dotfiles/nix/nixos-modules/erase-my-darlings.nix

{ config, lib, ... }:
with lib;
let
  cfg = config.magic_rb.erase-my-darlings;
in
# Send love to https://grahamc.com/blog/erase-your-darlings
{
  options.magic_rb.erase-my-darlings = {
    zfs = {
      enable = mkEnableOption "Erase the root filesystem using ZFS snapshots on boot.";

      snapshot = mkOption {
        type = types.str;
        description = "Which snapshot to rollback to, also specifies which dataset to rollback.";
      };
    };

    btrfs = {
      enable = mkEnableOption "Erase the root filesystem using BTRFS snapshots on boot.";
      
      snapshot = mkOption {
        type = types.str;
        description = "Which snapshot to rollback to.";
      };

      target = mkOption {
        type = types.str;
        description = "Which subvolume to rollback.";
      };

      disk = mkOption {
        type = types.str;
        description = "Which disk contains these subvolumes.";
      };
    };
  };

  config = mkIf (cfg.zfs.enable || cfg.btrfs.enable)
    {
      boot.initrd.postDeviceCommands = mkAfter
        ((optionalString cfg.zfs.enable ''
          zfs rollback -r ${cfg.zfs.snapshot}
        '')
        +
        (optionalString cfg.btrfs.enable ''
          mkdir -p /mnt

          mount -o subvol=/ ${cfg.btrfs.disk} /mnt

          btrfs subvolume list -o /mnt/${cfg.btrfs.target} |
          cut -f9 -d' ' |
          while read subvolume; do
            echo "deleting /$subvolume subvolume..."
            btrfs subvolume delete "/mnt/$subvolume"
          done &&
          echo "deleting ${cfg.btrfs.target} subvolume..." &&
          btrfs subvolume delete /mnt/${cfg.btrfs.target}

          echo "restoring ${cfg.btrfs.snapshot} to ${cfg.btrfs.target}..."
          btrfs subvolume snapshot /mnt/${cfg.btrfs.snapshot} /mnt/${cfg.btrfs.target}

          umount /mnt
        ''));

      assertions =
        [ { assertion = (cfg.zfs.enable == true && cfg.btrfs.enable == false) ||
                        (cfg.zfs.enable == false && cfg.btrfs.enable == true) ||
                        (cfg.zfs.enable == false && cfg.btrfs.enable == false);
            message = "Erase my darlings can be enable either with zfs, btrfs or none, not both.";
          }
        ];
    };
}