[FFmpeg-trac] #6862(undetermined:new): SRT conversion failure depends on unrelated other tracks being converted

[FFmpeg]

#6862: SRT conversion failure depends on unrelated other tracks being converted
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             Reporter:  julian       |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:  git-
  undetermined                       |  master
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 there is something weird going on:

 1.) download the files here ... sorry i couldn't find any other or shorter
 samples exhibiting this issue: http://bit.ly/2gYksPx
 2.) for simplicity rename the mkv to mmm.mkv and the srt to mmm.srt
 3.) now start conversion using ffmpeg 3.4 or current git head .. this will
 fail because the SRT has an unknown encoding which can't be guessed
 {{{ffmpeg -fix_sub_duration -i mmm.mkv -i mmm.srt -map 0:0 -map 0:1 -map
 1:0 -scodec mov_text -vcodec copy -acodec copy  mmm.mp4}}}
 4.) if do do a completely unrelated change to the command line, conversion
 suddenly succeeds. we tell it to also encode the internal MKV subtitle:
 {{{ffmpeg -fix_sub_duration -i mmm.mkv -i mmm.srt -map 0:0 -map 0:1 -map
 1:0 -map 0:2 -scodec mov_text -vcodec copy -acodec copy  mmm.mp4}}}
 i see no reason why success of the conversion depends on whether the
 internal subtitle is converted, when the problem is with the external SRT
 subtitle. i think this must be a bug. if there is a problem with the SRT,
 ffmpeg should fail (or succeed) regardless of whether other subtitle
 tracks are also specified.

 attached are the full outputs of ffmpeg

 note that the behaviour is the same whether -sub_charenc is omitted or if
 its added but specifies a wrong encoding

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Ticket URL: <https://trac.ffmpeg.org/ticket/6862>
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