Make default Entity.getAddPacket automatically use Forge's enhanced spawn packet if nessasary.

This spawn packet needs to be re-designed, it is effectively only there to hook into creating the new entity instance. This will be re-evaluated during the cleanup in 1.21+

src/main/java/net/minecraftforge/common/ForgeHooks.java

@@ -30,6 +30,7 @@ import com.mojang.serialization.Dynamic;
 import com.mojang.serialization.DynamicOps;
 import com.mojang.serialization.Lifecycle;
 
+import io.netty.buffer.Unpooled;
 import it.unimi.dsi.fastutil.objects.ObjectArrayList;
 import net.minecraft.commands.CommandSourceStack;
 import net.minecraft.commands.Commands;
@@ -42,6 +43,7 @@ import net.minecraft.core.HolderSet;
 import net.minecraft.core.particles.ParticleTypes;
 import net.minecraft.core.registries.Registries;
 import net.minecraft.network.Connection;
+import net.minecraft.network.FriendlyByteBuf;
 import net.minecraft.network.chat.ClickEvent;
 import net.minecraft.network.chat.Component;
 import net.minecraft.network.chat.MutableComponent;
@@ -96,6 +98,7 @@ import net.minecraft.nbt.CompoundTag;
 import net.minecraft.nbt.ListTag;
 import net.minecraft.network.protocol.Packet;
 import net.minecraft.network.syncher.EntityDataSerializer;
+import net.minecraft.network.protocol.game.ClientGamePacketListener;
 import net.minecraft.network.protocol.game.ClientboundBlockUpdatePacket;
 import net.minecraft.network.protocol.game.ServerboundPlayerActionPacket;
 import net.minecraft.world.item.alchemy.Potion;
@@ -116,6 +119,7 @@ import net.minecraftforge.common.util.BrainBuilder;
 import net.minecraftforge.common.util.Lazy;
 import net.minecraftforge.common.util.MavenVersionStringHelper;
 import net.minecraftforge.common.util.MutableHashedLinkedMap;
+import net.minecraftforge.entity.IEntityAdditionalSpawnData;
 import net.minecraftforge.event.AnvilUpdateEvent;
 import net.minecraftforge.event.BuildCreativeModeTabContentsEvent;
 import net.minecraftforge.event.DifficultyChangeEvent;
@@ -166,7 +170,10 @@ import net.minecraftforge.fml.util.thread.EffectiveSide;
 import net.minecraftforge.network.ConnectionType;
 import net.minecraftforge.network.ICustomPacket;
 import net.minecraftforge.network.NetworkContext;
+import net.minecraftforge.network.NetworkDirection;
+import net.minecraftforge.network.NetworkInitialization;
 import net.minecraftforge.network.NetworkRegistry;
+import net.minecraftforge.network.packets.SpawnEntity;
 import net.minecraftforge.resource.ResourcePackLoader;
 import net.minecraftforge.registries.ForgeRegistries;
 import net.minecraftforge.registries.ForgeRegistry;
@@ -1251,4 +1258,16 @@ public class ForgeHooks {
             LOGGER.info("Connected to a modded server.");
     }
 
+    @ApiStatus.Internal
+    public static Packet<ClientGamePacketListener> getEntitySpawnPacket(Entity entity) {
+        if (!(entity instanceof IEntityAdditionalSpawnData add))
+            throw new IllegalArgumentException(entity.getClass() + " is not an instance of " + IEntityAdditionalSpawnData.class);
+
+        var play = NetworkInitialization.PLAY;
+        var msg = new SpawnEntity(entity);
+        var data = play.toBuffer(msg);
+        @SuppressWarnings("unchecked")
+        var pkt = (Packet<ClientGamePacketListener>)NetworkDirection.PLAY_TO_CLIENT.buildPacket(data, play.getName()).getThis();
+        return pkt;
+    }
 }

src/main/java/net/minecraftforge/network/Channel.java

@@ -38,7 +38,7 @@ public abstract class Channel<MSG> {
         return NetworkContext.get(connection).getRemoteChannels().contains(getName());
     }
 
-    protected abstract FriendlyByteBuf toBuffer(MSG message);
+    public abstract FriendlyByteBuf toBuffer(MSG message);
 
     // Package private so we can call from ourselves.
     Packet<?> toVanillaPacket(Connection connection, MSG message) {

src/main/java/net/minecraftforge/network/EventNetworkChannel.java

@@ -42,7 +42,7 @@ public class EventNetworkChannel extends Channel<FriendlyByteBuf> {
     }
 
     @Override
-    protected FriendlyByteBuf toBuffer(FriendlyByteBuf message) {
+    public FriendlyByteBuf toBuffer(FriendlyByteBuf message) {
         return message;
     }
 }

src/main/java/net/minecraftforge/network/NetworkInitialization.java

@@ -94,16 +94,16 @@ public class NetworkInitialization {
             .consumerNetworkThread(CONTEXT, ForgePacketHandler::handleModMismatchData)
             .add()
 
-        .messageBuilder(SpawnEntity.class)
+        .messageBuilder(SpawnEntity.class, NetworkDirection.PLAY_TO_CLIENT)
            .decoder(SpawnEntity::decode)
            .encoder(SpawnEntity::encode)
-           .consumerNetworkThread(SpawnEntity::handle)
+           .consumerMainThread(SpawnEntity::handle)
            .add()
 
        .messageBuilder(OpenContainer.class)
            .decoder(OpenContainer::decode)
            .encoder(OpenContainer::encode)
-           .consumerNetworkThread(OpenContainer::handle)
+           .consumerMainThread(OpenContainer::handle)
            .add();
 
     public static void init() {

src/main/java/net/minecraftforge/network/SimpleChannel.java

@@ -334,7 +334,7 @@ public class SimpleChannel extends Channel<Object> {
 
     @SuppressWarnings("unchecked")
     @Override
-    protected FriendlyByteBuf toBuffer(Object message) {
+    public FriendlyByteBuf toBuffer(Object message) {
         var msg = byType.get(message.getClass());
 
         if (msg == null) {

src/main/java/net/minecraftforge/network/packets/OpenContainer.java

@@ -10,8 +10,6 @@ import org.jetbrains.annotations.ApiStatus;
 import io.netty.buffer.Unpooled;
 import net.minecraft.client.Minecraft;
 import net.minecraft.client.gui.screens.MenuScreens;
-import net.minecraft.client.gui.screens.Screen;
-import net.minecraft.client.gui.screens.inventory.MenuAccess;
 import net.minecraft.core.registries.BuiltInRegistries;
 import net.minecraft.network.FriendlyByteBuf;
 import net.minecraft.network.chat.Component;
@@ -55,23 +53,20 @@ public class OpenContainer {
 
     @SuppressWarnings("unchecked")
     public static void handle(OpenContainer msg, CustomPayloadEvent.Context ctx) {
-        ctx.enqueueWork(() -> {
-            try {
-                var mc = Minecraft.getInstance();
-                var inv = mc.player.getInventory();
-                MenuScreens.getScreenFactory(msg.getType(), mc, msg.getWindowId(), msg.getName()).ifPresent(f -> {
-                    AbstractContainerMenu c = msg.getType().create(msg.getWindowId(), inv, msg.getAdditionalData());
+        try {
+            var mc = Minecraft.getInstance();
+            var inv = mc.player.getInventory();
+            var factory = MenuScreens.getScreenFactory(msg.getType(), mc, msg.getWindowId(), msg.getName());
+            factory.ifPresent(f -> {
+                var c = msg.getType().create(msg.getWindowId(), inv, msg.getAdditionalData());
 
-                    Screen s = ((MenuScreens.ScreenConstructor<AbstractContainerMenu, ?>)f).create(c, inv, msg.getName());
-                    mc.player.containerMenu = ((MenuAccess<?>)s).getMenu();
-                    mc.setScreen(s);
-                });
-            } finally {
-                msg.getAdditionalData().release();
-            }
-
-        });
-        ctx.setPacketHandled(true);
+                var s = ((MenuScreens.ScreenConstructor<AbstractContainerMenu, ?>)f).create(c, inv, msg.getName());
+                mc.player.containerMenu = s.getMenu();
+                mc.setScreen(s);
+            });
+        } finally {
+            msg.getAdditionalData().release();
+        }
     }
 
     @SuppressWarnings("deprecation")

src/main/java/net/minecraftforge/network/packets/SpawnEntity.java

@@ -8,6 +8,8 @@ package net.minecraftforge.network.packets;
 import java.util.Optional;
 import java.util.UUID;
 
+import org.jetbrains.annotations.ApiStatus;
+
 import io.netty.buffer.Unpooled;
 import net.minecraft.client.multiplayer.ClientLevel;
 import net.minecraft.core.registries.BuiltInRegistries;
@@ -30,6 +32,7 @@ import net.minecraftforge.event.network.CustomPayloadEvent;
  * {@link EntityType})
  * see {@link EntityType.Builder#setCustomClientFactory}.
  */
+// TODO: Re-write this packet into something simpler. This is literally just ClientboundAddEntityPacket with an extra byte[]
 public class SpawnEntity {
     private final Entity entity;
     private final int typeId;
@@ -41,7 +44,8 @@ public class SpawnEntity {
     private final FriendlyByteBuf buf;
 
     @SuppressWarnings("deprecation")
-    SpawnEntity(Entity e) {
+    @ApiStatus.Internal
+    public SpawnEntity(Entity e) {
         this.entity = e;
         this.typeId = BuiltInRegistries.ENTITY_TYPE.getId(e.getType()); //TODO: Codecs
         this.entityId = e.getId();
@@ -123,36 +127,33 @@ public class SpawnEntity {
     }
 
     public static void handle(SpawnEntity msg, CustomPayloadEvent.Context ctx) {
-        ctx.enqueueWork(() -> {
-            try {
-                @SuppressWarnings("deprecation")
-                EntityType<?> type = BuiltInRegistries.ENTITY_TYPE.byId(msg.typeId);
-                Optional<Level> world = LogicalSidedProvider.CLIENTWORLD.get(ctx.getDirection().getReceptionSide());
-                Entity e = world.map(w -> type.customClientSpawn(msg, w)).orElse(null);
-                if (e == null)
-                    return;
+        try {
+            @SuppressWarnings("deprecation")
+            EntityType<?> type = BuiltInRegistries.ENTITY_TYPE.byId(msg.typeId);
+            Optional<Level> world = LogicalSidedProvider.CLIENTWORLD.get(ctx.getDirection().getReceptionSide());
+            Entity e = world.map(w -> type.customClientSpawn(msg, w)).orElse(null);
+            if (e == null)
+                return;
 
-                /*
-                 * Sets the postiion on the client, Mirrors what
-                 * Entity#recreateFromPacket and LivingEntity#recreateFromPacket does.
-                 */
-                e.syncPacketPositionCodec(msg.posX, msg.posY, msg.posZ);
-                e.absMoveTo(msg.posX, msg.posY, msg.posZ, (msg.yaw * 360) / 256.0F, (msg.pitch * 360) / 256.0F);
-                e.setYHeadRot((msg.headYaw * 360) / 256.0F);
-                e.setYBodyRot((msg.headYaw * 360) / 256.0F);
+            /*
+             * Sets the postiion on the client, Mirrors what
+             * Entity#recreateFromPacket and LivingEntity#recreateFromPacket does.
+             */
+            e.syncPacketPositionCodec(msg.posX, msg.posY, msg.posZ);
+            e.absMoveTo(msg.posX, msg.posY, msg.posZ, (msg.yaw * 360) / 256.0F, (msg.pitch * 360) / 256.0F);
+            e.setYHeadRot((msg.headYaw * 360) / 256.0F);
+            e.setYBodyRot((msg.headYaw * 360) / 256.0F);
 
-                e.setId(msg.entityId);
-                e.setUUID(msg.uuid);
-                if (world.orElse(null) instanceof ClientLevel cworld)
-                    cworld.addEntity(e);
-                e.lerpMotion(msg.velX / 8000.0, msg.velY / 8000.0, msg.velZ / 8000.0);
-                if (e instanceof IEntityAdditionalSpawnData entityAdditionalSpawnData)
-                    entityAdditionalSpawnData.readSpawnData(msg.buf);
-            } finally {
-                msg.buf.release();
-            }
-        });
-        ctx.setPacketHandled(true);
+            e.setId(msg.entityId);
+            e.setUUID(msg.uuid);
+            if (world.orElse(null) instanceof ClientLevel cworld)
+                cworld.addEntity(e);
+            e.lerpMotion(msg.velX / 8000.0, msg.velY / 8000.0, msg.velZ / 8000.0);
+            if (e instanceof IEntityAdditionalSpawnData entityAdditionalSpawnData)
+                entityAdditionalSpawnData.readSpawnData(msg.buf);
+        } finally {
+            msg.buf.release();
+        }
     }
 
     public Entity getEntity() {