allow automatic rebooting on kernel updates

auto-upgrade.nix

@@ -0,0 +1,119 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let cfg = config.system.autoUpgrade; in
+
+{
+
+  options = {
+
+    system.autoUpgrade = {
+
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to periodically upgrade NixOS to the latest
+          version. If enabled, a systemd timer will run
+          <literal>nixos-rebuild switch --upgrade</literal> once a
+          day.
+        '';
+      };
+
+      channel = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        example = https://nixos.org/channels/nixos-14.12-small;
+        description = ''
+          The URI of the NixOS channel to use for automatic
+          upgrades. By default, this is the channel set using
+          <command>nix-channel</command> (run <literal>nix-channel
+          --list</literal> to see the current value).
+        '';
+      };
+
+      flags = mkOption {
+        type = types.listOf types.str;
+        default = [];
+        example = [ "-I" "stuff=/home/alice/nixos-stuff" "--option" "extra-binary-caches" "http://my-cache.example.org/" ];
+        description = ''
+          Any additional flags passed to <command>nixos-rebuild</command>.
+        '';
+      };
+
+      dates = mkOption {
+        default = "04:40";
+        type = types.str;
+        description = ''
+          Specification (in the format described by
+          <citerefentry><refentrytitle>systemd.time</refentrytitle>
+          <manvolnum>7</manvolnum></citerefentry>) of the time at
+          which the update will occur.
+        '';
+      };
+
+      allowReboot = mkOption {
+        default = false;
+        type = types.bool;
+        description = ''
+          Shall the system be rebooted if the new configuration
+          uses a different kernel, kernel modules or initrd
+          than the booted system?
+        '';
+      };
+
+    };
+
+  };
+
+  config = {
+
+    system.autoUpgrade.flags =
+      [ "--no-build-output" ]
+      ++ (if cfg.channel == null
+          then [ "--upgrade" ]
+          else [ "-I" "nixpkgs=${cfg.channel}/nixexprs.tar.xz" ]);
+
+    systemd.services.nixos-upgrade = {
+      description = "NixOS Upgrade";
+
+      restartIfChanged = false;
+      unitConfig.X-StopOnRemoval = false;
+
+      serviceConfig.Type = "oneshot";
+
+      environment = config.nix.envVars //
+        { inherit (config.environment.sessionVariables) NIX_PATH;
+          HOME = "/root";
+        } // config.networking.proxy.envVars;
+
+      path = [ pkgs.gnutar pkgs.xz.bin pkgs.gitMinimal config.nix.package.out ];
+
+      script = ''
+        ${lib.optionalString cfg.allowReboot ''
+          set -euo pipefail
+          set -x
+          T=$(mktemp -d)
+          cd "$T"
+          ${config.system.build.nixos-rebuild}/bin/nixos-rebuild build ${toString cfg.flags}
+          booted="$(${pkgs.coreutils}/bin/readlink /run/booted-system/{initrd,kernel,kernel-modules})"
+          built="$(${pkgs.coreutils}/bin/readlink result/{initrd,kernel,kernel-modules})"
+          cd /tmp
+          rm -rf "$T"
+          if [ "$booted" = "$built" ]; then
+            ${config.system.build.nixos-rebuild}/bin/nixos-rebuild switch ${toString cfg.flags}
+          else
+            ${config.system.build.nixos-rebuild}/bin/nixos-rebuild boot ${toString cfg.flags}
+            /run/current-system/sw/bin/shutdown -r +1
+          fi
+          ''}
+          ${lib.optionalString (!cfg.allowReboot) "${config.system.build.nixos-rebuild}/bin/nixos-rebuild switch ${toString cfg.flags}"}
+        '';
+
+      startAt = optional cfg.enable cfg.dates;
+    };
+
+  };
+
+}

configuration.nix

@@ -5,8 +5,10 @@
 { config, pkgs, ... }:
 
 {
+  disabledModules = [ "tasks/auto-upgrade.nix" ];
   imports =
     [
+      ./auto-upgrade.nix
       ./hardware-configuration.nix
       ./sshusers.nix
       ./variables.nix
@@ -51,6 +53,7 @@
 
   # Automatic update each day at 04:40. Will not restart the system, so a reboot every now and then is a good idea.
   system.autoUpgrade.enable = true;
+  system.autoUpgrade.allowReboot = true;
   nix = {
     autoOptimiseStore = true;
     gc.automatic = true;