Fix players disconnecting when updating boss bars

proxy/src/main/java/com/velocitypowered/proxy/adventure/VelocityBossBarImplementation.java

@@ -53,15 +53,23 @@ public boolean viewerAdd(final ConnectedPlayer viewer) {
           viewer.getProtocolVersion(),
           viewer.translateMessage(this.bar.name())
       );
-      viewer.getConnection().write(BossBarPacket.createAddPacket(this.id, this.bar, name));
+      viewer.getBossBarManager().writeUpdate(this, BossBarPacket.createAddPacket(this.id, this.bar, name));
       return true;
     }
     return false;
   }
 
+  public void createDirect(final ConnectedPlayer viewer) {
+    final ComponentHolder name = new ComponentHolder(
+        viewer.getProtocolVersion(),
+        viewer.translateMessage(this.bar.name())
+    );
+    viewer.getConnection().write(BossBarPacket.createAddPacket(this.id, this.bar, name));
+  }
+
   public boolean viewerRemove(final ConnectedPlayer viewer) {
     if (this.viewers.remove(viewer)) {
-      viewer.getConnection().write(BossBarPacket.createRemovePacket(this.id, this.bar));
+      viewer.getBossBarManager().remove(this, BossBarPacket.createRemovePacket(this.id, this.bar));
       return true;
     }
     return false;
@@ -84,7 +92,7 @@ public void bossBarNameChanged(
           this.bar,
           new ComponentHolder(viewer.getProtocolVersion(), translated)
       );
-      viewer.getConnection().write(packet);
+      viewer.getBossBarManager().writeUpdate(this, packet);
     }
   }
 
@@ -96,7 +104,7 @@ public void bossBarProgressChanged(
   ) {
     final BossBarPacket packet = BossBarPacket.createUpdateProgressPacket(this.id, this.bar);
     for (final ConnectedPlayer viewer : this.viewers) {
-      viewer.getConnection().write(packet);
+      viewer.getBossBarManager().writeUpdate(this, packet);
     }
   }
 
@@ -108,7 +116,7 @@ public void bossBarColorChanged(
   ) {
     final BossBarPacket packet = BossBarPacket.createUpdateStylePacket(this.id, this.bar);
     for (final ConnectedPlayer viewer : this.viewers) {
-      viewer.getConnection().write(packet);
+      viewer.getBossBarManager().writeUpdate(this, packet);
     }
   }
 
@@ -120,7 +128,7 @@ public void bossBarOverlayChanged(
   ) {
     final BossBarPacket packet = BossBarPacket.createUpdateStylePacket(this.id, this.bar);
     for (final ConnectedPlayer viewer : this.viewers) {
-      viewer.getConnection().write(packet);
+      viewer.getBossBarManager().writeUpdate(this, packet);
     }
   }
 
@@ -132,7 +140,7 @@ public void bossBarFlagsChanged(
   ) {
     final BossBarPacket packet = BossBarPacket.createUpdatePropertiesPacket(this.id, this.bar);
     for (final ConnectedPlayer viewer : this.viewers) {
-      viewer.getConnection().write(packet);
+      viewer.getBossBarManager().writeUpdate(this, packet);
     }
   }
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -177,10 +177,12 @@ public boolean handle(DisconnectPacket packet) {
 
   @Override
   public boolean handle(BossBarPacket packet) {
-    if (packet.getAction() == BossBarPacket.ADD) {
-      playerSessionHandler.getServerBossBars().add(packet.getUuid());
-    } else if (packet.getAction() == BossBarPacket.REMOVE) {
-      playerSessionHandler.getServerBossBars().remove(packet.getUuid());
+    if (serverConn.getPlayer().getProtocolVersion().lessThan(ProtocolVersion.MINECRAFT_1_20_2)) {
+      if (packet.getAction() == BossBarPacket.ADD) {
+        playerSessionHandler.getServerBossBars().add(packet.getUuid());
+      } else if (packet.getAction() == BossBarPacket.REMOVE) {
+        playerSessionHandler.getServerBossBars().remove(packet.getUuid());
+      }
     }
     return false; // forward
   }
@@ -516,4 +518,4 @@ public void writabilityChanged() {
 
     playerConnection.setAutoReading(writable);
   }
-}
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -535,9 +535,13 @@ public CompletableFuture<Void> doSwitch() {
 
       // Config state clears everything in the client. No need to clear later.
       spawned = false;
-      serverBossBars.clear();
       player.clearPlayerListHeaderAndFooterSilent();
       player.getTabList().clearAllSilent();
+      if (player.getProtocolVersion().noLessThan(ProtocolVersion.MINECRAFT_1_20_2)) {
+        player.getBossBarManager().dropPackets();
+      } else {
+        serverBossBars.clear();
+      }
     }
 
     player.switchToConfigState();
@@ -576,16 +580,19 @@ public void handleBackendJoinGame(JoinGamePacket joinGame, VelocityServerConnect
     }
 
     destination.setEntityId(joinGame.getEntityId()); // used for sound api
-
-    // Remove previous boss bars. These don't get cleared when sending JoinGame, thus the need to
-    // track them.
-    for (UUID serverBossBar : serverBossBars) {
-      BossBarPacket deletePacket = new BossBarPacket();
-      deletePacket.setUuid(serverBossBar);
-      deletePacket.setAction(BossBarPacket.REMOVE);
-      player.getConnection().delayedWrite(deletePacket);
+    if (player.getProtocolVersion().noLessThan(ProtocolVersion.MINECRAFT_1_20_2)) {
+      player.getBossBarManager().sendBossBars();
+    } else {
+      // Remove previous boss bars. These don't get cleared when sending JoinGame (up until 1.20.2),
+      // thus the need to track them.
+      for (UUID serverBossBar : serverBossBars) {
+        BossBarPacket deletePacket = new BossBarPacket();
+        deletePacket.setUuid(serverBossBar);
+        deletePacket.setAction(BossBarPacket.REMOVE);
+        player.getConnection().delayedWrite(deletePacket);
+      }
+      serverBossBars.clear();
     }
-    serverBossBars.clear();
 
     // Tell the server about the proxy's plugin message channels.
     ProtocolVersion serverVersion = serverMc.getProtocolVersion();

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -62,6 +62,7 @@
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftConnectionAssociation;
 import com.velocitypowered.proxy.connection.backend.VelocityServerConnection;
+import com.velocitypowered.proxy.connection.player.bossbar.BossBarManager;
 import com.velocitypowered.proxy.connection.player.bundle.BundleDelimiterHandler;
 import com.velocitypowered.proxy.connection.player.resourcepack.VelocityResourcePackInfo;
 import com.velocitypowered.proxy.connection.player.resourcepack.handler.ResourcePackHandler;
@@ -201,6 +202,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player,
   private @Nullable ClientSettingsPacket clientSettingsPacket;
   private volatile ChatQueue chatQueue;
   private final ChatBuilderFactory chatBuilderFactory;
+  private final BossBarManager bossBarManager;
 
   ConnectedPlayer(VelocityServer server, GameProfile profile, MinecraftConnection connection,
                   @Nullable InetSocketAddress virtualHost, @Nullable String rawVirtualHost, boolean onlineMode,
@@ -227,6 +229,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player,
     this.chatQueue = new ChatQueue(this);
     this.chatBuilderFactory = new ChatBuilderFactory(this.getProtocolVersion());
     this.resourcePackHandler = ResourcePackHandler.create(this, server);
+    this.bossBarManager = new BossBarManager(this);
   }
 
   /**
@@ -1431,6 +1434,10 @@ public HandshakeIntent getHandshakeIntent() {
     return handshakeIntent;
   }
 
+  public BossBarManager getBossBarManager() {
+    return bossBarManager;
+  }
+
   private final class ConnectionRequestBuilderImpl implements ConnectionRequestBuilder {
 
     private final RegisteredServer toConnect;

proxy/src/main/java/com/velocitypowered/proxy/connection/player/bossbar/BossBarManager.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright (C) 2019-2023 Velocity Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.velocitypowered.proxy.connection.player.bossbar;
+
+import com.velocitypowered.proxy.adventure.VelocityBossBarImplementation;
+import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
+import com.velocitypowered.proxy.protocol.packet.BossBarPacket;
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * Handles dropping and resending boss bar packets on versions 1.20.2 and newer because the client now
+ * deletes all boss bars during the login phase, and sending update packets would cause the client to be disconnected.
+ */
+public final class BossBarManager {
+
+  private final ConnectedPlayer player;
+  private final Set<VelocityBossBarImplementation> bossBars = new HashSet<>();
+
+  private boolean dropPackets = false;
+
+  public BossBarManager(ConnectedPlayer player) {
+    this.player = player;
+  }
+
+  /**
+   * Records the specified boss bar to be re-sent when a player changes server, and sends the update packet
+   * if the client is able to receive it and not be disconnected.
+   */
+  public synchronized void writeUpdate(VelocityBossBarImplementation bar, BossBarPacket packet) {
+    this.bossBars.add(bar);
+    if (!this.dropPackets) {
+      this.player.getConnection().write(packet);
+    }
+  }
+
+  /**
+   * Removes the specified boss bar from the player to ensure it is not re-sent.
+   */
+  public synchronized void remove(VelocityBossBarImplementation bar, BossBarPacket packet) {
+    this.bossBars.remove(bar);
+    if (!this.dropPackets) {
+      this.player.getConnection().write(packet);
+    }
+  }
+
+  /**
+   * Re-creates the boss bars the player can see with any updates that may have occurred in the meantime,
+   * and allows update packets for those boss bars to be sent.
+   */
+  public synchronized void sendBossBars() {
+    for (VelocityBossBarImplementation bossBar : bossBars) {
+      bossBar.createDirect(player);
+    }
+    this.dropPackets = false;
+  }
+
+  /**
+   * Prevents the player from receiving boss bar update packets while logging in to a new server.
+   */
+  public synchronized void dropPackets() {
+    this.dropPackets = true;
+  }
+}