nixos/services/minecraft.nix

# Configuration from https://dataswamp.org/~solene/2022-08-20-on-demand-minecraft-with-systemd.html
{
  config,
  lib,
  pkgs,
  modulesPath,
  ...
}: let
  # check every 15 minutes if the server
  # need to be stopped
  frequency-check-players = "*-*-* *:0/15";

  # time in second before we could stop the server
  # this should let it time to spawn
  minimum-server-lifetime = 300;

  # minecraft port
  # used in a few places in the code
  # this is not the port that should be used publicly
  # don't need to open it on the firewall
  minecraft-port = 25564;

  # this is the port that will trigger the server start
  # and the one that should be used by players
  # you need to open it in the firewall
  public-port = 25565;

  # a rcon password used by the local systemd commands
  # to get information about the server such as the
  # player list
  # this will be stored plaintext in the store
  rcon-password = "260a368f55f4fb4fa";

  # a script used by hook-minecraft.service
  # to start minecraft and the timer regularly
  # polling for stopping it
  start-mc = pkgs.writeShellScriptBin "start-mc" ''
    systemctl start minecraft-server.service
    systemctl start stop-minecraft.timer
  '';

  # wait 60s for a TCP socket to be available
  # to wait in the proxifier
  # idea found in https://blog.developer.atlassian.com/docker-systemd-socket-activation/
  wait-tcp = pkgs.writeShellScriptBin "wait-tcp" ''
    for i in `seq 60`; do
      if ${pkgs.libressl.nc}/bin/nc -z 127.0.0.1 ${toString minecraft-port} > /dev/null ; then
        exit 0
      fi
      ${pkgs.busybox.out}/bin/sleep 1
    done
    exit 1
  '';

  # script returning true if the server has to be shutdown
  # for minecraft, uses rcon to get the player list
  # skips the checks if the service started less than minimum-server-lifetime
  no-player-connected = pkgs.writeShellScriptBin "no-player-connected" ''
    servicestartsec=$(date -d "$(systemctl show --property=ActiveEnterTimestamp minecraft-server.service | cut -d= -f2)" +%s)
    serviceelapsedsec=$(( $(date +%s) - servicestartsec))

    # exit if the server started less than 5 minutes ago
    if [ $serviceelapsedsec -lt ${toString minimum-server-lifetime} ]
    then
      echo "server is too young to be stopped"
      exit 1
    fi

    PLAYERS=`printf "list\n" | ${pkgs.rcon.out}/bin/rcon -m -H 127.0.0.1 -p 25575 -P ${rcon-password}`
    if echo "$PLAYERS" | grep "are 0 of a"
    then
      exit 0
    else
      exit 1
    fi
  '';
in {
  networking.firewall = {
    allowedTCPPorts = [public-port];
    allowedUDPPorts = [public-port];
  };

  # use NixOS module to declare your Minecraft
  # rcon is mandatory for no-player-connected
  services.minecraft-server = {
    enable = true;
    eula = true;
    openFirewall = false;
    declarative = true;
    whitelist = {
      "Tux922" = "b1848d60-4ddd-4fa4-a328-fb269910b6ae";
      "HomeInTheClouds" = "d49f0aee-f217-477b-9516-9d5906f7fc87";
    };
    serverProperties = {
      server-port = minecraft-port;
      difficulty = "easy";
      gamemode = "survival";
      force-gamemode = true;
      max-players = 10;
      # level-seed = 238902389203;
      motd = "NixOS Minecraft server!";
      white-list = true;
      enable-rcon = true;
      "rcon.password" = rcon-password;
    };
  };

  # don't start Minecraft on startup
  systemd.services.minecraft-server = {
    wantedBy = pkgs.lib.mkForce [];
  };

  # this waits for incoming connection on public-port
  # and triggers listen-minecraft.service upon connection
  systemd.sockets.listen-minecraft = {
    enable = true;
    wantedBy = ["sockets.target"];
    requires = ["network.target"];
    listenStreams = ["${toString public-port}"];
  };

  # this is triggered by a connection on TCP port public-port
  # start hook-minecraft if not running yet and wait for it to return
  # then, proxify the TCP connection to the real Minecraft port on localhost
  systemd.services.listen-minecraft = {
    path = with pkgs; [systemd];
    enable = true;
    requires = ["hook-minecraft.service" "listen-minecraft.socket"];
    after = ["hook-minecraft.service" "listen-minecraft.socket"];
    serviceConfig.ExecStart = "${pkgs.systemd.out}/lib/systemd/systemd-socket-proxyd 127.0.0.1:${toString minecraft-port}";
  };

  # this starts Minecraft is required
  # and wait for it to be available over TCP
  # to unlock listen-minecraft.service proxy
  systemd.services.hook-minecraft = {
    path = with pkgs; [systemd libressl busybox];
    enable = true;
    serviceConfig = {
      ExecStartPost = "${wait-tcp.out}/bin/wait-tcp";
      ExecStart = "${start-mc.out}/bin/start-mc";
    };
  };

  # create a timer running every frequency-check-players
  # that runs stop-minecraft.service script on a regular
  # basis to check if the server needs to be stopped
  systemd.timers.stop-minecraft = {
    enable = true;
    timerConfig = {
      OnCalendar = "${frequency-check-players}";
      Unit = "stop-minecraft.service";
    };
    wantedBy = ["timers.target"];
  };

  # run the script no-player-connected
  # and if it returns true, stop the minecraft-server
  # but also the timer and the hook-minecraft service
  # to prepare a working state ready to resume the
  # server again
  systemd.services.stop-minecraft = {
    enable = true;
    serviceConfig.Type = "oneshot";
    script = ''
      if ${no-player-connected}/bin/no-player-connected
      then
        echo "stopping server"
        systemctl stop minecraft-server.service hook-minecraft.service stop-minecraft.timer
      fi
    '';
  };
}