Validate state transition

proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java

@@ -84,6 +84,7 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
   private static final Logger logger = LogManager.getLogger(MinecraftConnection.class);
 
   private final Channel channel;
+  public boolean pendingConfigurationSwitch = false;
   private SocketAddress remoteAddress;
   private StateRegistry state;
   private Map<StateRegistry, MinecraftSessionHandler> sessionHandlers;

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

@@ -72,6 +72,7 @@ import com.velocitypowered.proxy.protocol.packet.config.FinishedUpdatePacket;
 import com.velocitypowered.proxy.protocol.packet.title.GenericTitlePacket;
 import com.velocitypowered.proxy.protocol.util.PluginMessageUtil;
 import com.velocitypowered.proxy.util.CharacterUtil;
+import com.velocitypowered.proxy.util.except.QuietRuntimeException;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufUtil;
 import io.netty.buffer.Unpooled;
@@ -392,6 +393,9 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
 
   @Override
   public boolean handle(FinishedUpdatePacket packet) {
+    if (!player.getConnection().pendingConfigurationSwitch) {
+      throw new QuietRuntimeException("Not expecting reconfiguration");
+    }
     // Complete client switch
     player.getConnection().setActiveSessionHandler(StateRegistry.CONFIG);
     VelocityServerConnection serverConnection = player.getConnectedServer();

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

@@ -1318,6 +1318,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player,
             connection.write(BundleDelimiterPacket.INSTANCE);
           }
           connection.write(StartUpdatePacket.INSTANCE);
+          connection.pendingConfigurationSwitch = true;
           connection.getChannel().pipeline().get(MinecraftEncoder.class).setState(StateRegistry.CONFIG);
           // Make sure we don't send any play packets to the player after update start
           connection.addPlayPacketQueueHandler();