autoupdate: set up a systemd timer

hostsModules/laptop/nixos/autoUpdate.nix

@@ -0,0 +1,47 @@
+{ pkgs, ... }:
+
+{
+  # Ensure your script is available system-wide
+  environment.systemPackages = [
+    (pkgs.callPackage ../scripts/autoUpdate.nix {})
+  ];
+  users.users.tomasr = {
+    linger = true;  # lingering is required 
+  };
+
+  systemd.user.services.custom-autoupdate = {
+    description = "NixOS flake auto update";
+
+    serviceConfig = {
+      Type = "oneshot";
+
+      ExecStart = "/run/current-system/sw/bin/custom-autoupdate";
+
+      # safety for long rebuilds
+      TimeoutStartSec = "45min";
+      TimeoutStopSec = "10min";
+
+      # avoid overlap
+      RemainAfterExit = true;
+
+      # tweaks that should make the system run normally during the rebuilds
+      Nice = 10;
+      IOSchedulingClass = "best-effort";
+      IOSchedulingPriority = 7;
+    };
+  };
+
+  # Systemd USER timer
+  systemd.user.timers.custom-autoupdate = {
+    wantedBy = [ "timers.target" ];
+
+    timerConfig = {
+      OnCalendar = "Fri *-*-* 20:00:00"; # runs friday night. If for whatever reason something breaks. I have whole week-end to fix it.
+
+      Persistent = true; # if it happens during shutted down
+
+      # avoids thundering herd on boot
+      RandomizedDelaySec = "2h";
+    };
+  };
+}