[FFmpeg-trac] #10623(ffmpeg:new): Metadata is written at the end of the process, making piped output miss metadata

[FFmpeg]

#10623: Metadata is written at the end of the process, making piped output miss
metadata
-------------------------------------+-------------------------------------
             Reporter:  vortex       |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  ffmpeg       |                  Version:  git-
             Keywords:  pipe         |  master
  matroska -map                      |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 I use ffmpeg to modify (convert, copy or remove) some subtitles on mkv
 files. The data is piped through stdin and stdout using this command:

 {{{
 % ffmpeg -i pipe:0 -map 0:a -c:a copy -map 0:v -c:v copy -map 0:s? -c:s:0
 subrip -c:s:1 copy -hide_banner -loglevel warning -stats -f matroska
 pipe:1
 }}}

 The result I get from stdout is missing metadata.

 So I tried to let ffmpeg write the output to a file and I was reading the
 file asynchronously while it was still be written.

 {{{
 % ffmpeg -i pipe:0 -map 0:a -c:a copy -map 0:v -c:v copy -map 0:s? -c:s:0
 subrip -c:s:1 copy -hide_banner -loglevel warning -stats -f matroska
 out.mkv
 }}}

 I notified the metadata in the asynchronously read file was still missing,
 but the final output file had the metadata. I binary compared them and
 they were mostly identical, but differed from address 0x300D to 0x700C (I
 guess this is part the metadata).

 So it looks like ffmpeg writes the metadata at the end of processing which
 does not work well with pipes.

 I also tried to build ffmpeg from master branch (commit 5ddabd49d) and use
 it, but the problem persisted.

 Is this a bug or are my ffmpeg commandline args not optimal?
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/10623>
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