amazon-init.nix

{ config, pkgs, ... }:

let
  script = ''
    #!${pkgs.runtimeShell} -eu

    echo "attempting to fetch configuration from EC2 user data..."

    export HOME=/root
    export PATH=${pkgs.lib.makeBinPath [ config.nix.package pkgs.systemd pkgs.gnugrep pkgs.gnused config.system.build.nixos-rebuild]}:$PATH
    export NIX_PATH=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels

    userData=/etc/ec2-metadata/user-data

    if [ -s "$userData" ]; then
      # If the user-data looks like it could be a nix expression,
      # copy it over. Also, look for a magic three-hash comment and set
      # that as the channel.
      if sed '/^\(#\|SSH_HOST_.*\)/d' < "$userData" | grep -q '\S'; then
        channels="$(grep '^###' "$userData" | sed 's|###\s*||')"
        printf "%s" "$channels" | while read channel; do
          echo "writing channel: $channel"
        done

        if [[ -n "$channels" ]]; then
          printf "%s" "$channels" > /root/.nix-channels
          nix-channel --update
        fi

        echo "setting configuration from EC2 user data"
        cp "$userData" /etc/nixos/configuration.nix
      else
        echo "user data does not appear to be a Nix expression; ignoring"
        exit
      fi
    else
      echo "no user data is available"
      exit
    fi

    nixos-rebuild switch
  '';
in {
  systemd.services.amazon-init = {
    inherit script;
    description = "Reconfigure the system from EC2 userdata on startup";

    wantedBy = [ "multi-user.target" ];
    after = [ "multi-user.target" ];
    requires = [ "network-online.target" ];
 
    restartIfChanged = false;
    unitConfig.X-StopOnRemoval = false;

    serviceConfig = {
      Type = "oneshot";
      RemainAfterExit = true;
    };
  };
}