satellite/hosts/nixos/calypso/filesystems/default.nix

{ lib, ... }:
{
  imports = [ (import ./partitions.nix { }) ];

  boot.supportedFilesystems = [ "btrfs" ];
  services.btrfs.autoScrub.enable = true;

  # {{{ Mark a bunch of paths as needed for boot
  fileSystems =
    lib.attrsets.genAttrs
      [
        "/"
        "/nix"
        "/persist/data"
        "/persist/state"
        "/persist/local/cache"
        "/boot"
      ]
      (p: {
        neededForBoot = true;
      });
  # }}}
  # {{{ Rollback
  boot.initrd.systemd.services.rollback = {
    description = "Rollback BTRFS root subvolume to a pristine state";
    wantedBy = [ "initrd.target" ];
    after = [ "systemd-cryptsetup@crypted.service" ];
    before = [ "sysroot.mount" ];
    unitConfig.DefaultDependencies = "no";
    serviceConfig.Type = "oneshot";
    script = ''
      mkdir -p /mnt

      # We first mount the btrfs root to /mnt
      # so we can manipulate btrfs subvolumes.
      mount -o subvol=/ /dev/mapper/crypted /mnt

      # While we're tempted to just delete /root and create
      # a new snapshot from /root-blank, /root is already
      # populated at this point with a number of subvolumes,
      # which makes `btrfs subvolume delete` fail.
      # So, we remove them first.
      #
      # /root contains subvolumes:
      # - /root/var/lib/portables
      # - /root/var/lib/machines
      #
      # I suspect these are related to systemd-nspawn, but
      # since I don't use it I'm not 100% sure.
      # Anyhow, deleting these subvolumes hasn't resulted
      # in any issues so far, except for fairly
      # benign-looking errors from systemd-tmpfiles.
      btrfs subvolume list -o /mnt/root |
        cut -f9 -d' ' |
        while read subvolume; do
          echo "deleting /$subvolume subvolume..."
          btrfs subvolume delete "/mnt/$subvolume"
        done &&
        echo "deleting /root subvolume..." &&
        btrfs subvolume delete /mnt/root

      echo "restoring blank /root subvolume..."
      btrfs subvolume snapshot /mnt/blank /mnt/root

      # Once we're done rolling back to a blank snapshot,
      # we can unmount /mnt and continue on the boot process.
      umount /mnt
    '';
  };
  # }}}
}