autostart and stop minecraft server

modules/gaming/minecraft-server.nix

@@ -1,4 +1,13 @@
-{ ... }: {
+{ pkgs, ... }:
+
+let
+
+  localPort = 25564;
+  publicPort = 25565;
+  rconPort = 25575;
+  rconPassword = "thiscanbeanything";
+
+in {
 
   unfreePackages = [ "minecraft-server" ];
 
@@ -7,9 +16,9 @@
     eula = true;
     declarative = true;
     whitelist = { };
-    openFirewall = true;
+    openFirewall = false;
     serverProperties = {
-      server-port = 25565;
+      server-port = localPort;
       difficulty = "normal";
       gamemode = "survival";
       white-list = false;
@@ -26,7 +35,112 @@
       spawn-monsters = true;
       allow-nether = true;
       allow-flight = false;
+      enable-rcon = true;
+      "rcon.port" = rconPort;
+      "rcon.password" = rconPassword;
     };
   };
 
+  networking.firewall.allowedTCPPorts = [ publicPort ];
+
+  ## Automatically start and stop Minecraft server based on player connections
+
+  # Adapted shamelessly from:
+  # https://dataswamp.org/~solene/2022-08-20-on-demand-minecraft-with-systemd.html
+
+  # Prevent Minecraft from starting by default
+  systemd.services.minecraft-server = { wantedBy = pkgs.lib.mkForce [ ]; };
+
+  # Listen for connections on the public port, to trigger the actual
+  # listen-minecraft service.
+  systemd.sockets.listen-minecraft = {
+    wantedBy = [ "sockets.target" ];
+    requires = [ "network.target" ];
+    listenStreams = [ "${toString publicPort}" ];
+  };
+
+  # Proxy traffic to local port, and trigger hook-minecraft
+  systemd.services.listen-minecraft = {
+    path = [ pkgs.systemd ];
+    requires = [ "hook-minecraft.service" "listen-minecraft.socket" ];
+    after = [ "hook-minecraft.service" "listen-minecraft.socket" ];
+    serviceConfig.ExecStart =
+      "${pkgs.systemd.out}/lib/systemd/systemd-socket-proxyd 127.0.0.1:${
+        toString localPort
+      }";
+  };
+
+  # Start Minecraft if required and wait for it to be available
+  # Then unlock the listen-minecraft.service
+  systemd.services.hook-minecraft = {
+    path = with pkgs; [ systemd libressl busybox ];
+
+    # Start Minecraft and the auto-shutdown timer
+    script = ''
+      systemctl start minecraft-server.service
+      systemctl start stop-minecraft.timer
+    '';
+
+    # Keep checking until the service is available
+    postStart = ''
+      for i in $(seq 60); do
+        if ${pkgs.libressl.nc}/bin/nc -z 127.0.0.1 ${
+          toString localPort
+        } > /dev/null ; then
+          exit 0
+        fi
+        ${pkgs.busybox.out}/bin/sleep 1
+      done
+      exit 1
+    '';
+  };
+
+  # Run a player check on a schedule for auto-shutdown
+  systemd.timers.stop-minecraft = {
+    timerConfig = {
+      OnCalendar = "*-*-* *:*:0/20"; # Every 20 seconds
+      Unit = "stop-minecraft.service";
+    };
+    wantedBy = [ "timers.target" ];
+  };
+
+  # If no players are connected, then stop services and prepare to resume again
+  systemd.services.stop-minecraft = {
+    serviceConfig.Type = "oneshot";
+    script = ''
+      # Check when service was launched
+      servicestartsec=$(
+        date -d \
+          "$(systemctl show \
+               --property=ActiveEnterTimestamp \
+               minecraft-server.service \
+               | cut -d= -f2)" \
+        +%s)
+
+      # Calculate elapsed time
+      serviceelapsedsec=$(( $(date +%s) - servicestartsec))
+
+      # Ignore if service just started
+      if [ $serviceelapsedsec -lt 180 ]
+      then
+        echo "Server was just started"
+        exit 0
+      fi
+
+      PLAYERS=$(
+        printf "list\n" \
+          | ${pkgs.rcon.out}/bin/rcon -m \
+          -H 127.0.0.1 -p ${builtins.toString rconPort} -P ${rconPassword} \
+      )
+
+      if echo "$PLAYERS" | grep "are 0 of a"
+      then
+        echo "Stopping server"
+        systemctl stop minecraft-server.service
+        systemctl stop hook-minecraft.service
+        systemctl stop stop-minecraft.timer
+      fi
+    '';
+  };
+
 }