workstation/detect-reboot-needed.nix | add new service to nag when reboot needed

loki/configuration.nix

@@ -7,6 +7,7 @@
       /etc/nixos/hardware-configuration.nix
       /home/don/nixos/workstation/configuration.nix
       /home/don/nixos/workstation/sway.nix
+      /home/don/nixos/workstation/detect-reboot-needed.nix
     ];
   networking.hostName = "loki";
 }

workstation/detect-reboot-needed.nix

@@ -0,0 +1,30 @@
+{ pkgs, ... }:
+
+let
+  readlink = "${pkgs.coreutils}/bin/readlink";
+  notify-send = "${pkgs.libnotify}/bin/notify-send";
+in {
+  systemd.user.services.detect-reboot-for-upgrade = {
+    script = ''
+      set -eu -o pipefail
+      booted="$(${readlink} /run/booted-system/{initrd,kernel,kernel-modules})"
+      built="$(${readlink} /nix/var/nix/profiles/system/{initrd,kernel,kernel-modules})"
+      if [[ "''${booted}" != "''${built}" ]];
+      then
+        echo "Looks like we need a reboot!"
+        ${notify-send} --urgency=low --icon=system-reboot "Reboot is needed for a NixOS upgrade."
+      fi
+    '';
+    serviceConfig = {
+      Type = "oneshot";
+    };
+  };
+  systemd.user.timers.detect-reboot-for-upgrade = {
+    wantedBy = [ "timers.target" ];
+    partOf = [ "detect-reboot-for-upgrade.service" ];
+    timerConfig = {
+      OnCalendar = "hourly";
+      Unit = "detect-reboot-for-upgrade.service";
+    };
+  };
+}