This will build on ostreedev/ostree#3420 and we're hoping to land this by at least the Fedora 43 timeframe.

TODO:

• update to newer ostree rust APIs
• Expose in status whether we are capable of soft reboots to the staged deployment
• use api to detect soft reboot capable (ostree)
• handle case where kargs are different on the container image than local state /usr/lib/bootc/kargs.d. (bootc)
• Debug prepare-root: boot is kept writable ostreedev/ostree#3365
• Debug [ 388.410231] systemd[1]: Failed to load new SELinux policy. Continuing with old policy. (this may be a serious problem!)
• UNDO: bootc upgrade --apply should automatically use soft reboots if capable (detect via new API); though we may need an opt-out flag for unusual cases bootc upgrade --apply --skip-soft-reboot?
• Handle cases (RHEL9) where soft reboot is not supported by systemd (detect missing soft-reboot.target)
• Opt-in and opt-out mechanisms (see below)

Detailed API

We need more sophistication for agent-driven cases that want precise control over the flow of every step in:

• download image (also xref upgrade/switch: Support trickle-fetch #1375 )
• queue for update (xref Use finalization locking #1320)
• detect when update can or cannot be soft-rebooted (on bare metal, the difference in downtime can be minutes)
• Control whether or not soft reboot is used

Opt-in soft reboots

• systemctl enable bootc-soft-reboot.service
• CLI flag bootc upgrade --queue-soft-reboot? (Problem: tools like flightctl, gnome-software etc. would all need to learn how to opt in to this) - this just calls systemctl enable --runtime bootc-soft-reboot.service

Precedent: rhsm-publish-facts.service which monitors bootc state for modifications