Implement OpenToLAN system

src/main/java/net/minestom/server/event/server/ServerListPingEvent.java

@@ -39,7 +39,7 @@ public class ServerListPingEvent extends Event implements CancellableEvent {
      * @param version the ping version to respond with
      */
     public ServerListPingEvent(@Nullable PlayerConnection connection, @NotNull ServerListPingVersion version) {
-        //noinspection deprecation we need to continue doing this until the consumer is removed
+        //noinspection deprecation we need to continue doing this until the consumer is removed - todo remove
         ResponseDataConsumer consumer = MinecraftServer.getResponseDataConsumer();
         this.responseData = new ResponseData();
 
@@ -97,7 +97,8 @@ public class ServerListPingEvent extends Event implements CancellableEvent {
     }
 
     /**
-     * Cancelling this event will cause you server to appear offline in the vanilla server list.
+     * Cancelling this event will cause the server to appear offline in the vanilla server list.
+     * Note that this will have no effect if the ping version is {@link ServerListPingVersion#OPEN_TO_LAN}.
      *
      * @param cancel true if the event should be cancelled, false otherwise
      */

src/main/java/net/minestom/server/extras/lan/OpenToLAN.java

@@ -0,0 +1,135 @@
+package net.minestom.server.extras.lan;
+
+import net.minestom.server.MinecraftServer;
+import net.minestom.server.event.server.ServerListPingEvent;
+import net.minestom.server.timer.Task;
+import net.minestom.server.utils.time.Cooldown;
+import org.jetbrains.annotations.NotNull;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.net.*;
+import java.nio.charset.StandardCharsets;
+
+import static net.minestom.server.ping.ServerListPingVersion.OPEN_TO_LAN;
+
+/**
+ * Utility class to manage opening the server to LAN. Note that this <b>doesn't</b> actually
+ * open your server to LAN if it isn't already visible to anyone on your local network.
+ * Instead it simply sends the packets needed to trick the Minecraft client into thinking
+ * that this is a single-player world that has been opened to LANfor it to be displayed on
+ * the bottom of the server list.
+ * @see <a href="https://wiki.vg/Server_List_Ping#Ping_via_LAN_.28Open_to_LAN_in_Singleplayer.29">wiki.vg</a>
+ */
+public class OpenToLAN {
+    private static final InetSocketAddress PING_ADDRESS = new InetSocketAddress("224.0.2.60", 4445);
+
+    private static final Logger LOGGER = LoggerFactory.getLogger(OpenToLAN.class);
+
+    private static volatile Cooldown eventCooldown;
+    private static volatile DatagramSocket socket = null;
+    private static volatile DatagramPacket packet = null;
+    private static volatile Task task = null;
+
+    private OpenToLAN() { }
+
+    /**
+     * Opens the server to LAN with the default config.
+     *
+     * @return {@code true} if it was open successfully, {@code false} otherwise
+     */
+    public static boolean open() {
+        return open(new OpenToLANConfig());
+    }
+
+    /**
+     * Opens the server to LAN.
+     *
+     * @param config the configuration
+     * @return {@code true} if it was open successfully, {@code false} otherwise
+     */
+    public static boolean open(@NotNull OpenToLANConfig config) {
+        if (socket != null) {
+            return false;
+        } else {
+            int port = config.port;
+
+            if (port == 0) {
+                try {
+                    final ServerSocket socket = new ServerSocket(0);
+                    port = socket.getLocalPort();
+                    socket.close();
+                } catch (IOException e) {
+                    LOGGER.warn("Could not find an open port!", e);
+                    return false;
+                }
+            }
+
+            try {
+                socket = new DatagramSocket(port);
+            } catch (SocketException e) {
+                LOGGER.warn("Could not bind to the port!", e);
+                return false;
+            }
+
+            eventCooldown = new Cooldown(config.delayBetweenEvent);
+            task = MinecraftServer.getSchedulerManager().buildTask(OpenToLAN::ping)
+                    .repeat(config.delayBetweenPings.getValue(), config.delayBetweenPings.getTimeUnit())
+                    .schedule();
+            return true;
+        }
+    }
+
+    /**
+     * Closes the server to LAN.
+     *
+     * @return {@code true} if it was closed, {@code false} if it was already closed
+     */
+    public static boolean close() {
+        if (socket == null) {
+            return false;
+        } else {
+            task.cancel();
+            socket.close();
+
+            task = null;
+            socket = null;
+
+            return true;
+        }
+    }
+
+    /**
+     * Checks if the server is currently opened to LAN.
+     *
+     * @return {@code true} if it is, {@code false} otherwise
+     */
+    public static boolean isOpen() {
+        return socket != null;
+    }
+
+    /**
+     * Performs the ping.
+     */
+    private static void ping() {
+        if (MinecraftServer.getNettyServer().getPort() != 0) {
+            if (packet == null || eventCooldown.isReady(System.currentTimeMillis())) {
+                final ServerListPingEvent event = new ServerListPingEvent(OPEN_TO_LAN);
+                MinecraftServer.getGlobalEventHandler().callEvent(ServerListPingEvent.class, event);
+
+                final byte[] data = OPEN_TO_LAN.getPingResponse(event.getResponseData()).getBytes(StandardCharsets.UTF_8);
+                packet = new DatagramPacket(data, data.length, PING_ADDRESS);
+
+                eventCooldown.refreshLastUpdate(System.currentTimeMillis());
+            }
+
+
+            try {
+                socket.send(packet);
+            } catch (IOException e) {
+                LOGGER.warn("Could not send Open to LAN packet!", e);
+            }
+        }
+    }
+}

src/main/java/net/minestom/server/extras/lan/OpenToLANConfig.java

@@ -0,0 +1,64 @@
+package net.minestom.server.extras.lan;
+
+import java.util.Objects;
+
+import net.minestom.server.event.server.ServerListPingEvent;
+import net.minestom.server.utils.time.TimeUnit;
+import net.minestom.server.utils.time.UpdateOption;
+import org.jetbrains.annotations.Contract;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Configuration for opening the server to LAN.
+ * @see OpenToLAN#open(OpenToLANConfig)
+ */
+public class OpenToLANConfig {
+    int port;
+    UpdateOption delayBetweenPings, delayBetweenEvent;
+
+    /**
+     * Creates a new config with the port set to random and the delay between pings set
+     * to 1.5 seconds and the delay between event calls set to 30 seconds.
+     */
+    public OpenToLANConfig() {
+        this.port = 0;
+        this.delayBetweenPings = new UpdateOption(1500, TimeUnit.MILLISECOND);
+        this.delayBetweenEvent = new UpdateOption(30, TimeUnit.SECOND);
+    }
+
+    /**
+     * Sets the port used to send pings from. Use {@code 0} to pick a random free port.
+     *
+     * @param port the port
+     * @return {@code this}, for chaining
+     */
+    @Contract("_ -> this")
+    public @NotNull OpenToLANConfig setPort(int port) {
+        this.port = port;
+        return this;
+    }
+
+    /**
+     * Sets the delay between outgoing pings.
+     *
+     * @param delay the delay
+     * @return {@code this}, for chaining
+     */
+    @Contract("_ -> this")
+    public @NotNull OpenToLANConfig setDelayBetweenPings(@NotNull UpdateOption delay) {
+        this.delayBetweenPings = Objects.requireNonNull(delay, "delay");
+        return this;
+    }
+
+    /**
+     * Sets the delay between calls of {@link ServerListPingEvent}.
+     *
+     * @param delay the delay
+     * @return {@code this}, for chaining
+     */
+    @Contract("_ -> this")
+    public @NotNull OpenToLANConfig setDelayBetweenEventCalls(@NotNull UpdateOption delay) {
+        this.delayBetweenEvent = Objects.requireNonNull(delay, "delay");
+        return this;
+    }
+}

src/main/java/net/minestom/server/ping/ServerListPingVersion.java

@@ -5,6 +5,7 @@ import com.google.gson.JsonObject;
 import net.kyori.adventure.text.serializer.gson.GsonComponentSerializer;
 import net.kyori.adventure.text.serializer.legacy.LegacyComponentSerializer;
 import net.minestom.server.MinecraftServer;
+import net.minestom.server.extras.lan.OpenToLAN;
 import net.minestom.server.utils.identity.NamedAndIdentified;
 import org.jetbrains.annotations.NotNull;
 
@@ -34,7 +35,14 @@ public enum ServerListPingVersion {
     /**
      * The client is on version 1.5 or lower and supports a description and the player count.
      */
-    LEGACY(data -> getLegacyPingResponse(data, false));
+    LEGACY(data -> getLegacyPingResponse(data, false)),
+
+    /**
+     * The ping that is sent when {@link OpenToLAN} is enabled and sending packets.
+     * Only the description formatted as a legacy string is sent.
+     * Ping events with this ping version are <b>not</b> cancellable.
+     */
+    OPEN_TO_LAN(ServerListPingVersion::getOpenToLANPing);
 
     private final Function<ResponseData, String> pingResponseCreator;
 
@@ -52,10 +60,22 @@ public enum ServerListPingVersion {
         return this.pingResponseCreator.apply(responseData);
     }
 
+    private static final String LAN_PING_FORMAT = "[MOTD]%s[/MOTD][AD]%s[/AD]";
     private static final GsonComponentSerializer FULL_RGB = GsonComponentSerializer.gson(),
             NAMED_RGB = GsonComponentSerializer.colorDownsamplingGson();
     private static final LegacyComponentSerializer SECTION = LegacyComponentSerializer.legacySection();
 
+    /**
+     * Creates a ping sent when the server is sending {@link OpenToLAN} packets.
+     *
+     * @param data the response data
+     * @return the ping
+     * @see OpenToLAN
+     */
+    public static @NotNull String getOpenToLANPing(@NotNull ResponseData data) {
+        return String.format(LAN_PING_FORMAT, SECTION.serialize(data.getDescription()), MinecraftServer.getNettyServer().getPort());
+    }
+
     /**
      * Creates a legacy ping response for client versions below the Netty rewrite (1.6-).
      *

src/test/java/demo/Main.java

@@ -11,6 +11,8 @@ import net.kyori.adventure.text.format.TextDecoration;
 import net.minestom.server.MinecraftServer;
 import net.minestom.server.command.CommandManager;
 import net.minestom.server.event.server.ServerListPingEvent;
+import net.minestom.server.extras.lan.OpenToLAN;
+import net.minestom.server.extras.lan.OpenToLANConfig;
 import net.minestom.server.extras.optifine.OptifineSupport;
 import net.minestom.server.instance.block.BlockManager;
 import net.minestom.server.instance.block.rule.vanilla.RedstonePlacementRule;
@@ -103,6 +105,9 @@ public class Main {
 
         //MojangAuth.init();
 
+        // useful for testing - we don't need to worry about event calls so just set this to a long time
+        OpenToLAN.open(new OpenToLANConfig().setDelayBetweenEventCalls(new UpdateOption(1, TimeUnit.DAY)));
+
         minecraftServer.start("0.0.0.0", 25565);
         //Runtime.getRuntime().addShutdownHook(new Thread(MinecraftServer::stopCleanly));
     }