Transactional Overlay Yob

Overview

Operating systems in transactional-update mode such as MicroOS require a reboot for package installations to take effect. That can be annyoing for installing tools, e.g. for debugging. Especially when the tools are not actually mean to be installed on the host permanently. Installing a fat container runtime and as well as some container that contain another operating system just for that purpose is wasteful. Also some systems such as 32bit ARM devices also may not support fancy containers at all.

Therefore this tool simply launches a shell in a writable overlayfs over the current root fs. It leverages systemd-nspawn for that so no extra software needed.

The rpm database is a single file though, so overlays normally would be one-shot. As soon as the host installs updates, the overlay would see the files but not the changed database. To solve that, the tool dumps the host's rpm database to individual files per rpm header. The container does the same, then a new database is created from the combination of rpm headers.

Running

Initialize dump of rpm database as individual headers:

# transactional-update --continue shell
# toy --init

Reboot. Need to execute toy --init after each package installation or update.

Launch the overlay shell and install some packages

# toy
# zypper in hello 

That's it :-)

Caveats

• reconstructing the rpm database from header files is time consuming as rpm has to copy the full content of headers into the database.. Ideally rpm itself would support such a file based rpm database that allows overlays but upstream doesn't see the value yet. Hopefully this tool shows a use case.
• the overlay can't really remove packages
• toy --init has to be called manually atm as transactional-update has no custom hooks

How it works

The "container" is created empty in /var/lib/machines/toy. An overlayfs with a base of the current snapshot (in /.snapshots/) is mounted over it. Files written in the container end up in /var/lib/machines/.toy/overlay/ The updated rpm database is in /var/lib/machines/.toy/rpmdb Also, /var is stored persistently in /var/lib/machines/.toy/var