Add optional registries & refactor registry sync (#4233)

* Refactor registry sync

* Remove old file

* Checkstyle

* Fixes

* Improve error message

* Fix

* Fix test

fabric-networking-api-v1/src/main/java/net/fabricmc/fabric/impl/networking/server/ServerConfigurationNetworkAddon.java

@@ -20,8 +20,11 @@ import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
 
+import org.jetbrains.annotations.Nullable;
+
 import net.minecraft.network.NetworkPhase;
 import net.minecraft.network.PacketCallbacks;
+import net.minecraft.network.packet.BrandCustomPayload;
 import net.minecraft.network.packet.CustomPayload;
 import net.minecraft.network.packet.Packet;
 import net.minecraft.network.packet.s2c.common.CommonPingS2CPacket;
@@ -44,6 +47,8 @@ public final class ServerConfigurationNetworkAddon extends AbstractChanneledNetw
 	private final MinecraftServer server;
 	private final ServerConfigurationNetworking.Context context;
 	private RegisterState registerState = RegisterState.NOT_SENT;
+	@Nullable
+	private String clientBrand = null;
 
 	public ServerConfigurationNetworkAddon(ServerConfigurationNetworkHandler handler, MinecraftServer server) {
 		super(ServerNetworkingImpl.CONFIGURATION, ((ServerCommonNetworkHandlerAccessor) handler).getConnection(), "ServerConfigurationNetworkAddon for " + handler.getDebugProfile().getName());
@@ -55,6 +60,16 @@ public final class ServerConfigurationNetworkAddon extends AbstractChanneledNetw
 		this.registerPendingChannels((ChannelInfoHolder) this.connection, NetworkPhase.CONFIGURATION);
 	}
 
+	@Override
+	public boolean handle(CustomPayload payload) {
+		if (payload instanceof BrandCustomPayload brandCustomPayload) {
+			clientBrand = brandCustomPayload.brand();
+			return false;
+		}
+
+		return super.handle(payload);
+	}
+
 	@Override
 	protected void invokeInitEvent() {
 	}
@@ -169,6 +184,10 @@ public final class ServerConfigurationNetworkAddon extends AbstractChanneledNetw
 		handler.send(packet, callback);
 	}
 
+	public @Nullable String getClientBrand() {
+		return clientBrand;
+	}
+
 	private enum RegisterState {
 		NOT_SENT,
 		SENT,

fabric-registry-sync-v0/src/client/java/net/fabricmc/fabric/impl/client/registry/sync/ClientRegistrySyncHandler.java

@@ -0,0 +1,240 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 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.client.registry.sync;
+
+import java.util.ArrayList;
+import java.util.EnumSet;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionException;
+
+import it.unimi.dsi.fastutil.objects.Object2IntMap;
+import org.jetbrains.annotations.VisibleForTesting;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import net.minecraft.registry.Registries;
+import net.minecraft.registry.Registry;
+import net.minecraft.screen.ScreenTexts;
+import net.minecraft.text.MutableText;
+import net.minecraft.text.Text;
+import net.minecraft.util.Formatting;
+import net.minecraft.util.Identifier;
+import net.minecraft.util.thread.ThreadExecutor;
+
+import net.fabricmc.fabric.api.event.registry.RegistryAttribute;
+import net.fabricmc.fabric.impl.registry.sync.RegistrySyncManager;
+import net.fabricmc.fabric.impl.registry.sync.RemapException;
+import net.fabricmc.fabric.impl.registry.sync.RemappableRegistry;
+import net.fabricmc.fabric.impl.registry.sync.packet.RegistryPacketHandler;
+
+public final class ClientRegistrySyncHandler {
+	private static final Logger LOGGER = LoggerFactory.getLogger(ClientRegistrySyncHandler.class);
+
+	private ClientRegistrySyncHandler() {
+	}
+
+	public static <T extends RegistryPacketHandler.RegistrySyncPayload> CompletableFuture<Boolean> receivePacket(ThreadExecutor<?> executor, RegistryPacketHandler<T> handler, T payload, boolean accept) {
+		handler.receivePayload(payload);
+
+		if (!handler.isPacketFinished()) {
+			return CompletableFuture.completedFuture(false);
+		}
+
+		if (RegistrySyncManager.DEBUG) {
+			String handlerName = handler.getClass().getSimpleName();
+			LOGGER.info("{} total packet: {}", handlerName, handler.getTotalPacketReceived());
+			LOGGER.info("{} raw size: {}", handlerName, handler.getRawBufSize());
+			LOGGER.info("{} deflated size: {}", handlerName, handler.getDeflatedBufSize());
+		}
+
+		RegistryPacketHandler.SyncedPacketData data = handler.getSyncedPacketData();
+
+		if (!accept) {
+			return CompletableFuture.completedFuture(true);
+		}
+
+		return executor.submit(() -> {
+			if (data == null) {
+				throw new CompletionException(new RemapException("Received null map in sync packet!"));
+			}
+
+			try {
+				apply(data);
+				return true;
+			} catch (RemapException e) {
+				throw new CompletionException(e);
+			}
+		});
+	}
+
+	public static void apply(RegistryPacketHandler.SyncedPacketData data) throws RemapException {
+		// First check that all of the data provided is valid before making any changes
+		checkRemoteRemap(data);
+
+		for (Map.Entry<Identifier, Object2IntMap<Identifier>> entry : data.idMap().entrySet()) {
+			final Identifier registryId = entry.getKey();
+
+			Registry<?> registry = Registries.REGISTRIES.get(registryId);
+
+			// Registry was not found on the client, is it optional?
+			// If so we can just ignore it.
+			// Otherwise we throw an exception and disconnect.
+			if (registry == null) {
+				if (isRegistryOptional(registryId, data)) {
+					LOGGER.info("Received registry data for unknown optional registry: {}", registryId);
+					continue;
+				}
+			}
+
+			if (registry instanceof RemappableRegistry remappableRegistry) {
+				remappableRegistry.remap(entry.getValue(), RemappableRegistry.RemapMode.REMOTE);
+				return;
+			}
+
+			throw new RemapException("Registry " + registryId + " is not remappable");
+		}
+	}
+
+	@VisibleForTesting
+	public static void checkRemoteRemap(RegistryPacketHandler.SyncedPacketData data) throws RemapException {
+		Map<Identifier, Object2IntMap<Identifier>> map = data.idMap();
+		ArrayList<Identifier> missingRegistries = new ArrayList<>();
+		Map<Identifier, List<Identifier>> missingEntries = new HashMap<>();
+
+		for (Identifier registryId : map.keySet()) {
+			final Object2IntMap<Identifier> remoteRegistry = map.get(registryId);
+			Registry<?> registry = Registries.REGISTRIES.get(registryId);
+
+			if (registry == null) {
+				if (!isRegistryOptional(registryId, data)) {
+					// Registry was not found on the client, and is not optional.
+					missingRegistries.add(registryId);
+				}
+
+				continue;
+			}
+
+			for (Identifier remoteId : remoteRegistry.keySet()) {
+				if (!registry.containsId(remoteId)) {
+					// Found a registry entry from the server that is missing on the client
+					missingEntries.computeIfAbsent(registryId, i -> new ArrayList<>()).add(remoteId);
+				}
+			}
+		}
+
+		if (missingRegistries.isEmpty() && missingEntries.isEmpty()) {
+			// All good :)
+			return;
+		}
+
+		// Print out details to the log
+		if (!missingRegistries.isEmpty()) {
+			LOGGER.error("Received unknown remote registries from server");
+
+			for (Identifier registryId : missingRegistries) {
+				LOGGER.error("Received unknown remote registry ({}) from server", registryId);
+			}
+		}
+
+		if (!missingEntries.isEmpty()) {
+			LOGGER.error("Received unknown remote registry entries from server");
+
+			for (Map.Entry<Identifier, List<Identifier>> entry : missingEntries.entrySet()) {
+				for (Identifier identifier : entry.getValue()) {
+					LOGGER.error("Registry entry ({}) is missing from local registry ({})", identifier, entry.getKey());
+				}
+			}
+		}
+
+		if (!missingRegistries.isEmpty()) {
+			throw new RemapException(missingRegistriesError(missingRegistries));
+		}
+
+		throw new RemapException(missingEntriesError(missingEntries));
+	}
+
+	private static Text missingRegistriesError(List<Identifier> missingRegistries) {
+		MutableText text = Text.empty();
+
+		final int count = missingRegistries.size();
+
+		if (count == 1) {
+			text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-registry.title.singular"));
+		} else {
+			text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-registry.title.plural", count));
+		}
+
+		text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-registry.subtitle.1").formatted(Formatting.GREEN));
+		text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-registry.subtitle.2"));
+
+		final int toDisplay = 4;
+
+		for (int i = 0; i < Math.min(missingRegistries.size(), toDisplay); i++) {
+			text = text.append(Text.literal(missingRegistries.get(i).toString()).formatted(Formatting.YELLOW));
+			text = text.append(ScreenTexts.LINE_BREAK);
+		}
+
+		if (missingRegistries.size() > toDisplay) {
+			text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-registry.footer", missingRegistries.size() - toDisplay));
+		}
+
+		return text;
+	}
+
+	private static Text missingEntriesError(Map<Identifier, List<Identifier>> missingEntries) {
+		MutableText text = Text.empty();
+
+		final int count = missingEntries.values().stream().mapToInt(List::size).sum();
+
+		if (count == 1) {
+			text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.title.singular"));
+		} else {
+			text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.title.plural", count));
+		}
+
+		text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.subtitle.1").formatted(Formatting.GREEN));
+		text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.subtitle.2"));
+
+		final int toDisplay = 4;
+		// Get the distinct missing namespaces
+		final List<String> namespaces = missingEntries.values().stream()
+				.flatMap(List::stream)
+				.map(Identifier::getNamespace)
+				.distinct()
+				.sorted()
+				.toList();
+
+		for (int i = 0; i < Math.min(namespaces.size(), toDisplay); i++) {
+			text = text.append(Text.literal(namespaces.get(i)).formatted(Formatting.YELLOW));
+			text = text.append(ScreenTexts.LINE_BREAK);
+		}
+
+		if (namespaces.size() > toDisplay) {
+			text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.footer", namespaces.size() - toDisplay));
+		}
+
+		return text;
+	}
+
+	private static boolean isRegistryOptional(Identifier registryId, RegistryPacketHandler.SyncedPacketData data) {
+		EnumSet<RegistryAttribute> registryAttributes = data.attributes().get(registryId);
+		return registryAttributes.contains(RegistryAttribute.OPTIONAL);
+	}
+}

fabric-registry-sync-v0/src/client/java/net/fabricmc/fabric/impl/client/registry/sync/FabricRegistryClientInit.java

@@ -40,7 +40,7 @@ public class FabricRegistryClientInit implements ClientModInitializer {
 
 	private <T extends RegistryPacketHandler.RegistrySyncPayload> void registerSyncPacketReceiver(RegistryPacketHandler<T> packetHandler) {
 		ClientConfigurationNetworking.registerGlobalReceiver(packetHandler.getPacketId(), (payload, context) -> {
-			RegistrySyncManager.receivePacket(context.client(), packetHandler, payload, RegistrySyncManager.DEBUG || !context.client().isInSingleplayer())
+			ClientRegistrySyncHandler.receivePacket(context.client(), packetHandler, payload, RegistrySyncManager.DEBUG || !context.client().isInSingleplayer())
 					.whenComplete((complete, throwable) -> {
 						if (throwable != null) {
 							LOGGER.error("Registry remapping failed!", throwable);

fabric-registry-sync-v0/src/client/java/net/fabricmc/fabric/mixin/registry/sync/client/MinecraftClientMixin.java

@@ -20,6 +20,7 @@ import org.slf4j.Logger;
 import org.spongepowered.asm.mixin.Final;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.Unique;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
@@ -28,9 +29,11 @@ import net.minecraft.client.MinecraftClient;
 import net.minecraft.client.gui.screen.Screen;
 import net.minecraft.item.ItemGroups;
 import net.minecraft.registry.Registries;
+import net.minecraft.registry.Registry;
+import net.minecraft.util.Identifier;
 
-import net.fabricmc.fabric.impl.registry.sync.RegistrySyncManager;
 import net.fabricmc.fabric.impl.registry.sync.RemapException;
+import net.fabricmc.fabric.impl.registry.sync.RemappableRegistry;
 import net.fabricmc.fabric.impl.registry.sync.trackers.vanilla.BlockInitTracker;
 
 @Mixin(MinecraftClient.class)
@@ -43,7 +46,7 @@ public class MinecraftClientMixin {
 	@Inject(at = @At("RETURN"), method = "disconnect(Lnet/minecraft/client/gui/screen/Screen;Z)V")
 	public void disconnectAfter(Screen disconnectionScreen, boolean bl, CallbackInfo ci) {
 		try {
-			RegistrySyncManager.unmap();
+			unmap();
 		} catch (RemapException e) {
 			LOGGER.warn("Failed to unmap Fabric registries!", e);
 		}
@@ -57,4 +60,15 @@ public class MinecraftClientMixin {
 		BlockInitTracker.postFreeze();
 		ItemGroups.collect();
 	}
+
+	@Unique
+	private static void unmap() throws RemapException {
+		for (Identifier registryId : Registries.REGISTRIES.getIds()) {
+			Registry<?> registry = Registries.REGISTRIES.get(registryId);
+
+			if (registry instanceof RemappableRegistry) {
+				((RemappableRegistry) registry).unmap();
+			}
+		}
+	}
 }

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/api/event/registry/RegistryAttribute.java

@@ -25,5 +25,10 @@ public enum RegistryAttribute {
 	/**
 	 * Registry has been modded.
 	 */
-	MODDED
+	MODDED,
+
+	/**
+	 * Registry is optional, any connecting client will not be disconnected if the registry is not present.
+	 */
+	OPTIONAL
 }

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/impl/registry/sync/RegistryAttributeImpl.java

@@ -20,10 +20,13 @@ import java.util.EnumSet;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 
+import org.jetbrains.annotations.VisibleForTesting;
+
 import net.minecraft.registry.RegistryKey;
 
 import net.fabricmc.fabric.api.event.registry.RegistryAttribute;
 import net.fabricmc.fabric.api.event.registry.RegistryAttributeHolder;
+import net.fabricmc.loader.api.FabricLoader;
 
 public final class RegistryAttributeImpl implements RegistryAttributeHolder {
 	private static final Map<RegistryKey<?>, RegistryAttributeHolder> HOLDER_MAP = new ConcurrentHashMap<>();
@@ -43,6 +46,15 @@ public final class RegistryAttributeImpl implements RegistryAttributeHolder {
 		return this;
 	}
 
+	@VisibleForTesting
+	public void removeAttribute(RegistryAttribute attribute) {
+		if (!FabricLoader.getInstance().isDevelopmentEnvironment()) {
+			throw new AssertionError();
+		}
+
+		attributes.remove(attribute);
+	}
+
 	@Override
 	public boolean hasAttribute(RegistryAttribute attribute) {
 		return attributes.contains(attribute);

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/impl/registry/sync/RegistrySyncManager.java

@@ -20,31 +20,24 @@ import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
-import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
-import java.util.concurrent.CompletableFuture;
-import java.util.concurrent.CompletionException;
 import java.util.function.Consumer;
 
-import com.google.common.base.Joiner;
-import com.google.common.collect.Sets;
 import it.unimi.dsi.fastutil.ints.IntOpenHashSet;
 import it.unimi.dsi.fastutil.ints.IntSet;
 import it.unimi.dsi.fastutil.objects.Object2IntLinkedOpenHashMap;
 import it.unimi.dsi.fastutil.objects.Object2IntMap;
 import org.jetbrains.annotations.Nullable;
-import org.jetbrains.annotations.VisibleForTesting;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import net.minecraft.network.packet.Packet;
 import net.minecraft.registry.Registries;
 import net.minecraft.registry.Registry;
-import net.minecraft.registry.RegistryKey;
 import net.minecraft.screen.ScreenTexts;
 import net.minecraft.server.MinecraftServer;
 import net.minecraft.server.network.ServerConfigurationNetworkHandler;
@@ -53,13 +46,12 @@ import net.minecraft.text.MutableText;
 import net.minecraft.text.Text;
 import net.minecraft.util.Formatting;
 import net.minecraft.util.Identifier;
-import net.minecraft.util.thread.ThreadExecutor;
 
 import net.fabricmc.fabric.api.event.registry.RegistryAttribute;
 import net.fabricmc.fabric.api.event.registry.RegistryAttributeHolder;
 import net.fabricmc.fabric.api.networking.v1.ServerConfigurationNetworking;
+import net.fabricmc.fabric.impl.networking.server.ServerNetworkingImpl;
 import net.fabricmc.fabric.impl.registry.sync.packet.DirectRegistryPacketHandler;
-import net.fabricmc.fabric.impl.registry.sync.packet.RegistryPacketHandler;
 
 public final class RegistrySyncManager {
 	public static final boolean DEBUG = Boolean.getBoolean("fabric.registry.debug");
@@ -80,11 +72,6 @@ public final class RegistrySyncManager {
 			return;
 		}
 
-		if (!ServerConfigurationNetworking.canSend(handler, DIRECT_PACKET_HANDLER.getPacketId())) {
-			// Don't send if the client cannot receive
-			return;
-		}
-
 		final Map<Identifier, Object2IntMap<Identifier>> map = RegistrySyncManager.createAndPopulateRegistryMap();
 
 		if (map == null) {
@@ -92,9 +79,62 @@ public final class RegistrySyncManager {
 			return;
 		}
 
+		if (!ServerConfigurationNetworking.canSend(handler, DIRECT_PACKET_HANDLER.getPacketId())) {
+			if (areAllRegistriesOptional(map)) {
+				// Allow the client to connect if all of the registries we want to sync are optional
+				return;
+			}
+
+			// Disconnect incompatible clients
+			Text message = getIncompatibleClientText(ServerNetworkingImpl.getAddon(handler).getClientBrand(), map);
+			handler.disconnect(message);
+			return;
+		}
+
 		handler.addTask(new SyncConfigurationTask(handler, map));
 	}
 
+	private static Text getIncompatibleClientText(@Nullable String brand, Map<Identifier, Object2IntMap<Identifier>> map) {
+		String brandText = switch (brand) {
+		case "fabric" -> "Fabric API";
+		case null, default -> "Fabric Loader and Fabric API";
+		};
+
+		final int toDisplay = 4;
+
+		List<String> namespaces = map.values().stream()
+				.map(Object2IntMap::keySet)
+				.flatMap(Set::stream)
+				.map(Identifier::getNamespace)
+				.filter(s -> !s.equals(Identifier.DEFAULT_NAMESPACE))
+				.distinct()
+				.sorted()
+				.toList();
+
+		MutableText text = Text.literal("The following registry entry namespaces may be related:\n\n");
+
+		for (int i = 0; i < Math.min(namespaces.size(), toDisplay); i++) {
+			text = text.append(Text.literal(namespaces.get(i)).formatted(Formatting.YELLOW));
+			text = text.append(ScreenTexts.LINE_BREAK);
+		}
+
+		if (namespaces.size() > toDisplay) {
+			text = text.append(Text.literal("And %d more...".formatted(namespaces.size() - toDisplay)));
+		}
+
+		return Text.literal("This server requires ").append(Text.literal(brandText).formatted(Formatting.GREEN)).append(" installed on your client!")
+				.append(ScreenTexts.LINE_BREAK).append(text)
+				.append(ScreenTexts.LINE_BREAK).append(ScreenTexts.LINE_BREAK).append(Text.literal("Contact the server's administrator for more information!").formatted(Formatting.GOLD));
+	}
+
+	private static boolean areAllRegistriesOptional(Map<Identifier, Object2IntMap<Identifier>> map) {
+		return map.keySet().stream()
+				.map(Registries.REGISTRIES::get)
+				.filter(Objects::nonNull)
+				.map(RegistryAttributeHolder::get)
+				.allMatch(attributes -> attributes.hasAttribute(RegistryAttribute.OPTIONAL));
+	}
+
 	public record SyncConfigurationTask(
 			ServerConfigurationNetworkHandler handler,
 			Map<Identifier, Object2IntMap<Identifier>> map
@@ -112,40 +152,6 @@ public final class RegistrySyncManager {
 		}
 	}
 
-	public static <T extends RegistryPacketHandler.RegistrySyncPayload> CompletableFuture<Boolean> receivePacket(ThreadExecutor<?> executor, RegistryPacketHandler<T> handler, T payload, boolean accept) {
-		handler.receivePayload(payload);
-
-		if (!handler.isPacketFinished()) {
-			return CompletableFuture.completedFuture(false);
-		}
-
-		if (DEBUG) {
-			String handlerName = handler.getClass().getSimpleName();
-			LOGGER.info("{} total packet: {}", handlerName, handler.getTotalPacketReceived());
-			LOGGER.info("{} raw size: {}", handlerName, handler.getRawBufSize());
-			LOGGER.info("{} deflated size: {}", handlerName, handler.getDeflatedBufSize());
-		}
-
-		Map<Identifier, Object2IntMap<Identifier>> map = handler.getSyncedRegistryMap();
-
-		if (!accept) {
-			return CompletableFuture.completedFuture(true);
-		}
-
-		return executor.submit(() -> {
-			if (map == null) {
-				throw new CompletionException(new RemapException("Received null map in sync packet!"));
-			}
-
-			try {
-				apply(map, RemappableRegistry.RemapMode.REMOTE);
-				return true;
-			} catch (RemapException e) {
-				throw new CompletionException(e);
-			}
-		});
-	}
-
 	/**
 	 * Creates a {@link Map} used to sync the registry ids.
 	 *
@@ -254,121 +260,6 @@ public final class RegistrySyncManager {
 		return map;
 	}
 
-	public static void apply(Map<Identifier, Object2IntMap<Identifier>> map, RemappableRegistry.RemapMode mode) throws RemapException {
-		if (mode == RemappableRegistry.RemapMode.REMOTE) {
-			checkRemoteRemap(map);
-		}
-
-		Set<Identifier> containedRegistries = Sets.newHashSet(map.keySet());
-
-		for (Identifier registryId : Registries.REGISTRIES.getIds()) {
-			if (!containedRegistries.remove(registryId)) {
-				continue;
-			}
-
-			Object2IntMap<Identifier> registryMap = map.get(registryId);
-			Registry<?> registry = Registries.REGISTRIES.get(registryId);
-
-			RegistryAttributeHolder attributeHolder = RegistryAttributeHolder.get(registry.getKey());
-
-			if (!attributeHolder.hasAttribute(RegistryAttribute.MODDED)) {
-				LOGGER.debug("Not applying registry data to vanilla registry {}", registryId.toString());
-				continue;
-			}
-
-			if (registry instanceof RemappableRegistry remappableRegistry) {
-				remappableRegistry.remap(registryId.toString(), registryMap, mode);
-			} else {
-				throw new RemapException("Registry " + registryId + " is not remappable");
-			}
-		}
-
-		if (!containedRegistries.isEmpty()) {
-			LOGGER.warn("[fabric-registry-sync] Could not find the following registries: " + Joiner.on(", ").join(containedRegistries));
-		}
-	}
-
-	@VisibleForTesting
-	public static void checkRemoteRemap(Map<Identifier, Object2IntMap<Identifier>> map) throws RemapException {
-		Map<Identifier, List<Identifier>> missingEntries = new HashMap<>();
-
-		for (Map.Entry<? extends RegistryKey<? extends Registry<?>>, ? extends Registry<?>> entry : Registries.REGISTRIES.getEntrySet()) {
-			final Registry<?> registry = entry.getValue();
-			final Identifier registryId = entry.getKey().getValue();
-			final Object2IntMap<Identifier> remoteRegistry = map.get(registryId);
-
-			if (remoteRegistry == null) {
-				// Registry sync does not contain data for this registry, will print a warning when applying.
-				continue;
-			}
-
-			for (Identifier remoteId : remoteRegistry.keySet()) {
-				if (!registry.containsId(remoteId)) {
-					// Found a registry entry from the server that is
-					missingEntries.computeIfAbsent(registryId, i -> new ArrayList<>()).add(remoteId);
-				}
-			}
-		}
-
-		if (missingEntries.isEmpty()) {
-			// All good :)
-			return;
-		}
-
-		// Print out details to the log
-		LOGGER.error("Received unknown remote registry entries from server");
-
-		for (Map.Entry<Identifier, List<Identifier>> entry : missingEntries.entrySet()) {
-			for (Identifier identifier : entry.getValue()) {
-				LOGGER.error("Registry entry ({}) is missing from local registry ({})", identifier, entry.getKey());
-			}
-		}
-
-		// Create a nice user friendly error message.
-		MutableText text = Text.empty();
-
-		final int count = missingEntries.values().stream().mapToInt(List::size).sum();
-
-		if (count == 1) {
-			text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.title.singular"));
-		} else {
-			text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.title.plural", count));
-		}
-
-		text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.subtitle.1").formatted(Formatting.GREEN));
-		text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.subtitle.2"));
-
-		final int toDisplay = 4;
-		// Get the distinct missing namespaces
-		final List<String> namespaces = missingEntries.values().stream()
-				.flatMap(List::stream)
-				.map(Identifier::getNamespace)
-				.distinct()
-				.sorted()
-				.toList();
-
-		for (int i = 0; i < Math.min(namespaces.size(), toDisplay); i++) {
-			text = text.append(Text.literal(namespaces.get(i)).formatted(Formatting.YELLOW));
-			text = text.append(ScreenTexts.LINE_BREAK);
-		}
-
-		if (namespaces.size() > toDisplay) {
-			text = text.append(Text.translatable("fabric-registry-sync-v0.unknown-remote.footer", namespaces.size() - toDisplay));
-		}
-
-		throw new RemapException(text);
-	}
-
-	public static void unmap() throws RemapException {
-		for (Identifier registryId : Registries.REGISTRIES.getIds()) {
-			Registry registry = Registries.REGISTRIES.get(registryId);
-
-			if (registry instanceof RemappableRegistry) {
-				((RemappableRegistry) registry).unmap(registryId.toString());
-			}
-		}
-	}
-
 	public static void bootstrapRegistries() {
 		postBootstrap = true;
 	}

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/impl/registry/sync/RemappableRegistry.java

@@ -37,13 +37,9 @@ public interface RemappableRegistry {
 		 * client).
 		 */
 		REMOTE,
-		/**
-		 * No differences in entry sets are allowed.
-		 */
-		EXACT
 	}
 
-	void remap(String name, Object2IntMap<Identifier> remoteIndexedEntries, RemapMode mode) throws RemapException;
+	void remap(Object2IntMap<Identifier> remoteIndexedEntries, RemapMode mode) throws RemapException;
 
-	void unmap(String name) throws RemapException;
+	void unmap() throws RemapException;
 }

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/impl/registry/sync/packet/DirectRegistryPacketHandler.java

@@ -17,7 +17,9 @@
 package net.fabricmc.fabric.impl.registry.sync.packet;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Comparator;
+import java.util.EnumSet;
 import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.List;
@@ -33,8 +35,12 @@ import org.jetbrains.annotations.Nullable;
 import net.minecraft.network.PacketByteBuf;
 import net.minecraft.network.codec.PacketCodec;
 import net.minecraft.network.packet.CustomPayload;
+import net.minecraft.registry.Registries;
+import net.minecraft.registry.Registry;
 import net.minecraft.util.Identifier;
 
+import net.fabricmc.fabric.api.event.registry.RegistryAttribute;
+import net.fabricmc.fabric.api.event.registry.RegistryAttributeHolder;
 import net.fabricmc.fabric.api.networking.v1.PacketByteBufs;
 
 /**
@@ -65,6 +71,9 @@ public class DirectRegistryPacketHandler extends RegistryPacketHandler<DirectReg
 	@Nullable
 	private Map<Identifier, Object2IntMap<Identifier>> syncedRegistryMap;
 
+	@Nullable
+	private Map<Identifier, EnumSet<RegistryAttribute>> syncedRegistryAttributes;
+
 	private boolean isPacketFinished = false;
 	private int totalPacketReceived = 0;
 
@@ -89,6 +98,7 @@ public class DirectRegistryPacketHandler extends RegistryPacketHandler<DirectReg
 
 			for (Identifier regId : regIds) {
 				buf.writeString(regId.getPath());
+				buf.writeByte(encodeRegistryAttributes(regId));
 
 				Object2IntMap<Identifier> idMap = registryMap.get(regId);
 
@@ -181,6 +191,7 @@ public class DirectRegistryPacketHandler extends RegistryPacketHandler<DirectReg
 
 		computeBufSize(combinedBuf);
 		syncedRegistryMap = new LinkedHashMap<>();
+		syncedRegistryAttributes = new LinkedHashMap<>();
 		int regNamespaceGroupAmount = combinedBuf.readVarInt();
 
 		for (int i = 0; i < regNamespaceGroupAmount; i++) {
@@ -189,6 +200,7 @@ public class DirectRegistryPacketHandler extends RegistryPacketHandler<DirectReg
 
 			for (int j = 0; j < regNamespaceGroupLength; j++) {
 				String regPath = combinedBuf.readString();
+				EnumSet<RegistryAttribute> attributes = decodeRegistryAttributes(combinedBuf.readByte());
 				Object2IntMap<Identifier> idMap = new Object2IntLinkedOpenHashMap<>();
 				int idNamespaceGroupAmount = combinedBuf.readVarInt();
 
@@ -214,7 +226,9 @@ public class DirectRegistryPacketHandler extends RegistryPacketHandler<DirectReg
 					}
 				}
 
-				syncedRegistryMap.put(Identifier.of(regNamespace, regPath), idMap);
+				Identifier registryId = Identifier.of(regNamespace, regPath);
+				syncedRegistryMap.put(registryId, idMap);
+				syncedRegistryAttributes.put(registryId, attributes);
 			}
 		}
 
@@ -235,13 +249,22 @@ public class DirectRegistryPacketHandler extends RegistryPacketHandler<DirectReg
 
 	@Override
 	@Nullable
-	public Map<Identifier, Object2IntMap<Identifier>> getSyncedRegistryMap() {
+	public SyncedPacketData getSyncedPacketData() {
 		Preconditions.checkState(isPacketFinished);
-		Map<Identifier, Object2IntMap<Identifier>> map = syncedRegistryMap;
+
+		if (syncedRegistryMap == null || syncedRegistryAttributes == null) {
+			return null;
+		}
+
+		Map<Identifier, Object2IntMap<Identifier>> map = Collections.unmodifiableMap(syncedRegistryMap);
+		Map<Identifier, EnumSet<RegistryAttribute>> attributes = Collections.unmodifiableMap(syncedRegistryAttributes);
+
 		isPacketFinished = false;
 		totalPacketReceived = 0;
 		syncedRegistryMap = null;
-		return map;
+		syncedRegistryAttributes = null;
+
+		return new SyncedPacketData(map, attributes);
 	}
 
 	private DirectRegistryPacketHandler.Payload createPayload(PacketByteBuf buf) {
@@ -283,4 +306,32 @@ public class DirectRegistryPacketHandler extends RegistryPacketHandler<DirectReg
 			return ID;
 		}
 	}
+
+	private static byte encodeRegistryAttributes(Identifier identifier) {
+		Registry<?> registry = Registries.REGISTRIES.get(identifier);
+
+		if (registry == null) {
+			return 0;
+		}
+
+		RegistryAttributeHolder holder = RegistryAttributeHolder.get(registry);
+		byte encoded = 0;
+
+		// Only send the optional marker.
+		if (holder.hasAttribute(RegistryAttribute.OPTIONAL)) {
+			encoded |= 0x1;
+		}
+
+		return encoded;
+	}
+
+	private static EnumSet<RegistryAttribute> decodeRegistryAttributes(byte encoded) {
+		EnumSet<RegistryAttribute> attributes = EnumSet.noneOf(RegistryAttribute.class);
+
+		if ((encoded & 0x1) != 0) {
+			attributes.add(RegistryAttribute.OPTIONAL);
+		}
+
+		return attributes;
+	}
 }

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/impl/registry/sync/packet/RegistryPacketHandler.java

@@ -16,6 +16,7 @@
 
 package net.fabricmc.fabric.impl.registry.sync.packet;
 
+import java.util.EnumSet;
 import java.util.Map;
 import java.util.function.Consumer;
 import java.util.zip.Deflater;
@@ -28,6 +29,7 @@ import net.minecraft.network.PacketByteBuf;
 import net.minecraft.network.packet.CustomPayload;
 import net.minecraft.util.Identifier;
 
+import net.fabricmc.fabric.api.event.registry.RegistryAttribute;
 import net.fabricmc.fabric.api.networking.v1.PacketByteBufs;
 import net.fabricmc.fabric.impl.registry.sync.RegistrySyncManager;
 
@@ -46,7 +48,7 @@ public abstract class RegistryPacketHandler<T extends RegistryPacketHandler.Regi
 	public abstract boolean isPacketFinished();
 
 	@Nullable
-	public abstract Map<Identifier, Object2IntMap<Identifier>> getSyncedRegistryMap();
+	public abstract SyncedPacketData getSyncedPacketData();
 
 	protected final void computeBufSize(PacketByteBuf buf) {
 		if (!RegistrySyncManager.DEBUG) {
@@ -92,4 +94,9 @@ public abstract class RegistryPacketHandler<T extends RegistryPacketHandler.Regi
 
 	public interface RegistrySyncPayload extends CustomPayload {
 	}
+
+	public record SyncedPacketData(
+			Map<Identifier, Object2IntMap<Identifier>> idMap,
+			Map<Identifier, EnumSet<RegistryAttribute>> attributes
+	) { }
 }

fabric-registry-sync-v0/src/main/java/net/fabricmc/fabric/mixin/registry/sync/SimpleRegistryMixin.java

@@ -73,9 +73,6 @@ public abstract class SimpleRegistryMixin<T> implements MutableRegistry<T>, Rema
 	@Unique
 	private static final Set<String> VANILLA_NAMESPACES = Set.of("minecraft", "brigadier");
 
-	@Unique
-	private static final Logger LOGGER = LoggerFactory.getLogger("FabricRegistrySync");
-
 	@Shadow
 	@Final
 	private ObjectList<RegistryEntry.Reference<T>> rawIdToEntry;
@@ -123,7 +120,7 @@ public abstract class SimpleRegistryMixin<T> implements MutableRegistry<T>, Rema
 	}
 
 	@Inject(method = "<init>(Lnet/minecraft/registry/RegistryKey;Lcom/mojang/serialization/Lifecycle;Z)V", at = @At("RETURN"))
-	private void init(RegistryKey key, Lifecycle lifecycle, boolean intrusive, CallbackInfo ci) {
+	private void init(RegistryKey<?> key, Lifecycle lifecycle, boolean intrusive, CallbackInfo ci) {
 		fabric_addObjectEvent = EventFactory.createArrayBacked(RegistryEntryAddedCallback.class,
 			(callbacks) -> (rawId, id, object) -> {
 				for (RegistryEntryAddedCallback<T> callback : callbacks) {
@@ -165,7 +162,7 @@ public abstract class SimpleRegistryMixin<T> implements MutableRegistry<T>, Rema
 	}
 
 	@Override
-	public void remap(String name, Object2IntMap<Identifier> remoteIndexedEntries, RemapMode mode) throws RemapException {
+	public void remap(Object2IntMap<Identifier> remoteIndexedEntries, RemapMode mode) throws RemapException {
 		// Throw on invalid conditions.
 		switch (mode) {
 		case AUTHORITATIVE:
@@ -184,34 +181,7 @@ public abstract class SimpleRegistryMixin<T> implements MutableRegistry<T>, Rema
 			}
 
 			if (strings != null) {
-				StringBuilder builder = new StringBuilder("Received ID map for " + name + " contains IDs unknown to the receiver!");
+				StringBuilder builder = new StringBuilder("Received ID map for " + getKey() + " contains IDs unknown to the receiver!");
-
-				for (String s : strings) {
-					builder.append('\n').append(s);
-				}
-
-				throw new RemapException(builder.toString());
-			}
-
-			break;
-		}
-		case EXACT: {
-			if (!idToEntry.keySet().equals(remoteIndexedEntries.keySet())) {
-				List<String> strings = new ArrayList<>();
-
-				for (Identifier remoteId : remoteIndexedEntries.keySet()) {
-					if (!idToEntry.containsKey(remoteId)) {
-						strings.add(" - " + remoteId + " (missing on local)");
-					}
-				}
-
-				for (Identifier localId : getIds()) {
-					if (!remoteIndexedEntries.containsKey(localId)) {
-						strings.add(" - " + localId + " (missing on remote)");
-					}
-				}
-
-				StringBuilder builder = new StringBuilder("Local and remote ID sets for " + name + " do not match!");
 
 				for (String s : strings) {
 					builder.append('\n').append(s);
@@ -350,7 +320,7 @@ public abstract class SimpleRegistryMixin<T> implements MutableRegistry<T>, Rema
 	}
 
 	@Override
-	public void unmap(String name) throws RemapException {
+	public void unmap() throws RemapException {
 		if (fabric_prevIndexedEntries != null) {
 			List<Identifier> addedIds = new ArrayList<>();
 
@@ -372,7 +342,7 @@ public abstract class SimpleRegistryMixin<T> implements MutableRegistry<T>, Rema
 				keyToEntry.put(entryKey, entry.getValue());
 			}
 
-			remap(name, fabric_prevIndexedEntries, RemapMode.AUTHORITATIVE);
+			remap(fabric_prevIndexedEntries, RemapMode.AUTHORITATIVE);
 
 			for (Identifier id : addedIds) {
 				fabric_getAddObjectEvent().invoker().onEntryAdded(entryToRawId.getInt(idToEntry.get(id)), id, get(id));

fabric-registry-sync-v0/src/main/resources/assets/fabric-registry-sync-v0/lang/en_us.json

@@ -3,5 +3,10 @@
   "fabric-registry-sync-v0.unknown-remote.title.plural" : "Received %d registry entries that are unknown to this client.\n",
   "fabric-registry-sync-v0.unknown-remote.subtitle.1" : "This is usually caused by a mismatched mod set between the client and server.",
   "fabric-registry-sync-v0.unknown-remote.subtitle.2" : " See the client logs for more details.\nThe following registry entry namespaces may be related:\n\n",
-  "fabric-registry-sync-v0.unknown-remote.footer" : "And %d more..."
+  "fabric-registry-sync-v0.unknown-remote.footer" : "And %d more...",
+  "fabric-registry-sync-v0.unknown-registry.title.singular" : "Received a registry that is unknown to this client.\n",
+  "fabric-registry-sync-v0.unknown-registry.title.plural" : "Received %d registries that are unknown to this client.\n",
+  "fabric-registry-sync-v0.unknown-registry.subtitle.1" : "This is usually caused by a mismatched mod set between the client and server.",
+  "fabric-registry-sync-v0.unknown-registry.subtitle.2" : " See the client logs for more details.\nThe following registries are not present on the client:\n\n",
+  "fabric-registry-sync-v0.unknown-registry.footer" : "And %d more..."
 }

fabric-registry-sync-v0/src/test/java/net/fabricmc/fabric/test/registry/sync/DirectRegistryPacketHandlerTest.java

@@ -24,13 +24,22 @@ import java.util.Map;
 
 import it.unimi.dsi.fastutil.objects.Object2IntMap;
 import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap;
+import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 
+import net.minecraft.Bootstrap;
+import net.minecraft.SharedConstants;
 import net.minecraft.util.Identifier;
 
 import net.fabricmc.fabric.impl.registry.sync.packet.DirectRegistryPacketHandler;
 
 public class DirectRegistryPacketHandlerTest {
+	@BeforeAll
+	static void beforeAll() {
+		SharedConstants.createGameVersion();
+		Bootstrap.initialize();
+	}
+
 	@Test
 	void emptyRegistrySync() {
 		DirectRegistryPacketHandler handler = new DirectRegistryPacketHandler();
@@ -48,7 +57,7 @@ public class DirectRegistryPacketHandlerTest {
 			handler.receivePayload(payload);
 		}
 
-		assertMatchesDeep(registry, handler.getSyncedRegistryMap());
+		assertMatchesDeep(registry, handler.getSyncedPacketData().idMap());
 	}
 
 	@Test
@@ -69,7 +78,7 @@ public class DirectRegistryPacketHandlerTest {
 			handler.receivePayload(payload);
 		}
 
-		assertMatchesDeep(registry, handler.getSyncedRegistryMap());
+		assertMatchesDeep(registry, handler.getSyncedPacketData().idMap());
 	}
 
 	@Test
@@ -93,7 +102,7 @@ public class DirectRegistryPacketHandlerTest {
 			handler.receivePayload(payload);
 		}
 
-		assertMatchesDeep(registry, handler.getSyncedRegistryMap());
+		assertMatchesDeep(registry, handler.getSyncedPacketData().idMap());
 	}
 
 	private static Object2IntMap<Identifier> createRegistry(int size) {

fabric-registry-sync-v0/src/test/java/net/fabricmc/fabric/test/registry/sync/RegistryRemapTest.java

@@ -0,0 +1,310 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 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.test.registry.sync;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.UUID;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionException;
+import java.util.concurrent.ExecutionException;
+
+import it.unimi.dsi.fastutil.objects.Object2IntMap;
+import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import net.minecraft.Bootstrap;
+import net.minecraft.SharedConstants;
+import net.minecraft.registry.Registry;
+import net.minecraft.registry.RegistryKey;
+import net.minecraft.registry.SimpleRegistry;
+import net.minecraft.util.Identifier;
+import net.minecraft.util.thread.ThreadExecutor;
+
+import net.fabricmc.fabric.api.event.registry.FabricRegistryBuilder;
+import net.fabricmc.fabric.api.event.registry.RegistryAttribute;
+import net.fabricmc.fabric.api.event.registry.RegistryAttributeHolder;
+import net.fabricmc.fabric.impl.client.registry.sync.ClientRegistrySyncHandler;
+import net.fabricmc.fabric.impl.registry.sync.RegistryAttributeImpl;
+import net.fabricmc.fabric.impl.registry.sync.RegistrySyncManager;
+import net.fabricmc.fabric.impl.registry.sync.RemapException;
+import net.fabricmc.fabric.impl.registry.sync.RemappableRegistry;
+import net.fabricmc.fabric.impl.registry.sync.packet.DirectRegistryPacketHandler;
+
+public class RegistryRemapTest {
+	private RegistryKey<Registry<String>> testRegistryKey;
+	private SimpleRegistry<String> testRegistry;
+
+	@BeforeAll
+	static void beforeAll() {
+		SharedConstants.createGameVersion();
+		Bootstrap.initialize();
+	}
+
+	@BeforeEach
+	void beforeEach() {
+		testRegistryKey = RegistryKey.ofRegistry(id(UUID.randomUUID().toString()));
+		testRegistry = FabricRegistryBuilder.createSimple(testRegistryKey)
+				.attribute(RegistryAttribute.SYNCED)
+				.buildAndRegister();
+
+		Registry.register(testRegistry, id("zero"), "zero");
+		Registry.register(testRegistry, id("one"), "one");
+		Registry.register(testRegistry, id("two"), "two");
+	}
+
+	@AfterEach
+	void afterEach() throws RemapException {
+		// If a test fails, make sure we unmap the registry to avoid affecting other tests
+		RemappableRegistry remappableRegistry = (RemappableRegistry) testRegistry;
+		remappableRegistry.unmap();
+	}
+
+	@Test
+	void remapRegistry() throws RemapException {
+		RemappableRegistry remappableRegistry = (RemappableRegistry) testRegistry;
+
+		assertEquals(0, testRegistry.getRawId("zero"));
+		assertEquals(1, testRegistry.getRawId("one"));
+		assertEquals(2, testRegistry.getRawId("two"));
+
+		Map<Identifier, Integer> idMap = Map.of(
+				id("zero"), 2,
+				id("one"), 1,
+				id("two"), 0
+		);
+		remappableRegistry.remap(asFastMap(idMap), RemappableRegistry.RemapMode.AUTHORITATIVE);
+
+		assertEquals(2, testRegistry.getRawId("zero"));
+		assertEquals(1, testRegistry.getRawId("one"));
+		assertEquals(0, testRegistry.getRawId("two"));
+
+		remappableRegistry.unmap();
+
+		assertEquals(0, testRegistry.getRawId("zero"));
+		assertEquals(1, testRegistry.getRawId("one"));
+		assertEquals(2, testRegistry.getRawId("two"));
+	}
+
+	@Test
+	void remapRegistryViaPacket() throws RemapException {
+		RemappableRegistry remappableRegistry = (RemappableRegistry) testRegistry;
+
+		assertEquals(0, testRegistry.getRawId("zero"));
+		assertEquals(1, testRegistry.getRawId("one"));
+		assertEquals(2, testRegistry.getRawId("two"));
+
+		Map<Identifier, Integer> idMap = Map.of(
+				id("two"), 0,
+				id("one"), 1,
+				id("zero"), 2
+		);
+
+		var payloads = new ArrayList<DirectRegistryPacketHandler.Payload>();
+
+		RegistrySyncManager.DIRECT_PACKET_HANDLER.sendPacket(
+				payloads::add,
+				Map.of(testRegistryKey.getValue(), asFastMap(idMap))
+		);
+
+		List<Boolean> results = receivePayloads(payloads);
+
+		// Expect 2 packets, 1 with the data (as it fits in one packet) and 1 empty packet to signal the end
+		assertEquals(2, results.size());
+		assertFalse(results.getFirst());
+		assertTrue(results.get(1));
+
+		assertEquals(2, testRegistry.getRawId("zero"));
+		assertEquals(1, testRegistry.getRawId("one"));
+		assertEquals(0, testRegistry.getRawId("two"));
+
+		remappableRegistry.unmap();
+
+		assertEquals(0, testRegistry.getRawId("zero"));
+		assertEquals(1, testRegistry.getRawId("one"));
+		assertEquals(2, testRegistry.getRawId("two"));
+	}
+
+	@Test
+	void unknownEntry() {
+		Map<Identifier, Integer> idMap = Map.of(
+				id("two"), 0,
+				id("one"), 1,
+				id("zero"), 2,
+				id("unknown"), 3
+		);
+
+		var payloads = new ArrayList<DirectRegistryPacketHandler.Payload>();
+
+		RegistrySyncManager.DIRECT_PACKET_HANDLER.sendPacket(
+				payloads::add,
+				Map.of(testRegistryKey.getValue(), asFastMap(idMap))
+		);
+
+		RemapException remapException = assertThrows(RemapException.class, () -> receivePayloads(payloads));
+		assertTrue(remapException.getMessage().contains("unknown-remote"));
+	}
+
+	@Test
+	void unknownRegistry() {
+		Map<Identifier, Integer> idMap = Map.of(
+				id("two"), 0,
+				id("one"), 1,
+				id("zero"), 2
+		);
+
+		var payloads = new ArrayList<DirectRegistryPacketHandler.Payload>();
+
+		RegistrySyncManager.DIRECT_PACKET_HANDLER.sendPacket(
+				payloads::add,
+				Map.of(id("unknown"), asFastMap(idMap))
+		);
+
+		RemapException remapException = assertThrows(RemapException.class, () -> receivePayloads(payloads));
+		assertTrue(remapException.getMessage().contains("unknown-registry"));
+	}
+
+	@Test
+	void unknownOptionalRegistry() throws RemapException {
+		Map<Identifier, Integer> idMap = Map.of(
+				id("two"), 0,
+				id("one"), 1,
+				id("zero"), 2
+		);
+
+		RegistryAttributeImpl holder = (RegistryAttributeImpl) RegistryAttributeHolder.get(testRegistryKey);
+		holder.addAttribute(RegistryAttribute.OPTIONAL);
+
+		var payloads = new ArrayList<DirectRegistryPacketHandler.Payload>();
+
+		RegistrySyncManager.DIRECT_PACKET_HANDLER.sendPacket(
+				payloads::add,
+				Map.of(testRegistryKey.getValue(), asFastMap(idMap))
+		);
+
+		// Packet should be handled without issue.
+		List<Boolean> results = receivePayloads(payloads);
+		assertEquals(2, results.size());
+		assertFalse(results.getFirst());
+		assertTrue(results.get(1));
+
+		holder.removeAttribute(RegistryAttribute.OPTIONAL);
+	}
+
+	@Test
+	void missingRemoteEntries() throws RemapException {
+		RemappableRegistry remappableRegistry = (RemappableRegistry) testRegistry;
+
+		assertEquals(0, testRegistry.getRawId("zero"));
+		assertEquals(1, testRegistry.getRawId("one"));
+		assertEquals(2, testRegistry.getRawId("two"));
+
+		Map<Identifier, Integer> idMap = Map.of(
+				id("two"), 0,
+				id("zero"), 1
+		);
+
+		var payloads = new ArrayList<DirectRegistryPacketHandler.Payload>();
+
+		RegistrySyncManager.DIRECT_PACKET_HANDLER.sendPacket(
+				payloads::add,
+				Map.of(testRegistryKey.getValue(), asFastMap(idMap))
+		);
+
+		receivePayloads(payloads);
+
+		assertEquals(0, testRegistry.getRawId("two"));
+		assertEquals(1, testRegistry.getRawId("zero"));
+		// assigned an ID at the end of the registry
+		assertEquals(2, testRegistry.getRawId("one"));
+
+		remappableRegistry.unmap();
+
+		assertEquals(0, testRegistry.getRawId("zero"));
+		assertEquals(1, testRegistry.getRawId("one"));
+		assertEquals(2, testRegistry.getRawId("two"));
+	}
+
+	private static List<Boolean> receivePayloads(List<DirectRegistryPacketHandler.Payload> payloads) throws RemapException {
+		var results = new ArrayList<Boolean>();
+
+		try {
+			for (DirectRegistryPacketHandler.Payload payload : payloads) {
+				CompletableFuture<Boolean> future = ClientRegistrySyncHandler.receivePacket(
+						ThisThreadExecutor.INSTANCE,
+						RegistrySyncManager.DIRECT_PACKET_HANDLER,
+						payload,
+						true
+				);
+				results.add(future.get());
+			}
+		} catch (CompletionException e) {
+			if (e.getCause() instanceof RemapException remapException) {
+				throw remapException;
+			}
+
+			throw e;
+		} catch (ExecutionException | InterruptedException e) {
+			throw new RuntimeException(e);
+		}
+
+		return results;
+	}
+
+	private static Object2IntMap<Identifier> asFastMap(Map<Identifier, Integer> map) {
+		var fastMap = new Object2IntOpenHashMap<Identifier>();
+		fastMap.putAll(map);
+		return fastMap;
+	}
+
+	private static Identifier id(String path) {
+		return Identifier.of("registry_sync_test", path);
+	}
+
+	// Run the task on the current thread instantly
+	private static class ThisThreadExecutor extends ThreadExecutor<Runnable> {
+		public static final ThisThreadExecutor INSTANCE = new ThisThreadExecutor();
+
+		private ThisThreadExecutor() {
+			super("Test thread executor");
+		}
+
+		@Override
+		protected boolean canExecute(Runnable task) {
+			return true;
+		}
+
+		@Override
+		protected Thread getThread() {
+			return Thread.currentThread();
+		}
+
+		@Override
+		public Runnable createTask(Runnable runnable) {
+			return runnable;
+		}
+	}
+}

fabric-registry-sync-v0/src/testmod/java/net/fabricmc/fabric/test/registry/sync/RegistrySyncTest.java

@@ -18,13 +18,9 @@ package net.fabricmc.fabric.test.registry.sync;
 
 import java.util.ArrayList;
 import java.util.List;
-import java.util.Map;
-import java.util.Objects;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 import com.mojang.logging.LogUtils;
-import it.unimi.dsi.fastutil.objects.Object2IntMap;
-import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap;
 import org.apache.commons.lang3.Validate;
 import org.slf4j.Logger;
 
@@ -37,19 +33,14 @@ import net.minecraft.registry.Registry;
 import net.minecraft.registry.RegistryKey;
 import net.minecraft.registry.RegistryKeys;
 import net.minecraft.registry.SimpleRegistry;
-import net.minecraft.server.command.CommandManager;
-import net.minecraft.server.network.ServerPlayerEntity;
 import net.minecraft.util.Identifier;
 
 import net.fabricmc.api.ModInitializer;
-import net.fabricmc.fabric.api.command.v2.CommandRegistrationCallback;
 import net.fabricmc.fabric.api.event.lifecycle.v1.ServerLifecycleEvents;
 import net.fabricmc.fabric.api.event.registry.DynamicRegistrySetupCallback;
 import net.fabricmc.fabric.api.event.registry.FabricRegistryBuilder;
 import net.fabricmc.fabric.api.event.registry.RegistryAttribute;
 import net.fabricmc.fabric.api.event.registry.RegistryAttributeHolder;
-import net.fabricmc.fabric.impl.registry.sync.RegistrySyncManager;
-import net.fabricmc.fabric.impl.registry.sync.RemapException;
 
 public class RegistrySyncTest implements ModInitializer {
 	private static final Logger LOGGER = LogUtils.getLogger();
@@ -112,28 +103,6 @@ public class RegistrySyncTest implements ModInitializer {
 
 		// Vanilla status effects don't have an entry for the int id 0, test we can handle this.
 		RegistryAttributeHolder.get(Registries.STATUS_EFFECT).addAttribute(RegistryAttribute.MODDED);
-
-		CommandRegistrationCallback.EVENT.register((dispatcher, registryAccess, environment) ->
-				dispatcher.register(CommandManager.literal("remote_remap_error_test").executes(context -> {
-					Map<Identifier, Object2IntMap<Identifier>> registryData = Map.of(
-							RegistryKeys.BLOCK.getValue(), createFakeRegistryEntries(),
-							RegistryKeys.ITEM.getValue(), createFakeRegistryEntries()
-					);
-
-					try {
-						RegistrySyncManager.checkRemoteRemap(registryData);
-					} catch (RemapException e) {
-						final ServerPlayerEntity player = context.getSource().getPlayer();
-
-						if (player != null) {
-							player.networkHandler.disconnect(Objects.requireNonNull(e.getText()));
-						}
-
-						return 1;
-					}
-
-					throw new IllegalStateException();
-				})));
 	}
 
 	public static void checkSyncedRegistry(RegistryKey<? extends Registry<?>> registry) {
@@ -169,14 +138,4 @@ public class RegistrySyncTest implements ModInitializer {
 			}
 		}
 	}
-
-	private static Object2IntMap<Identifier> createFakeRegistryEntries() {
-		Object2IntMap<Identifier> map = new Object2IntOpenHashMap<>();
-
-		for (int i = 0; i < 12; i++) {
-			map.put(Identifier.of("mod_" + i, "entry"), 0);
-		}
-
-		return map;
-	}
 }

fabric-registry-sync-v0/src/testmod/resources/fabric.mod.json

@@ -17,7 +17,8 @@
       "net.fabricmc.fabric.test.registry.sync.RegistrySyncTest"
     ],
     "client": [
-      "net.fabricmc.fabric.test.registry.sync.client.DynamicRegistryClientTest"
+      "net.fabricmc.fabric.test.registry.sync.client.DynamicRegistryClientTest",
+      "net.fabricmc.fabric.test.registry.sync.client.RegistrySyncClientTest"
     ]
   }
 }

fabric-registry-sync-v0/src/testmodClient/java/net/fabricmc/fabric/test/registry/sync/client/RegistrySyncClientTest.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 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.test.registry.sync.client;
+
+import java.util.EnumSet;
+import java.util.Map;
+import java.util.Objects;
+
+import it.unimi.dsi.fastutil.objects.Object2IntMap;
+import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap;
+
+import net.minecraft.registry.RegistryKeys;
+import net.minecraft.server.command.CommandManager;
+import net.minecraft.server.network.ServerPlayerEntity;
+import net.minecraft.util.Identifier;
+
+import net.fabricmc.api.ClientModInitializer;
+import net.fabricmc.fabric.api.command.v2.CommandRegistrationCallback;
+import net.fabricmc.fabric.api.event.registry.RegistryAttribute;
+import net.fabricmc.fabric.impl.client.registry.sync.ClientRegistrySyncHandler;
+import net.fabricmc.fabric.impl.registry.sync.RemapException;
+import net.fabricmc.fabric.impl.registry.sync.packet.RegistryPacketHandler;
+
+public class RegistrySyncClientTest implements ClientModInitializer {
+	@Override
+	public void onInitializeClient() {
+		CommandRegistrationCallback.EVENT.register((dispatcher, registryAccess, environment) ->
+				dispatcher.register(CommandManager.literal("remote_remap_error_test").executes(context -> {
+					Map<Identifier, Object2IntMap<Identifier>> registryData = Map.of(
+							RegistryKeys.BLOCK.getValue(), createFakeRegistryEntries(),
+							RegistryKeys.ITEM.getValue(), createFakeRegistryEntries()
+					);
+					Map<Identifier, EnumSet<RegistryAttribute>> attributes = Map.of(
+							RegistryKeys.BLOCK.getValue(), EnumSet.noneOf(RegistryAttribute.class),
+							RegistryKeys.ITEM.getValue(), EnumSet.noneOf(RegistryAttribute.class)
+					);
+
+					try {
+						ClientRegistrySyncHandler.checkRemoteRemap(new RegistryPacketHandler.SyncedPacketData(registryData, attributes));
+					} catch (RemapException e) {
+						final ServerPlayerEntity player = context.getSource().getPlayer();
+
+						if (player != null) {
+							player.networkHandler.disconnect(Objects.requireNonNull(e.getText()));
+						}
+
+						return 1;
+					}
+
+					throw new IllegalStateException();
+				})));
+	}
+
+	private static Object2IntMap<Identifier> createFakeRegistryEntries() {
+		Object2IntMap<Identifier> map = new Object2IntOpenHashMap<>();
+
+		for (int i = 0; i < 12; i++) {
+			map.put(Identifier.of("mod_" + i, "entry"), 0);
+		}
+
+		return map;
+	}
+}