nixos-servers/modules/custom/backups.nix

{ pkgs, lib, config, ... }:
let
  cfg = config.custom.backups;
  snapshotFile = "/tmp/snapshot.qcow2";
  snapshotMount = "/tmp/snapshot";
  beforeEverything = pkgs.writeShellScriptBin "beforeEverything" ''
    ${pkgs.libvirt}/bin/virsh snapshot-create-as --domain ${cfg.domainName} --name backup-${cfg.domainName} --disk-only --quiesce --no-metadata --diskspec vda,snapshot=no --diskspec vdb,file=${snapshotFile} && ${pkgs.coreutils}/bin/sleep 1
    ${pkgs.coreutils}/bin/mkdir -p ${snapshotMount}
    ${pkgs.libguestfs-with-appliance}/bin/guestmount -a ${snapshotFile} -m /dev/sda1 --ro ${snapshotMount}
  '';

  afterEverything = pkgs.writeShellScriptBin "afterEverything" ''
    set +e
    ${pkgs.coreutils}/bin/sleep 10
    ${pkgs.libguestfs-with-appliance}/bin/guestunmount ${snapshotMount} && ${pkgs.coreutils}/bin/sleep 1
    ${pkgs.coreutils}/bin/rm -rf ${snapshotMount}
    ${pkgs.libvirt}/bin/virsh blockcommit ${cfg.domainName} vdb --active --verbose --pivot
    ${pkgs.coreutils}/bin/rm -f ${snapshotFile}
  '';

  borgmaticConfig = pkgs.writeTextFile {
    name = "borgmatic-config";
    text = ''
      source_directories:
      - ${snapshotMount}
      repositories:
      - path: ${cfg.repoLocation}
        label: ${cfg.domainName}
      keep_daily: 7
      keep_weekly: 4
      keep_monthly: 6
      unknown_unencrypted_repo_access_is_ok: true
      before_everything:
      - ${beforeEverything}/bin/beforeEverything
      after_everything:
      - ${afterEverything}/bin/afterEverything
    '';
  };
in
{
  options.custom.backups = {
    enable = lib.mkOption {
      default = false;
      type = lib.types.bool;
      description = ''
        Whether to enable backups of persistent data on this machine.
      '';
    };

    repoLocation = lib.mkOption {
      default = "${config.custom.dataDisk.mountPoint}/backups/thecloud-data.borg";
      type = lib.types.str;
      description = ''
        Location of the Borg repository to back up to.
      '';
    };

    domainName = lib.mkOption {
      default = "thecloud";
      type = lib.types.str;
      description = ''
        The name of the Libvirt domain with the data disk attached.
      '';
    };
  };

  config = lib.mkIf cfg.enable {
    environment.systemPackages = with pkgs; [ libguestfs-with-appliance borgbackup ];
    # Converted from:
    # https://github.com/borgmatic-collective/borgmatic/tree/84823dfb912db650936e3492f6ead7e0e0d32a0f/sample/systemd
    systemd.services.borgmatic = {
      description = "borgmatic backup";
      wants = [ "network-online.target" ];
      after = [ "network-online.target" ];
      unitConfig = {
        ConditionACPower = true;
      };
      serviceConfig = {
        Type = "oneshot";
        Nice = 19;
        CPUSchedulingPolicy = "batch";
        IOSchedulingClass = "best-effort";
        IOSchedulingPriority = 7;
        IOWeight = 100;
        Restart = "no";
        LogRateLimitIntervalSec = 0;
      };
      preStart = "${pkgs.coreutils}/bin/sleep 1m";
      script = "${pkgs.systemd}/bin/systemd-inhibit --who=\"borgmatic\" --what=\"sleep:shutdown\" --why=\"Prevent interrupting scheduled backup\" ${pkgs.borgmatic}/bin/borgmatic --verbosity -2 --syslog-verbosity 1";
    };

    environment.etc."borgmatic/config.yaml" = {
      source = borgmaticConfig;
    };

    # systemd.timers.borgmatic = {
    #   description = "Run borgmatic backup";
    #   wantedBy = [ "timers.target" ];
    #   timerConfig = {
    #     OnCalendar = "*-*-* 3:00:00";
    #     Persistent = true;
    #     RandomizedDelaySec = "3h";
    #   };
    # };
  };
}