additional hardening for registry desync issues

src/main/java/net/fabricmc/fabric/impl/FabricAPIClientInitiailizer.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2016, 2017, 2018 FabricMC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package net.fabricmc.fabric.impl;
+
+import net.fabricmc.api.ClientModInitializer;
+import net.fabricmc.fabric.networking.CustomPayloadPacketRegistry;
+import net.fabricmc.fabric.registry.RegistrySyncManager;
+import net.minecraft.client.MinecraftClient;
+import net.minecraft.server.integrated.IntegratedServer;
+
+public class FabricAPIClientInitiailizer implements ClientModInitializer {
+	@Override
+	public void onInitializeClient() {
+		CustomPayloadPacketRegistry.CLIENT.register(RegistrySyncManager.ID, (ctx, buf) -> {
+			// if not hosting server, apply packet
+			RegistrySyncManager.receivePacket(ctx, buf, !MinecraftClient.getInstance().method_1496());
+		});
+	}
+}

src/main/java/net/fabricmc/fabric/mixin/registry/MixinBootstrap.java

@@ -52,8 +52,5 @@ public class MixinBootstrap {
 		((ListenableRegistry<Block>) Registry.BLOCK).registerListener(new BootstrapBlockRegistryListener());
 		((ListenableRegistry<Fluid>) Registry.FLUID).registerListener(new BootstrapFluidRegistryListener());
 		((ListenableRegistry<Item>) Registry.ITEM).registerListener(new BootstrapItemRegistryListener());
-
-		// The packet code is not side-specific, so this is fine!
-		CustomPayloadPacketRegistry.CLIENT.register(RegistrySyncManager.ID, RegistrySyncManager::receivePacket);
 	}
 }

src/main/java/net/fabricmc/fabric/registry/RegistrySyncManager.java

@@ -51,9 +51,10 @@ public final class RegistrySyncManager {
 		return packet;
 	}
 
-	public static void receivePacket(PacketContext context, PacketByteBuf buf) {
+	public static void receivePacket(PacketContext context, PacketByteBuf buf, boolean accept) {
 		CompoundTag compound = buf.readCompoundTag();
 
+		if (accept) {
 			try {
 				context.getTaskQueue().executeFuture(() -> {
 					try {
@@ -70,6 +71,7 @@ public final class RegistrySyncManager {
 				new Exception("Failed to apply received packets in time!", e).printStackTrace();
 			}
 		}
+	}
 
 	public static CompoundTag toTag(boolean isClientSync) {
 		CompoundTag mainTag = new CompoundTag();

src/main/resources/fabric.mod.json

@@ -6,7 +6,8 @@
   "description": "Core API module providing key hooks and intercompatibility features.",
   "license": "Apache-2.0",
   "initializers": [
-    "net.fabricmc.fabric.impl.FabricAPIInitializer"
+    "net.fabricmc.fabric.impl.FabricAPIInitializer",
+    "net.fabricmc.fabric.impl.FabricAPIClientInitializer"
   ],
   "mixins": {
     "client": "net.fabricmc.fabric.mixins.client.json",