nixos/services/prebuild.nix

# services/prebuild.nix — nightly pre-build of the other hosts' system closures.
#
# Both laptops/desktops offload builds to this host (nix.buildMachines in
# common.nix), and the Macbook can't build at all (max-jobs = 0). Realising
# their toplevels here ahead of time turns `update` on those machines into a
# pure copy: the offloaded build finds the path already in this store and just
# ships it back over SSH.
#
# --out-link doubles as the gc root, so `clean` doesn't reap the closures
# before the other machines ever ask for them.
{ config, lib, pkgs, ... }:

let
  mkService = host: {
    description = "Pre-build the ${host} system closure";
    after = [ "network-online.target" ];
    wants = [ "network-online.target" ];
    path = [ pkgs.git ];
    environment.HOME = "/root";
    serviceConfig = {
      Type = "oneshot";
      StateDirectory = "prebuild";
    };
    # Gaming's kernel comes from lantian's attic (see hosts/hardware/
    # FredOS-Gaming.nix). That substituter isn't configured on this host, so
    # without it the cachyos kernel gets built from source here instead of
    # fetched — hours, not minutes. Harmless for the Macbook.
    script = ''
      ${config.nix.package}/bin/nix build --refresh \
        --option extra-substituters https://attic.xuyh0120.win/lantian \
        --option extra-trusted-public-keys lantian:EeAUQ+W+6r7EtwnmYjeVwx5kOGEBpjlBfPlzGlTNvHc= \
        --out-link /var/lib/prebuild/${host} \
        'git+https://forg.gregersen.it/rope/nixos#nixosConfigurations.${host}.config.system.build.toplevel'
    '';
  };

  # Staggered so the two builds don't fight over cores and bandwidth.
  mkTimer = startAt: {
    wantedBy = [ "timers.target" ];
    timerConfig = {
      OnCalendar = startAt;
      Persistent = true;
      RandomizedDelaySec = "20m";
    };
  };
in
{
  config = lib.mkIf (config.networking.hostName == "FredOS-Mediaserver") {
    systemd.services = {
      prebuild-macbook = mkService "FredOS-Macbook";
      prebuild-gaming = mkService "FredOS-Gaming";
    };

    systemd.timers = {
      prebuild-macbook = mkTimer "04:00";
      prebuild-gaming = mkTimer "05:00";
    };
  };
}