Allow players to send messages to each other over plugin channels

src/main/java/pw/kaboom/extras/Main.java

@@ -2,10 +2,10 @@ package pw.kaboom.extras;
 
 import org.bukkit.WorldCreator;
 import org.bukkit.WorldType;
-import org.bukkit.block.BlockFace;
 import org.bukkit.configuration.file.FileConfiguration;
 import org.bukkit.configuration.file.YamlConfiguration;
 import org.bukkit.plugin.java.JavaPlugin;
+import org.bukkit.plugin.messaging.Messenger;
 import pw.kaboom.extras.commands.*;
 import pw.kaboom.extras.modules.block.BlockCheck;
 import pw.kaboom.extras.modules.block.BlockPhysics;
@@ -18,7 +18,6 @@ import pw.kaboom.extras.modules.server.ServerGameRule;
 import pw.kaboom.extras.modules.server.ServerTabComplete;
 
 import java.io.File;
-import java.util.Collections;
 
 public final class Main extends JavaPlugin {
     private File prefixConfigFile;
@@ -91,6 +90,15 @@ public final class Main extends JavaPlugin {
         this.getServer().createWorld(
             new WorldCreator("world_flatlands").generateStructures(false).type(WorldType.FLAT)
         );
+
+        final Messenger messenger = this.getServer().getMessenger();
+        final PlayerMessaging playerMessaging = new PlayerMessaging(this);
+
+        messenger.registerIncomingPluginChannel(this, PlayerMessaging.REGISTER, playerMessaging);
+        messenger.registerIncomingPluginChannel(this, PlayerMessaging.UNREGISTER, playerMessaging);
+
+        messenger.registerIncomingPluginChannel(this, PlayerMessaging.MESSAGE, playerMessaging);
+        messenger.registerOutgoingPluginChannel(this, PlayerMessaging.MESSAGE);
     }
 
 	public File getPrefixConfigFile() {

src/main/java/pw/kaboom/extras/modules/player/PlayerMessaging.java

@@ -0,0 +1,180 @@
+package pw.kaboom.extras.modules.player;
+
+import com.google.common.primitives.Longs;
+import it.unimi.dsi.fastutil.io.FastByteArrayInputStream;
+import net.kyori.adventure.text.Component;
+import net.kyori.adventure.text.format.NamedTextColor;
+import org.bukkit.entity.Player;
+import org.bukkit.plugin.messaging.Messenger;
+import org.bukkit.plugin.messaging.PluginMessageListener;
+import org.jetbrains.annotations.NotNull;
+import pw.kaboom.extras.Main;
+
+import java.io.*;
+import java.nio.charset.StandardCharsets;
+import java.util.*;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+
+public final class PlayerMessaging implements PluginMessageListener {
+    public static final String REGISTER = "extras:register";
+    public static final String UNREGISTER = "extras:unregister";
+    public static final String MESSAGE = "extras:message";
+
+    private static final Component ERROR =
+            Component.text("Could not send plugin channel message.", NamedTextColor.RED);
+    private static final ScheduledExecutorService SCHEDULED_EXECUTOR_SERVICE =
+            Executors.newSingleThreadScheduledExecutor();
+
+    private final Main plugin;
+
+    public PlayerMessaging(Main plugin) {
+        this.plugin = plugin;
+
+        SCHEDULED_EXECUTOR_SERVICE.scheduleAtFixedRate(() -> {
+            synchronized (this.listening) {
+                final Iterator<Map.Entry<String, Set<Player>>> iterator =
+                        this.listening.entrySet().iterator();
+
+                while (iterator.hasNext()) {
+                    final Map.Entry<String, Set<Player>> entry = iterator.next();
+
+                    final Set<Player> players = entry.getValue();
+                    synchronized (players) {
+                        // try and avoid issues with other plugins causing player obj leaks
+                        int onlineCount = 0;
+
+                        for (final Player player: players) {
+                            if (!player.isOnline()) continue;
+                            onlineCount++;
+                        }
+
+                        if (onlineCount != 0) continue;
+                        iterator.remove();
+                    }
+                }
+            }
+        }, 1, 1, TimeUnit.MINUTES);
+    }
+
+    private final Map<String, Set<Player>> listening = Collections.synchronizedMap(new HashMap<>());
+
+    private static String readString(DataInput dataInput) throws IOException {
+        final int len = dataInput.readUnsignedByte();
+        final byte[] buf = new byte[len];
+        dataInput.readFully(buf);
+        return new String(buf, StandardCharsets.US_ASCII);
+    }
+
+    private void handleRegister(final Player player, final DataInput input) throws IOException {
+        this.listening.compute(readString(input), (k, v) -> {
+            v = v == null ?
+                            Collections.synchronizedSet(
+                                    Collections.newSetFromMap(
+                                            new WeakHashMap<>()
+                                    )
+                            )
+                            :
+                            v;
+            v.add(player);
+            return v;
+        });
+    }
+
+    private void handleUnregister(final Player player, final DataInput input) throws IOException {
+        this.listening.computeIfPresent(readString(input), (k, v) -> {
+            v.remove(player);
+            return v;
+        });
+    }
+
+    private void handleMessage(final Player player,
+                               final DataInputStream input)
+            throws IOException {
+        final String channelName = readString(input);
+        final Set<Player> players = this.listening.get(channelName);
+        if (players == null) return;
+
+        synchronized (players) {
+            // we start at null so that we do not read the bytes if the
+            // only player in the channel is the sender of the message
+            byte[] msg = null;
+
+            for (final Player playerInSet : players) {
+                if (playerInSet == player) continue;
+                if (msg == null) {
+                    final int remaining = input.available();
+
+                    // remaining count + channel name + channel length header + uuid
+                    // note: calls to channelName.length() are safe because we only read ASCII
+                    final int realLength = remaining + channelName.length() + 17;
+                    if (realLength > Messenger.MAX_MESSAGE_SIZE) {
+                        player.sendMessage(ERROR);
+                        return;
+                    }
+
+                    msg = new byte[realLength];
+                    msg[0] = (byte) channelName.length();
+                    int offset = 1;
+
+                    System.arraycopy(
+                            channelName.getBytes(StandardCharsets.US_ASCII),
+                            0,
+                            msg,
+                            offset,
+                            channelName.length()
+                    );
+                    offset += channelName.length();
+
+                    final UUID uuid = player.getUniqueId();
+                    System.arraycopy(
+                            Longs.toByteArray(uuid.getMostSignificantBits()),
+                            0,
+                            msg,
+                            offset,
+                            8)
+                    ;
+                    offset += 8;
+
+                    System.arraycopy(
+                            Longs.toByteArray(uuid.getLeastSignificantBits()),
+                            0,
+                            msg,
+                            offset,
+                            8
+                    );
+                    offset += 8;
+
+                    input.readFully(msg, offset, remaining);
+                }
+
+                playerInSet.sendPluginMessage(this.plugin, MESSAGE, msg);
+            }
+        }
+    }
+
+    @Override
+    public void onPluginMessageReceived(final @NotNull String channelName,
+                                        final @NotNull Player player,
+                                        final byte[] bytes) {
+        try {
+            switch (channelName) {
+                case REGISTER -> handleRegister(
+                        player,
+                        new DataInputStream(new FastByteArrayInputStream(bytes))
+                );
+                case UNREGISTER -> handleUnregister(
+                        player,
+                        new DataInputStream(new FastByteArrayInputStream(bytes))
+                );
+                case MESSAGE -> handleMessage(
+                        player,
+                        new DataInputStream(new FastByteArrayInputStream(bytes))
+                );
+            }
+        } catch (Exception ignored) {
+            player.sendMessage(ERROR);
+        }
+    }
+}