Re: wait until the file is done

[Marco Ippolito]

On 31/10/2020 18:41, João Eiras wrote:
> I'd solve this use case in bash using flock, if you are in a linux like
> environment (on windows, use cygwin).
> https://linux.die.net/man/1/flock
> e.g.

This type of invocation:

> flock -x video.mp4 -c poweroff
>
> The exclusive lock is acquired when no one else has opened the file with
> read or write access. That might mean your video editing software closed
> the file. Or not. For you to check.
presupposes *collaborative* locking.

The video editing program and the "shutdown watcher" script would both 
need to try to acquire an exclusive lock on the same pathname, such as:

$ flock -x video.mp4 video-editor --in some_file.mp4 --out video.mp4

$ flock -x video.mp4 sudo poweroff  # Run from another shell

It will not be enough for the video-editor program to open() the file, 
to contend the lock with its peer, it needs to do so with flock() semantics.

A typical opening mode unaware of this necessity (e.g. O_CREAT | O_TRUNC 
| O_WRONLY or something similar which you can expect the program to use 
internally) would not cut it on its own.

Marco Ippolito
maroloccio@gmail.com