[Feature request]: A flatpak command to wait for an application to exit

[g2p]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Suggestion

I would like to wait for an app to close from the CLI (example use case: run umount on an external disk after an app that uses it is done).

At the moment I found two awkward ways to do it:

• figure out the pid by grepping flatpak ps output for a matching application name ([Feature request]: JSON command output format #5499 would help), use tail -f /dev/null --pid $pid to wait for it (tail does kill(pid, 0) every second)
• figure out the pid, use it to build flatpak's systemd user unit scope name, and wait for that to finish using systemd: systemctl --user start --wait app-flatpak-$app-$pid.scope

The first option is technically racy, the second relies on those scope names being predictable.

RFE: provide flatpak wait INSTANCE to wait until an instance has exited. Non-zero exit if the instance doesn't exist. Possibly provide a --timeout flag.