I usually download videos using Youtube-dl. In fact, it's one of my default programs that's install on any new machine.

So earlier today, I was downloading a "live" youtube video which was to be live the whole day. I needed a specific part i.e. a single event so I ran youtube-dl on it's live URL at the beginning of the event, ran caffeinate -dismut 65535, set my mac brightness to 0 and left for my daily routine.

I came back after an hour when the event ended, and hit Ctrl+C hoping to get the final video but then I got this in my log:

^C[ffmpeg] Interrupted by user
[ffmpeg] Downloaded 370933808 bytes
[download] 100% of 353.75MiB in 01:02:40
Mohsin@Mozs-MacBook-Pro Desktop % [https @ 0x12800b800] Opening 'http://SomeLongURL/playlist/index.m3u8' for reading
Error writing trailer of file:SomeEvent.mp4: Immediate exit requested
frame=112801 fps= 21 q=-1.0 Lsize=  362240kB time=01:02:40.08 bitrate= 789.2kbits/s speed=0.717x
video:303393kB audio:59858kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Error closing file file:SomeEvent.mp4: Immediate exit requested
Exiting normally, received signal 2.

Now this Error writing trailer of file:SomeEvent.mp4 was not ideal. This meant that the mp4 file that the mp4 container of the video file that was saved did not have a proper terminating trailer. Videos have a header/trailer and it's specification on how that is, depends on it's format. And this meant that when I opened the video on VLC, it threw an error than playing. I needed to fix this in order to watch the video.

And that's when I found untrunc but that's originally written to work on Linux machines. I run MacOS and that too ARM (M1), so I would have to compile it from source. Fortunately, unlike the initial days since it's release, Docker works great on Apple Silicon machines and I can install this tool inside and use it.

The tool needs a working "reference" to fix the video so I ran youtube-dl again on the live video and took a small clip. This time, it terminated correctly and so I put them both in a videos folder with SomeEvent.mp4 and reference.mp4.

Then I cloned the untrunc repo, and ran the docker build using:

git clone git@github.com:ponchio/untrunc.git
docker build -t untrunc .

And once the docker build was done, I ran the final command to fix the video:

docker run -v ~/Desktop/videos:/files untrunc /files/reference.mp4 /files/SomeEvent.mp4

It gave an output:

Reading: /files/reference.mp4
[mov,mp4,m4a,3gp,3g2,mj2 @ 0xaaaacabcda80] multiple edit list entries, a/v desync might occur, patch welcome
Repair: /files/SomeEvent.mp4
Mdat not found!
Processed: 1%
...
...
Processed: 99%
Found 274677 packets.
Found 112779 chunks for avc1
Found 161898 chunks for mp4a
Saving to: /files/SomeEvent_fixed.mp4

And that was how I was able to recover the broken mp4 file. Hope this article helps anyone facing the same issue. Feel free to drop a comment if it does.