ffmpeg issues invalid argument error about ffmpeg-concat

[transitive-bullshit]

This is the problem ffmpeg -framerate NaN.

It's failing to calculate the correct framerate from your input videos.

If you can post your videos somewhere, I'll be able to inspect them and find the problem.

[NCPatrick]

The issue is that the promise resolves while the files are still being written to the filesystem, so that when the app moves on to another filter/function, the mp4 file they process from is still being written.

I'm adding async/await to pause until the filesystem can catch up. I just didn't know if there was a flag or another way to tell when ffmpeg-concat was finished processing files besides just assuming resolve.

I'm adding a check to make sure the files exist before resolving, otherwise I've had to resort to timeout/resolve, which is not ideal.

Thanks for the links!