32(AVIOContext *pb, AVIOContext **dyn_cp,
>                                    MatroskaMuxContext *mkv, uint32_t id,
> -                                  int length_size, int keep_buffer)
> +                                  int length_size, int keep_buffer,
> +                                  int add_seekentry)
>  {
>      uint8_t *buf, crc[4];
>      int size, skip = 0;
>  
> +    if (add_seekentry)
> +        mkv_add_seekhead_entry(mkv, id, avio_tell(pb));
> +
>      put_ebml_id(pb, id);
>      size = avio_get_dyn_buf(*dyn_cp, &buf);
>      put_ebml_length(pb, size, length_size);
> @@ -441,17 +456,6 @@ static void mkv_start_seekhead(MatroskaMuxContext *mkv, AVIOContext *pb)
>      put_ebml_void(pb, mkv->seekhead.reserved_size);
>  }
>  
> -static void mkv_add_seekhead_entry(MatroskaMuxContext *mkv, uint32_t elementid,
> -                                   uint64_t filepos)
> -{
> -    mkv_seekhead *seekhead = &mkv->seekhead;
> -
> -    av_assert1(seekhead->num_entries < MAX_SEEKHEAD_ENTRIES);
> -
> -    seekhead->entries[seekhead->num_entries].elementid    = elementid;
> -    seekhead->entries[seekhead->num_entries++].segmentpos = filepos - mkv->segment_offset;
> -}
> -
>  /**
>   * Write the SeekHead to the file at the location reserved for it
>   * and seek to destpos afterwards. When error_on_seek_failure
> @@ -489,7 +493,7 @@ static int mkv_write_seekhead(AVIOContext *pb, MatroskaMuxContext *mkv,
>          put_ebml_uint(dyn_cp, MATROSKA_ID_SEEKPOSITION, entry->segmentpos);
>          end_ebml_master(dyn_cp, seekentry);
>      }
> -    end_ebml_master_crc32(pb, &dyn_cp, mkv, MATROSKA_ID_SEEKHEAD, 0, 0);
> +    end_ebml_master_crc32(pb, &dyn_cp, mkv, MATROSKA_ID_SEEKHEAD, 0, 0, 0);
>  
>      remaining = seekhead->filepos + seekhead->reserved_size - avio_tell(pb);
>      put_ebml_void(pb, remaining);
> @@ -1421,7 +1425,8 @@ static int mkv_write_tracks(AVFormatContext *s)
>          end_ebml_master_crc32_preliminary(pb, mkv->tracks_bc,
>                                            MATROSKA_ID_TRACKS, &mkv->tracks_pos);
>      else
> -        end_ebml_master_crc32(pb, &mkv->tracks_bc, mkv, MATROSKA_ID_TRACKS, 0, 0);
> +        end_ebml_master_crc32(pb, &mkv->tracks_bc, mkv,
> +                              MATROSKA_ID_TRACKS, 0, 0, 0);
>  
>      return 0;
>  }
> @@ -1443,8 +1448,6 @@ static int mkv_write_chapters(AVFormatContext *s)
>              break;
>          }
>  
> -    mkv_add_seekhead_entry(mkv, MATROSKA_ID_CHAPTERS, avio_tell(pb));
> -
>      ret = start_ebml_master_crc32(&dyn_cp, mkv);
>      if (ret < 0)
>          return ret;
> @@ -1481,7 +1484,7 @@ static int mkv_write_chapters(AVFormatContext *s)
>          end_ebml_master(dyn_cp, chapteratom);
>      }
>      end_ebml_master(dyn_cp, editionentry);
> -    end_ebml_master_crc32(pb, &dyn_cp, mkv, MATROSKA_ID_CHAPTERS, 0, 0);
> +    end_ebml_master_crc32(pb, &dyn_cp, mkv, MATROSKA_ID_CHAPTERS, 0, 0, 1);
>  
>      mkv->wrote_chapters = 1;
>      return 0;
> @@ -1682,7 +1685,8 @@ static int mkv_write_tags(AVFormatContext *s)
>              end_ebml_master_crc32_preliminary(s->pb, mkv->tags_bc,
>                                                MATROSKA_ID_TAGS, &mkv->tags_pos);
>          else
> -            end_ebml_master_crc32(s->pb, &mkv->tags_bc, mkv, MATROSKA_ID_TAGS, 0, 0);
> +            end_ebml_master_crc32(s->pb, &mkv->tags_bc, mkv,
> +                                  MATROSKA_ID_TAGS, 0, 0, 0);
>      }
>      return 0;
>  }
> @@ -1713,8 +1717,6 @@ static int mkv_write_attachments(AVFormatContext *s)
>      if (!mkv->nb_attachments)
>          return 0;
>  
> -    mkv_add_seekhead_entry(mkv, MATROSKA_ID_ATTACHMENTS, avio_tell(pb));
> -
>      ret = start_ebml_master_crc32(&dyn_cp, mkv);
>      if (ret < 0)
>          return ret;
> @@ -1746,7 +1748,7 @@ static int mkv_write_attachments(AVFormatContext *s)
>          put_ebml_uid(dyn_cp, MATROSKA_ID_FILEUID, track->uid);
>          end_ebml_master(dyn_cp, attached_file);
>      }
> -    end_ebml_master_crc32(pb, &dyn_cp, mkv, MATROSKA_ID_ATTACHMENTS, 0, 0);
> +    end_ebml_master_crc32(pb, &dyn_cp, mkv, MATROSKA_ID_ATTACHMENTS, 0, 0, 1);
>  
>      return 0;
>  }
> @@ -1868,7 +1870,8 @@ static int mkv_write_header(AVFormatContext *s)
>          end_ebml_master_crc32_preliminary(s->pb, mkv->info_bc,
>                                            MATROSKA_ID_INFO, &mkv->info_pos);
>      else
> -        end_ebml_master_crc32(s->pb, &mkv->info_bc, mkv, MATROSKA_ID_INFO, 0, 0);
> +        end_ebml_master_crc32(s->pb, &mkv->info_bc, mkv,
> +                              MATROSKA_ID_INFO, 0, 0, 0);
>      pb = s->pb;
>  
>      ret = mkv_write_tracks(s);
> @@ -2153,7 +2156,8 @@ static void mkv_end_cluster(AVFormatContext *s)
>  {
>      MatroskaMuxContext *mkv = s->priv_data;
>  
> -    end_ebml_master_crc32(s->pb, &mkv->cluster_bc, mkv, MATROSKA_ID_CLUSTER, 0, 1);
> +    end_ebml_master_crc32(s->pb, &mkv->cluster_bc, mkv,
> +                          MATROSKA_ID_CLUSTER, 0, 1, 0);
>      if (!mkv->have_video) {
>          for (unsigned i = 0; i < s->nb_streams; i++)
>              mkv->tracks[i].has_cue = 0;
> @@ -2447,7 +2451,7 @@ static int mkv_write_trailer(AVFormatContext *s)
>  
>      if (mkv->cluster_bc) {
>          end_ebml_master_crc32(pb, &mkv->cluster_bc, mkv,
> -                              MATROSKA_ID_CLUSTER, 0, 0);
> +                              MATROSKA_ID_CLUSTER, 0, 0, 0);
>      }
>  
>      ret = mkv_write_chapters(s);
> @@ -2463,7 +2467,6 @@ static int mkv_write_trailer(AVFormatContext *s)
>          if (mkv->cues.num_entries) {
>              AVIOContext *cues = NULL;
>              uint64_t size;
> -            int64_t cuespos = endpos;
>              int length_size = 0;
>  
>              ret = start_ebml_master_crc32(&cues, mkv);
> @@ -2490,7 +2493,6 @@ static int mkv_write_trailer(AVFormatContext *s)
>                      ffio_free_dyn_buf(&cues);
>                      goto after_cues;
>                  } else {
> -                    cuespos = mkv->cues_pos;
>                      if ((ret64 = avio_seek(pb, mkv->cues_pos, SEEK_SET)) < 0) {
>                          ffio_free_dyn_buf(&cues);
>                          return ret64;
> @@ -2506,9 +2508,8 @@ static int mkv_write_trailer(AVFormatContext *s)
>                      }
>                  }
>              }
> -            mkv_add_seekhead_entry(mkv, MATROSKA_ID_CUES, cuespos);
>              end_ebml_master_crc32(pb, &cues, mkv, MATROSKA_ID_CUES,
> -                                  length_size, 0);
> +                                  length_size, 0, 1);
>              if (mkv->reserve_cues_space) {
>                  if (size < mkv->reserve_cues_space)
>                      put_ebml_void(pb, mkv->reserve_cues_space - size);
> @@ -2525,13 +2526,13 @@ static int mkv_write_trailer(AVFormatContext *s)
>          av_log(s, AV_LOG_DEBUG, "end duration = %" PRIu64 "\n", mkv->duration);
>          avio_seek(mkv->info_bc, mkv->duration_offset, SEEK_SET);
>          put_ebml_float(mkv->info_bc, MATROSKA_ID_DURATION, mkv->duration);
> -        end_ebml_master_crc32(pb, &mkv->info_bc, mkv, MATROSKA_ID_INFO, 0, 0);
> +        end_ebml_master_crc32(pb, &mkv->info_bc, mkv, MATROSKA_ID_INFO, 0, 0, 0);
>  
>          if (mkv->tracks_bc) {
>              // write Tracks master
>              avio_seek(pb, mkv->tracks_pos, SEEK_SET);
>              end_ebml_master_crc32(pb, &mkv->tracks_bc, mkv,
> -                                  MATROSKA_ID_TRACKS, 0, 0);
> +                                  MATROSKA_ID_TRACKS, 0, 0, 0);
>          }
>  
>          // update stream durations
> @@ -2559,7 +2560,8 @@ static int mkv_write_trailer(AVFormatContext *s)
>              }
>  
>              avio_seek(pb, mkv->tags_pos, SEEK_SET);
> -            end_ebml_master_crc32(pb, &mkv->tags_bc, mkv, MATROSKA_ID_TAGS, 0, 0);
> +            end_ebml_master_crc32(pb, &mkv->tags_bc, mkv,
> +                                  MATROSKA_ID_TAGS, 0, 0, 0);
>          }
>  
>          avio_seek(pb, endpos, SEEK_SET);
> 
Will apply the first five patches of this patchset (i.e. everything
except the cosmetics patch which has been superseded) tomorrow unless
there are objections.

- Andreas


More information about the ffmpeg-devel mailing list