lib/autoupdate: init, enable on mucbot

hosts/containers/mucbot/default.nix

@@ -1,6 +1,7 @@
 { zentralwerk, config, pkgs, lib, tigger, ... }:
 
 {
+  c3d2.autoUpdate = true;
   networking.hostName = "mucbot";
   networking.interfaces.eth0.ipv4.addresses = [{
     address = "172.20.73.27";

lib/autoupdate.nix

@@ -0,0 +1,53 @@
+{ config, lib, pkgs, ... }:
+
+{
+  options.c3d2.autoUpdate = with lib; mkOption {
+    description = ''
+      Enables a timer that periodically checks hydra.hq.c3d2.de for the last build of the local system, and switches to it if it is different.
+
+      Also enables periodical /nix/store GC.
+    '';
+    type = types.bool;
+    default = false;
+  };
+
+  config = lib.mkIf config.c3d2.autoUpdate {
+    systemd.services.autoupdate = {
+      wantedBy = [ "multi-user.target" ];
+      path = with pkgs; [ nixFlakes nettools curl jq ];
+      serviceConfig.Type = "oneshot";
+      script = ''
+        OLD=$(readlink /run/current-system)
+        NEW=$(curl -sLH "Accept: application/json" https://hydra.hq.c3d2.de/job/c3d2/nix-config/x86_64-linux.$(hostname)/latest | jq -r .buildoutputs.out.path)
+        if [ -z "$NEW" ]; then
+          echo "Unable to obtain updated system"
+          exit 1
+        fi
+
+        if [ "$OLD" != "$NEW" ]; then
+          echo "New system available: $NEW"
+          # this should fetch the new system from the binary cache
+          nix build --no-link "$NEW"
+          # switch to the new system
+          "$NEW/bin/switch-to-configuration" switch
+        else
+          echo "No update required"
+        fi
+      '';
+      # don't let the switch kill this service, aborting the switch
+      restartIfChanged = false;
+      unitConfig.X-StopOnRemoval = false;
+    };
+
+    systemd.timers.autoupdate = {
+      partOf = [ "autoupdate.service" ];
+      wantedBy = [ "timers.target" ];
+      timerConfig.OnCalendar = "hourly";
+    };
+
+    nix.gc = {
+      automatic = true;
+      randomizedDelaySec = "6h";
+    };
+  };
+}

lib/default.nix

@@ -34,6 +34,7 @@ in {
     ./pi-sensors.nix
     ./ceph-storage.nix
     ./cache.nix
+    ./autoupdate.nix
   ];
 
   options.c3d2 = with lib;