composefs-rs integration in bootc

[travier]

The composefs-rs project is a Rust implementation of composefs that is capable of generating composefs images from container images.

We should integrate it in bootc as an alternative to the ostree backend. This would help make progress on phasing out ostree, UKI support and unified storage:

• UKI/systemd-boot tracker #806
• Unified storage #20

To be able to do that, we need to make bootc capable of handling both repository formats and have it handle the transition from ostree to pure composefs.

A potential layout for this is discussed in containers/composefs-rs#38.

Here are suggested steps for creating a first proof of concept implementation:

• Add an option to bootc switch
  • bootc switch --composefs quay.io://foo:bar
  • bootc will import the container image using the composefs-rs library in a dedicated composefs repo
  • bootc will set up the repo as needed
• bootc will create a new "deployment" for this image
  • Do the three way merge for /etc, comparing previous image, new image, current changes
  • Or use overlayfs instead to do that for /etc
  • Setup /var so that it's shared with ostree deployments
• bootc will setup the new deployment for the next boot
  • Non-UKI case:
    • GRUB: Install the kernel & initrd in /boot and setup the BLS config
    • systemd-boot: Install the kernel & initrd in /boot/efi and setup the BLS config
  • UKI case:
    • GRUB: Generate GRUB config snippet to boot the UKIs in order
    • systemd-boot: Install the UKI in /boot/efi/EFI/Linux (order handling to be confirmed)

[cgwalters]

I added the area/composefs label to this, there's a few related things there, but especially #935 will step towards this from the other direction and I'd like to try to land that relatively early on.

progress on phasing out ostree

And yes while this is definitely a longer term plan there's A LOT of stuff there; for example, we document configuring ostree-prepare-root today, and so we'll need to continue honoring configs for /usr/lib/ostree/prepare-root.conf into the forseeable future.

https://bootc-dev.github.io/bootc/filesystem.html#enabling-state-overlays is another one.

[cgwalters]

BTW #935 is updated and working now at least.

#935 merged. Still TODO:

• Support for upgrades (we should check post fetching, before reboot, check if new image enables fsverity, if so do it before rebooting)
• bootc internals enable-fsverity or so