[FFmpeg-trac] #9409(undetermined:new): Embedding some mp4 subtitles causes ffmpeg to hang

FFmpeg trac at avcodec.org
Sun Sep 5 21:15:08 EEST 2021

#9409: Embedding some mp4 subtitles causes ffmpeg to hang
             Reporter:  Frank        |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:  git-
  undetermined                       |  master
             Keywords:  subtitles    |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 I've run into a bug when trying to embed some subtitles into an MP4 file,
 ffmpeg hangs indefinitely with 100% cpu usage on one thread. When this
 happens the output file stops growing and what is usually a few seconds
 job simply does not finish even if left for hours.

 This only started happening recently with some videos downloaded from
 youtube-dl. I've tested this and it seems to be an issue with the latest
 version of every 4.x branch - 4.0.6, 4.1.6, 4.2.4, 4.3.2, and the latest
 git build. ffmpeg completes if I try this with the latest 3.4.8 branch,
 but the subtitles are broken and do not appear when selected.

 Since I'm not sure where the problem is in the subtitle I can't cut out
 the problem part here for an example. I will attach one of the subtitle
 files that causes the issue and include the youtube-dl command that will
 download the smallest video.

 youtube-dl command:
 youtube-dl.exe -v -f 160+249 --merge-output-format mp4 --sub-lang bn
 --write-auto-sub --embed-subs 0UlUbeaBCTg

 ffmpeg command being run by youtube-dl:
 ffmpeg -y -loglevel "repeat+info" -i "file:Making Flavored Rum _ Easy and
 Cheap! - Part 1-0UlUbeaBCTg.mp4" -i "file:Making Flavored Rum _ Easy and
 Cheap! - Part 1-0UlUbeaBCTg.bn.vtt" -map 0 -c copy -map "-0:s" -map "-0:d"
 "-c:s" mov_text -map "1:0" "-metadata:s:s:0" "language=ben" "file:Making
 Flavored Rum _ Easy and Cheap! - Part 1-0UlUbeaBCTg.temp.mp4"
Ticket URL: <https://trac.ffmpeg.org/ticket/9409>
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