Initial remapping support, fix protocol crap.

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

@@ -2,6 +2,7 @@ package com.velocitypowered.proxy.connection.backend;
 
 import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.packets.*;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.protocol.util.PluginMessageUtil;
@@ -46,7 +47,7 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
             PluginMessage pm = (PluginMessage) packet;
             try {
                 PluginMessage newPacket = pm;
-                if (!canForwardMessage(newPacket)) {
+                if (!canForwardPluginMessage(newPacket)) {
                     return;
                 }
 
@@ -72,7 +73,10 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
 
     @Override
     public void handleUnknown(ByteBuf buf) {
-        connection.getProxyPlayer().getConnection().write(buf.retain());
+        ClientPlaySessionHandler playerHandler =
+                (ClientPlaySessionHandler) connection.getProxyPlayer().getConnection().getSessionHandler();
+        ByteBuf remapped = playerHandler.getIdRemapper().remap(buf, ProtocolConstants.Direction.CLIENTBOUND);
+        connection.getProxyPlayer().getConnection().write(remapped);
     }
 
     @Override
@@ -80,7 +84,7 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
         connection.getProxyPlayer().handleConnectionException(connection.getServerInfo(), throwable);
     }
 
-    private boolean canForwardMessage(PluginMessage message) {
+    private boolean canForwardPluginMessage(PluginMessage message) {
         // TODO: Update for 1.13
         ClientPlaySessionHandler playerHandler =
                 (ClientPlaySessionHandler) connection.getProxyPlayer().getConnection().getSessionHandler();

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

@@ -8,8 +8,10 @@ import com.velocitypowered.proxy.data.scoreboard.Score;
 import com.velocitypowered.proxy.data.scoreboard.Scoreboard;
 import com.velocitypowered.proxy.data.scoreboard.Team;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.packets.*;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
+import com.velocitypowered.proxy.protocol.remap.EntityIdRemapper;
 import com.velocitypowered.proxy.protocol.util.PluginMessageUtil;
 import com.velocitypowered.proxy.util.ThrowableUtils;
 import io.netty.buffer.ByteBuf;
@@ -39,6 +41,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     private PluginMessage brandMessage;
     private int currentDimension;
     private Scoreboard serverScoreboard = new Scoreboard();
+    private EntityIdRemapper idRemapper;
 
     public ClientPlaySessionHandler(ConnectedPlayer player) {
         this.player = player;
@@ -95,7 +98,8 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
 
     @Override
     public void handleUnknown(ByteBuf buf) {
-        player.getConnectedServer().getChannel().write(buf.retain());
+        ByteBuf remapped = idRemapper.remap(buf, ProtocolConstants.Direction.SERVERBOUND);
+        player.getConnectedServer().getChannel().write(remapped);
     }
 
     @Override
@@ -123,6 +127,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             spawned = true;
             currentDimension = joinGame.getDimension();
             player.getConnection().delayedWrite(joinGame);
+            idRemapper = EntityIdRemapper.getMapper(joinGame.getEntityId(), player.getConnection().getProtocolVersion());
         } else {
             // In order to handle switching to another server we will need send three packets:
             // - The join game packet from the backend server
@@ -130,6 +135,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             // - Another respawn with the correct dimension
             // We can't simply ignore the packet with the different dimension. If you try to be smart about it it doesn't
             // work.
+            idRemapper.setServerEntityId(joinGame.getEntityId());
             player.getConnection().delayedWrite(joinGame);
             int tempDim = joinGame.getDimension() == 0 ? -1 : 0;
             player.getConnection().delayedWrite(new Respawn(tempDim, joinGame.getDifficulty(), joinGame.getGamemode(), joinGame.getLevelType()));
@@ -304,4 +310,8 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     public Set<String> getClientPluginMsgChannels() {
         return clientPluginMsgChannels;
     }
+
+    public EntityIdRemapper getIdRemapper() {
+        return idRemapper;
+    }
 }

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

@@ -107,8 +107,7 @@ public enum StateRegistry {
         private static final IntObjectMap<int[]> LINKED_PROTOCOL_VERSIONS = new IntObjectHashMap<>();
 
         static {
-            LINKED_PROTOCOL_VERSIONS.put(MINECRAFT_1_10, new int[] { MINECRAFT_1_11 });
-            LINKED_PROTOCOL_VERSIONS.put(MINECRAFT_1_11, new int[] { MINECRAFT_1_11_1, MINECRAFT_1_12 });
+            LINKED_PROTOCOL_VERSIONS.put(MINECRAFT_1_10, new int[] { MINECRAFT_1_11, MINECRAFT_1_11_1, MINECRAFT_1_12 });
             LINKED_PROTOCOL_VERSIONS.put(MINECRAFT_1_12, new int[] { MINECRAFT_1_12_1 });
             LINKED_PROTOCOL_VERSIONS.put(MINECRAFT_1_12_1, new int[] { MINECRAFT_1_12_2 });
         }
@@ -142,6 +141,7 @@ public enum StateRegistry {
                 if (version == null) {
                     throw new IllegalArgumentException("Unknown protocol version " + mapping.protocolVersion);
                 }
+
                 version.packetIdToSupplier.put(mapping.id, packetSupplier);
                 version.packetClassToId.put(clazz, mapping.id);
 

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

@@ -58,7 +58,7 @@ public class ScoreboardSetScore implements MinecraftPacket {
         this.entity = ProtocolUtils.readString(buf, 40);
         this.action = buf.readByte();
         this.objective = ProtocolUtils.readString(buf, 16);
-        if (this.action == 0) {
+        if (this.action != 1) {
             value = ProtocolUtils.readVarInt(buf);
         }
     }
@@ -68,7 +68,7 @@ public class ScoreboardSetScore implements MinecraftPacket {
         ProtocolUtils.writeString(buf, entity);
         buf.writeByte(action);
         ProtocolUtils.writeString(buf, objective);
-        if (this.action == 0) {
+        if (this.action != 1) {
             ProtocolUtils.writeVarInt(buf, value);
         }
     }

src/main/java/com/velocitypowered/proxy/protocol/remap/DummyEntityIdRemapper.java

@@ -0,0 +1,32 @@
+package com.velocitypowered.proxy.protocol.remap;
+
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import io.netty.buffer.ByteBuf;
+
+public class DummyEntityIdRemapper implements EntityIdRemapper {
+    public static final DummyEntityIdRemapper INSTANCE = new DummyEntityIdRemapper();
+
+    private DummyEntityIdRemapper() {
+
+    }
+
+    @Override
+    public ByteBuf remap(ByteBuf original, ProtocolConstants.Direction direction) {
+        return original.retain();
+    }
+
+    @Override
+    public int getClientEntityId() {
+        return 0;
+    }
+
+    @Override
+    public int getServerEntityId() {
+        return 0;
+    }
+
+    @Override
+    public void setServerEntityId(int id) {
+
+    }
+}

src/main/java/com/velocitypowered/proxy/protocol/remap/EntityIdRemapper.java

@@ -0,0 +1,29 @@
+package com.velocitypowered.proxy.protocol.remap;
+
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import io.netty.buffer.ByteBuf;
+
+/**
+ * Represents a protocol-specific entity ID remapper for certain Minecraft packets. This is mostly required to support
+ * old versions of Minecraft. For Minecraft 1.9 clients and above, Velocity can use a more efficient method based on
+ * sending JoinGame packets multiple times.
+ */
+public interface EntityIdRemapper {
+    /**
+     * Remaps the entity IDs in this packet so that they apply to the player.
+     * @param original the packet to remap
+     * @param direction the direction of the packet
+     * @return a remapped packet, which may either be a retained version of the original buffer or an entirely new buffer
+     */
+    ByteBuf remap(ByteBuf original, ProtocolConstants.Direction direction);
+
+    int getClientEntityId();
+
+    int getServerEntityId();
+
+    void setServerEntityId(int id);
+
+    static EntityIdRemapper getMapper(int eid, int protocolVersion) {
+        return DummyEntityIdRemapper.INSTANCE;
+    }
+}

src/test/java/com/velocitypowered/proxy/protocol/PacketRegistryTest.java

@@ -4,32 +4,35 @@ import com.velocitypowered.proxy.protocol.packets.Handshake;
 import com.velocitypowered.proxy.protocol.packets.KeepAlive;
 import org.junit.jupiter.api.Test;
 
+import static com.velocitypowered.proxy.protocol.ProtocolConstants.MINECRAFT_1_12;
+import static com.velocitypowered.proxy.protocol.ProtocolConstants.MINECRAFT_1_12_1;
+import static com.velocitypowered.proxy.protocol.ProtocolConstants.MINECRAFT_1_12_2;
 import static org.junit.jupiter.api.Assertions.*;
 
 class PacketRegistryTest {
     private StateRegistry.PacketRegistry setupRegistry() {
         StateRegistry.PacketRegistry registry = new StateRegistry.PacketRegistry(ProtocolConstants.Direction.CLIENTBOUND);
-        registry.register(Handshake.class, Handshake::new, new StateRegistry.PacketMapping(0x00, ProtocolConstants.MINECRAFT_1_12));
+        registry.register(Handshake.class, Handshake::new, new StateRegistry.PacketMapping(0x00, MINECRAFT_1_12));
         return registry;
     }
 
     @Test
     void packetRegistryWorks() {
         StateRegistry.PacketRegistry registry = setupRegistry();
-        MinecraftPacket packet = registry.getVersion(ProtocolConstants.MINECRAFT_1_12).createPacket(0);
+        MinecraftPacket packet = registry.getVersion(MINECRAFT_1_12).createPacket(0);
         assertNotNull(packet, "Packet was not found in registry");
         assertEquals(Handshake.class, packet.getClass(), "Registry returned wrong class");
 
-        assertEquals(0, registry.getVersion(ProtocolConstants.MINECRAFT_1_12).getPacketId(packet), "Registry did not return the correct packet ID");
+        assertEquals(0, registry.getVersion(MINECRAFT_1_12).getPacketId(packet), "Registry did not return the correct packet ID");
     }
 
     @Test
     void packetRegistryLinkingWorks() {
         StateRegistry.PacketRegistry registry = setupRegistry();
-        MinecraftPacket packet = registry.getVersion(ProtocolConstants.MINECRAFT_1_12_1).createPacket(0);
+        MinecraftPacket packet = registry.getVersion(MINECRAFT_1_12_1).createPacket(0);
         assertNotNull(packet, "Packet was not found in registry");
         assertEquals(Handshake.class, packet.getClass(), "Registry returned wrong class");
-        assertEquals(0, registry.getVersion(ProtocolConstants.MINECRAFT_1_12_1).getPacketId(packet), "Registry did not return the correct packet ID");
+        assertEquals(0, registry.getVersion(MINECRAFT_1_12_1).getPacketId(packet), "Registry did not return the correct packet ID");
     }
 
     @Test
@@ -38,4 +41,14 @@ class PacketRegistryTest {
         assertThrows(IllegalArgumentException.class, () -> registry.register(Handshake.class, Handshake::new));
         assertThrows(IllegalArgumentException.class, () -> registry.getVersion(0).getPacketId(new Handshake()));
     }
+
+    @Test
+    void registrySuppliesCorrectPacketsByProtocol() {
+        StateRegistry.PacketRegistry registry = new StateRegistry.PacketRegistry(ProtocolConstants.Direction.CLIENTBOUND);
+        registry.register(Handshake.class, Handshake::new, new StateRegistry.PacketMapping(0x00, MINECRAFT_1_12),
+                new StateRegistry.PacketMapping(0x01, MINECRAFT_1_12_1));
+        assertEquals(Handshake.class, registry.getVersion(MINECRAFT_1_12).createPacket(0x00).getClass());
+        assertEquals(Handshake.class, registry.getVersion(MINECRAFT_1_12_1).createPacket(0x01).getClass());
+        assertEquals(Handshake.class, registry.getVersion(MINECRAFT_1_12_2).createPacket(0x01).getClass());
+    }
 }