machine_setup/nix/configuration/roles/hydra/default.nix

{
  config,
  lib,
  pkgs,
  ...
}:

let
  patchScriptBin =
    {
      filename,
      contents,
      path ? [ ],
    }:
    ((pkgs.writeScriptBin filename contents).overrideAttrs (old: {
      buildInputs = [ pkgs.makeWrapper ];
      buildCommand = "${old.buildCommand}\n patchShebangs $out\nwrapProgram $out/bin/${filename} --prefix PATH : ${lib.makeBinPath path}";
    }));
  build_odo = (
    patchScriptBin {
      filename = "build_odo";
      contents = (builtins.readFile ./files/build_odo.bash);
      path = with pkgs; [
        bash
        git
        nix
        nix-output-monitor
        nixos-rebuild
      ];
    }
  );
in
{
  imports = [ ];

  options.me = {
    hydra.enable = lib.mkOption {
      type = lib.types.bool;
      default = false;
      example = true;
      description = "Whether we want to install hydra.";
    };
  };

  config = lib.mkIf config.me.hydra.enable {
    environment.systemPackages = with pkgs; [
      build_odo
    ];

    environment.persistence."/persist" = lib.mkIf (config.me.mountPersistence) {
      hideMounts = true;
      users.nixworker = {
        directories = [
          {
            directory = "persist";
            user = "nixworker";
            group = "nixworker";
            mode = "0700";
          }
        ];
      };
    };

    # Nix 2.30.0 (2025-07-07) changed the build directory from /tmp to /nix/var/nix/builds which broke a number of builds because my ZFS datasets were utf8only.
    fileSystems."/home/nixworker/persist/root/nix/var/nix/builds" = {
      device = "tmpfs";
      fsType = "tmpfs";
      options = [
        "size=40G" # adjust for your situation and needs
        "mode=700"
        "uid=11400"
        "gid=11400"
      ];
    };

    systemd.timers."build-cache" = {
      wantedBy = [ "timers.target" ];
      timerConfig = {
        OnCalendar = "Mon *-*-* 02:00:00 America/New_York";
        Unit = "build-cache.service";
      };
    };

    systemd.services."build-cache" = {
      script = ''
        set -euo pipefail
        IFS=$'\n\t'
        DIR="$( cd "$( dirname "''${BASH_SOURCE[0]}" )" && pwd )"

        ${build_odo}/bin/build_odo
      '';
      restartIfChanged = false;
      serviceConfig = {
        Type = "simple";
        User = "nixworker";
        RemainAfterExit = true; # Prevents the service from automatically starting on rebuild. See https://discourse.nixos.org/t/how-to-prevent-custom-systemd-service-from-restarting-on-nixos-rebuild-switch/43431
        LimitNOFILE = 8192;
      };
    };
  };
}