gistfile1.txt

{ pkgs, ... }:
{
  systemd.services.auto-upgrade = {
    serviceConfig.Type = "oneshot";
    startAt = "hourly";

    # This is required as otherwise the call to "switch" will stop this very
    # unit, preventing the upgrade from completing.
    unitConfig.X-StopOnRemoval = false;

    script = ''
      # Fetch the name of the latest output
      OUT=$( curl --location --silent 'http://hydra/job/<name>/'"$(< /etc/tracking)"'/latest/api/get-info' | jq -r .outPath )

      # Realise the output and make it valid. This will fetch the output from
      # Hydra.
      nix-store --realise --ignore-unknown "$OUT"

      # Run "switch" to activate the configuration. If the fetch failed, this
      # file won't exist and the unit will fail.
      "$OUT/bin/switch-to-configuration" boot

      # Add a GC root.
      nix-env --profile /nix/var/nix/profiles/system --set "$OUT"
    '';

    path = with pkgs; [ curl jq nix ];
  };
}