Only send FML/FML|MP plugin messages if the player has joined the server.

See #78

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

@@ -8,6 +8,8 @@ public class VelocityConstants {
     public static final String VELOCITY_IP_FORWARDING_CHANNEL = "velocity:player_info";
 
     public static final String FORGE_LEGACY_HANDSHAKE_CHANNEL = "FML|HS";
+    public static final String FORGE_LEGACY_CHANNEL = "FML";
+    public static final String FORGE_MULTIPART_LEGACY_CHANNEL = "FML|MP";
 
     public static final byte[] FORGE_LEGACY_HANDSHAKE_RESET_DATA = new byte[] { -2, 0 };
 }

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

@@ -127,6 +127,12 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
         minecraftConnection.write(login);
     }
 
+    public void writeIfJoined(PluginMessage message) {
+        if (hasCompletedJoin) {
+            minecraftConnection.write(message);
+        }
+    }
+
     public MinecraftConnection getMinecraftConnection() {
         return minecraftConnection;
     }

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

@@ -4,6 +4,7 @@ import com.velocitypowered.api.event.connection.DisconnectEvent;
 import com.velocitypowered.api.proxy.messages.ChannelSide;
 import com.velocitypowered.api.proxy.messages.MessageHandler;
 import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.proxy.connection.VelocityConstants;
 import com.velocitypowered.proxy.connection.backend.VelocityServerConnection;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
@@ -275,10 +276,20 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
         MessageHandler.ForwardStatus status = server.getChannelRegistrar().handlePluginMessage(player,
                 ChannelSide.FROM_CLIENT, packet);
         if (status == MessageHandler.ForwardStatus.FORWARD) {
+            String channel = packet.getChannel();
+
             // We're going to forward on the original packet.
+            //
+            // If we have Forge messages, we may need to drop them if the server switch has
+            // not completed yet.
+            if (channel.equals(VelocityConstants.FORGE_LEGACY_CHANNEL)
+                || channel.equals(VelocityConstants.FORGE_MULTIPART_LEGACY_CHANNEL)) {
+                player.getConnectedServer().writeIfJoined(packet);
+            } else {
                 player.getConnectedServer().getMinecraftConnection().write(packet);
             }
         }
+    }
 
     public Set<String> getClientPluginMsgChannels() {
         return clientPluginMsgChannels;