{ config, pkgs, ... }:
let
  backup-source = "rootNas";
  # The "stay-awake" file is located at `${backup-source}:${stay-awake-file}`.
  stay-awake-file = "${config.networking.hostName}.stay-awake";
in
{
  # Suspend:
  #   sudo systemctl suspend
  # Suspend for 60 seconds:
  #   sudo rtcwake -m mem -s 60

  # Power consumption:
  #    2.4W powered off, with 1 RAM, 1 SSD
  #    2.6W suspended, with 1 RAM, 1 SSD
  #    2.6W suspended, with 1 RAM, 1 SSD, 2 HDDs
  #   18.9W idle, with 1 RAM, 1 SSD, 2 HDDs

  # journalctl -u daily-backup-and-suspend

  assertions = [{
    assertion = config.services.openssh.enable;
    message = "systemd service daily-backup-and-suspend requires SSH.";
  } {
    assertion = config.services.journalwatch.enable;
    message = "systemd service daily-backup-and-suspend requires journalwatch.";
  }];

  systemd.timers."daily-backup-and-suspend" = {
    wantedBy = [ "multi-user.target" ];
    timerConfig = {
      OnCalendar = [
        # Daily
        "*-*-* 00:00:05"
      ];
      WakeSystem = true;
    };
  };
  systemd.services."daily-backup-and-suspend" = {
    # Packages required for this script.
    # For `ssh` and `journalwatch`, there are assertions above.
    path = with pkgs; [
      # Provides `echo`, `sleep`, `printf`.
      coreutils
      # Provides `ssh`
      openssh
    ];
    # Script to execute as main process.
    script = ''
      set -eu -o pipefail
      printf '%s\n' 'Starting backup script.'

      # TODO: Backup: Pull BTRFS snapshots from ${backup-source}.

      # Don't suspend as long as `${backup-source}:${stay-awake-file}` exists.
      while :; do
        result="$(ssh ${backup-source} 'ls ${stay-awake-file} 2>&1')"
        case "$${result}" in
          *"No such file or directory")
            break
            ;;
          "${stay-awake-file}")
            printf '%s\n' 'Delaying suspend due to ${stay-awake-file} file.'
            ;;
          *)
            printf '%s\n' 'Delaying suspend due to SSH connectivity problems.'
            ;;
        esac
        sleep 10s
      done

      # Wait until no BTRFS scrub service is running.
      while systemctl list-units --type=service --plain --quiet | awk '{ print $1 }' | grep '^btrfs-scrub'; do
        printf '%s\n' 'Delaying suspend due to running BTRFS scrub service.'
        sleep 60s
      done

      # Send filtered journal by email.
      systemctl start journalwatch.service ||:
      # Short delay to let sendmail send the email.
      sleep 15s

      printf '%s\n' 'Finished backup script.'

      # Suspend to save power.
      # TODO
      #systemctl suspend
    '';
  };
}