previously, we had a singleton 'main' flag to indicate a site can
be used for updates. with this new approach, we can get rid of the
'primary' concept. each site can be used for updates or not.
the noop backend is migrated into 0 sites config.
when the updater code sees that there is no site to backup, it will
just fail. user has to manually update with skipBackup flag.
we generate a signing key pair for each target. Initially, I had this
as global. We needed a route to return the public key and putting it
under backup target seemed natural. Since we delete the backups when
we delete a target, we lose all the signing hashes. So, it's fine to lose
the key pair on target delete.
snapshot file tracks the snapshot directory. when app gets deleted,
the cleaner will remove the upstream snapshot directory when it runs.
cache files are used in rsync logic to track what was uploading into
snapshot in the previous run without needing to rescan upstream.
format is part of the backup target
in the future, if we want per-app format or schedule, we can add this
separately to the apps table itself. the full box backup can ignore
apps with a set backup target and use the latest backup (like an errored app).
the nice thing is restore will work correctly.
what's important:
* if task code ran, it exits with 0. this code is regardless of (error, result)
* when it exited cleanly, we will get the values from the database
* if task timed out, the box code kills it and it has a flag tracking timedOut. we can
ignore exit code in this case.
* if task code was stopped, box code will send SIGTERM which ideally it will handle and end with 70.
* if task code crashed and it caught the exception, it will return 50
* if task code crashed and node nuked us, it will exit with 1
* if task code was killed with some unhandleabe signal, taskworker.sh will return the signal (9=SIGKILL)
