Very basic multi-server switching functionality

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

@@ -33,11 +33,17 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
         }
 
         if (packet instanceof ServerLoginSuccess) {
-            // the player has been logged on.
+            // The player has been logged on to the backend server.
             connection.getChannel().setState(StateRegistry.PLAY);
-            connection.getProxyPlayer().setConnectedServer(connection);
+            if (connection.getProxyPlayer().getConnectedServer() == null) {
-            connection.getProxyPlayer().getConnection().setSessionHandler(new com.velocitypowered.proxy.connection.client.PlaySessionHandler(connection.getProxyPlayer()));
+                // Strap on the play session handler
+                connection.getProxyPlayer().getConnection().setSessionHandler(new com.velocitypowered.proxy.connection.client.PlaySessionHandler(connection.getProxyPlayer()));
+            } else {
+                // The previous server connection should become obsolete.
+                connection.getProxyPlayer().getConnectedServer().disconnect();
+            }
             connection.getChannel().setSessionHandler(new PlaySessionHandler(connection));
+            connection.getProxyPlayer().setConnectedServer(connection);
         }
     }
 

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

@@ -2,6 +2,7 @@ package com.velocitypowered.proxy.connection.backend;
 
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.packets.Disconnect;
+import com.velocitypowered.proxy.protocol.packets.JoinGame;
 import com.velocitypowered.proxy.protocol.packets.Ping;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import io.netty.buffer.ByteBuf;
@@ -31,6 +32,10 @@ public class PlaySessionHandler implements MinecraftSessionHandler {
                     .append(ComponentSerializers.JSON.deserialize(original.getReason()))
                     .build();
             connection.getProxyPlayer().close(reason);
+        } else if (packet instanceof JoinGame) {
+            com.velocitypowered.proxy.connection.client.PlaySessionHandler playerHandler =
+                    (com.velocitypowered.proxy.connection.client.PlaySessionHandler) connection.getProxyPlayer().getConnection().getSessionHandler();
+            playerHandler.handleBackendJoinGame((JoinGame) packet);
         } else {
             // Just forward the packet on. We don't have anything to handle at this time.
             connection.getProxyPlayer().getConnection().write(packet);

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

@@ -1,11 +1,18 @@
 package com.velocitypowered.proxy.connection.client;
 
+import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.proxy.connection.backend.ServerConnection;
+import com.velocitypowered.proxy.data.ServerInfo;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.packets.Chat;
+import com.velocitypowered.proxy.protocol.packets.JoinGame;
 import com.velocitypowered.proxy.protocol.packets.Ping;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
+import com.velocitypowered.proxy.protocol.packets.Respawn;
 import io.netty.buffer.ByteBuf;
 import io.netty.channel.EventLoop;
 
+import java.net.InetSocketAddress;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.ThreadLocalRandom;
 import java.util.concurrent.TimeUnit;
@@ -14,6 +21,8 @@ public class PlaySessionHandler implements MinecraftSessionHandler {
     private final ConnectedPlayer player;
     private ScheduledFuture<?> pingTask;
     private long lastPing = -1;
+    private boolean spawned = false;
+    private int currentDimension;
 
     public PlaySessionHandler(ConnectedPlayer player) {
         this.player = player;
@@ -38,12 +47,22 @@ public class PlaySessionHandler implements MinecraftSessionHandler {
         if (packet instanceof Ping) {
             Ping ping = (Ping) packet;
             if (ping.getRandomId() != lastPing) {
-                throw new IllegalStateException("Client sent invalid ping; expected " + lastPing + ", got " + ping.getRandomId());
+//                throw new IllegalStateException("Client sent invalid ping; expected " + lastPing + ", got " + ping.getRandomId());
             }
 
             // Do not forward the packet to the player's server, because we handle pings for all servers already.
             return;
         }
+
+        if (packet instanceof Chat) {
+            Chat chat = (Chat) packet;
+            if (chat.getMessage().equals("/connect")) {
+                ServerInfo info = new ServerInfo("test", new InetSocketAddress("localhost", 25566));
+                ServerConnection connection = new ServerConnection(info, player, VelocityServer.getServer());
+                connection.connect();
+            }
+        }
+
         // If we don't want to handle this packet, just forward it on.
         player.getConnectedServer().getChannel().write(packet);
     }
@@ -62,4 +81,25 @@ public class PlaySessionHandler implements MinecraftSessionHandler {
             pingTask = null;
         }
     }
+
+    public void handleBackendJoinGame(JoinGame joinGame) {
+        if (!spawned) {
+            // nothing special to do here
+            spawned = true;
+            currentDimension = joinGame.getDimension();
+            player.getConnection().write(joinGame);
+        } else {
+            // In order to handle switching to another server we will need send three packets:
+            // - The join game packet
+            // - A respawn packet, with a different dimension, if it differs
+            // - Another respawn with the correct dimension
+            player.getConnection().write(joinGame);
+            if (joinGame.getDimension() == currentDimension) {
+                int tempDim = joinGame.getDimension() == 0 ? -1 : 0;
+                player.getConnection().write(new Respawn(tempDim, joinGame.getDifficulty(), joinGame.getGamemode(), joinGame.getLevelType()));
+            }
+            player.getConnection().write(new Respawn(joinGame.getDimension(), joinGame.getDifficulty(), joinGame.getGamemode(), joinGame.getLevelType()));
+            currentDimension = joinGame.getDimension();
+        }
+    }
 }

src/main/java/com/velocitypowered/proxy/protocol/StateRegistry.java

@@ -47,6 +47,10 @@ public enum StateRegistry {
                     map(0x1A, MINECRAFT_1_12));
             TO_CLIENT.register(Ping.class, Ping::new,
                     map(0x1F, MINECRAFT_1_12));
+            TO_CLIENT.register(JoinGame.class, JoinGame::new,
+                    map(0x23, MINECRAFT_1_12));
+            TO_CLIENT.register(Respawn.class, Respawn::new,
+                    map(0x35, MINECRAFT_1_12));
         }
     },
     LOGIN {

src/main/java/com/velocitypowered/proxy/protocol/packets/JoinGame.java

@@ -0,0 +1,107 @@
+package com.velocitypowered.proxy.protocol.packets;
+
+import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import io.netty.buffer.ByteBuf;
+
+public class JoinGame implements MinecraftPacket {
+    private int entityId;
+    private short gamemode;
+    private int dimension;
+    private short difficulty;
+    private short maxPlayers;
+    private String levelType;
+    private boolean reducedDebugInfo;
+
+    public int getEntityId() {
+        return entityId;
+    }
+
+    public void setEntityId(int entityId) {
+        this.entityId = entityId;
+    }
+
+    public short getGamemode() {
+        return gamemode;
+    }
+
+    public void setGamemode(short gamemode) {
+        this.gamemode = gamemode;
+    }
+
+    public int getDimension() {
+        return dimension;
+    }
+
+    public void setDimension(int dimension) {
+        this.dimension = dimension;
+    }
+
+    public short getDifficulty() {
+        return difficulty;
+    }
+
+    public void setDifficulty(short difficulty) {
+        this.difficulty = difficulty;
+    }
+
+    public short getMaxPlayers() {
+        return maxPlayers;
+    }
+
+    public void setMaxPlayers(short maxPlayers) {
+        this.maxPlayers = maxPlayers;
+    }
+
+    public String getLevelType() {
+        return levelType;
+    }
+
+    public void setLevelType(String levelType) {
+        this.levelType = levelType;
+    }
+
+    public boolean isReducedDebugInfo() {
+        return reducedDebugInfo;
+    }
+
+    public void setReducedDebugInfo(boolean reducedDebugInfo) {
+        this.reducedDebugInfo = reducedDebugInfo;
+    }
+
+    @Override
+    public String toString() {
+        return "JoinGame{" +
+                "entityId=" + entityId +
+                ", gamemode=" + gamemode +
+                ", dimension=" + dimension +
+                ", difficulty=" + difficulty +
+                ", maxPlayers=" + maxPlayers +
+                ", levelType='" + levelType + '\'' +
+                ", reducedDebugInfo=" + reducedDebugInfo +
+                '}';
+    }
+
+    @Override
+    public void decode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        this.entityId = buf.readInt();
+        this.gamemode = buf.readUnsignedByte();
+        this.dimension = buf.readInt();
+        this.difficulty = buf.readUnsignedByte();
+        this.maxPlayers = buf.readUnsignedByte();
+        this.levelType = ProtocolUtils.readString(buf, 16);
+        this.reducedDebugInfo = buf.readBoolean();
+    }
+
+    @Override
+    public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        buf.writeInt(entityId);
+        buf.writeByte(gamemode);
+        buf.writeInt(dimension);
+        buf.writeByte(difficulty);
+        buf.writeByte(maxPlayers);
+        ProtocolUtils.writeString(buf, levelType);
+        buf.writeBoolean(reducedDebugInfo);
+    }
+}

src/main/java/com/velocitypowered/proxy/protocol/packets/Respawn.java

@@ -0,0 +1,81 @@
+package com.velocitypowered.proxy.protocol.packets;
+
+import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import io.netty.buffer.ByteBuf;
+
+public class Respawn implements MinecraftPacket {
+    private int dimension;
+    private short difficulty;
+    private short gamemode;
+    private String levelType;
+
+    public Respawn() {
+    }
+
+    public Respawn(int dimension, short difficulty, short gamemode, String levelType) {
+        this.dimension = dimension;
+        this.difficulty = difficulty;
+        this.gamemode = gamemode;
+        this.levelType = levelType;
+    }
+
+    public int getDimension() {
+        return dimension;
+    }
+
+    public void setDimension(int dimension) {
+        this.dimension = dimension;
+    }
+
+    public short getDifficulty() {
+        return difficulty;
+    }
+
+    public void setDifficulty(short difficulty) {
+        this.difficulty = difficulty;
+    }
+
+    public short getGamemode() {
+        return gamemode;
+    }
+
+    public void setGamemode(short gamemode) {
+        this.gamemode = gamemode;
+    }
+
+    public String getLevelType() {
+        return levelType;
+    }
+
+    public void setLevelType(String levelType) {
+        this.levelType = levelType;
+    }
+
+    @Override
+    public String toString() {
+        return "Respawn{" +
+                "dimension=" + dimension +
+                ", difficulty=" + difficulty +
+                ", gamemode=" + gamemode +
+                ", levelType='" + levelType + '\'' +
+                '}';
+    }
+
+    @Override
+    public void decode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        this.dimension = buf.readInt();
+        this.difficulty = buf.readUnsignedByte();
+        this.gamemode = buf.readUnsignedByte();
+        this.levelType = ProtocolUtils.readString(buf, 16);
+    }
+
+    @Override
+    public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        buf.writeInt(dimension);
+        buf.writeByte(difficulty);
+        buf.writeByte(gamemode);
+        ProtocolUtils.writeString(buf, levelType);
+    }
+}