Fix server game data snapshot being restored in singleplayer (fixes #44)

src/main/java/com/replaymod/recording/handler/FMLHandshakeFilter.java

@@ -0,0 +1,25 @@
+package com.replaymod.recording.handler;
+
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.channel.SimpleChannelInboundHandler;
+import net.minecraftforge.fml.common.network.handshake.FMLHandshakeMessage;
+
+/**
+ * Filters out all handshake packets that were sent for recording but must
+ * not actually be handled.
+ * This handler is only present when connected to the integrated server as
+ * otherwise all packets must be handled.
+ *
+ * When in single player, the game state packets must never be handled
+ * otherwise wired bugs related to semi-singletons can occur.
+ * See https://bugs.replaymod.com/show_bug.cgi?id=44
+ */
+public class FMLHandshakeFilter extends SimpleChannelInboundHandler<FMLHandshakeMessage> {
+    @Override
+    protected void channelRead0(ChannelHandlerContext ctx, FMLHandshakeMessage msg) throws Exception {
+        if (!(msg instanceof FMLHandshakeMessage.RegistryData)) {
+            // Pass on everything but RegistryData messages
+            ctx.fireChannelRead(msg);
+        }
+    }
+}

src/main/java/com/replaymod/recording/mixin/MixinNetworkDispatcher.java

@@ -1,7 +1,11 @@
 package com.replaymod.recording.mixin;
 
+import com.replaymod.recording.handler.FMLHandshakeFilter;
+import io.netty.channel.ChannelPipeline;
 import io.netty.channel.embedded.EmbeddedChannel;
+import net.minecraftforge.fml.common.network.handshake.FMLHandshakeCodec;
 import net.minecraftforge.fml.common.network.handshake.NetworkDispatcher;
+import net.minecraftforge.fml.relauncher.Side;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.Shadow;
 import org.spongepowered.asm.mixin.injection.At;
@@ -11,17 +15,32 @@ import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
 @Mixin(value = NetworkDispatcher.class, remap = false)
 public abstract class MixinNetworkDispatcher {
 
+    @Shadow
+    private Side side;
+
     @Shadow
     private EmbeddedChannel handshakeChannel;
 
     /**
-     * Always sets fml:isLocal to false.
+     * Always sets fml:isLocal to false on the server side.
      * This effectively removes the difference in the FML handshake between SP and MP
      * and forces the block/item ids, etc. to always be send.
-     * This might have undesired side effects but at least it works at all.
+     * Injects a {@link FMLHandshakeFilter} on the client side to filter out
+     * those extra, unexpected packets.
      */
     @Inject(method = "insertIntoChannel", at=@At("HEAD"))
-    public void replayModRecording_forceIsLocalToFalse(CallbackInfo cb) {
+    public void replayModRecording_setupForLocalRecording(CallbackInfo cb) {
+        // If we're in multiplayer, everything is fine as is
+        if (!handshakeChannel.attr(NetworkDispatcher.IS_LOCAL).get()) return;
+
+        if (side == Side.SERVER) {
+            // On the server side, force all packets to be sent
             handshakeChannel.attr(NetworkDispatcher.IS_LOCAL).set(false);
+        } else {
+            // On the client side, discard additional packets
+            ChannelPipeline pipeline = handshakeChannel.pipeline();
+            pipeline.addAfter(pipeline.context(FMLHandshakeCodec.class).name(),
+                    "replaymod_filter", new FMLHandshakeFilter());
+        }
     }
 }