Embedding some mp4 subtitles causes ffmpeg to hang

[Frank]

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, including some that worked before with no changes in my ffmpeg version, so I think youtube must've change the way they write their subs. Additionally 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 does complete without errors if I try this with the latest 3.4.8 branch, but the subtitles are broken and do not appear when selected during playback.

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

[mkver]

Regression since af043b839c38e850af1184fd6be691f8475c048e, will look into it.

[mkver]

Fixed in bb89a2f64fd1d74cc51af62d5ef133be274dd7af.