Workaround for MinecraftForge#7519

arclight-common/src/main/java/io/izzel/arclight/common/mixin/core/world/server/ServerWorldMixin.java

@@ -164,6 +164,16 @@ public abstract class ServerWorldMixin extends WorldMixin implements ServerWorld
         return this.convertable;
     }
 
+    @Inject(method = "addEntityIfNotDuplicate", at = @At("HEAD"))
+    private void arclight$worldGenAddStart(Entity entityIn, CallbackInfoReturnable<Boolean> cir) {
+        ArclightCaptures.captureWorldGenAddEntity();
+    }
+
+    @Inject(method = "addEntityIfNotDuplicate", at = @At("RETURN"))
+    private void arclight$worldGenAddEnd(Entity entityIn, CallbackInfoReturnable<Boolean> cir) {
+        ArclightCaptures.resetWorldGenAddEntity();
+    }
+
     @Inject(method = "onEntityAdded", at = @At("RETURN"))
     private void arclight$validEntity(Entity entityIn, CallbackInfo ci) {
         ((EntityBridge) entityIn).bridge$setValid(true);

arclight-common/src/main/java/io/izzel/arclight/common/mixin/forge/ForgeInternalHandlerMixin.java

@@ -0,0 +1,22 @@
+package io.izzel.arclight.common.mixin.forge;
+
+import io.izzel.arclight.common.mod.util.ArclightCaptures;
+import net.minecraft.entity.Entity;
+import net.minecraft.world.World;
+import net.minecraft.world.server.ServerWorld;
+import net.minecraftforge.common.ForgeInternalHandler;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Redirect;
+
+@Mixin(ForgeInternalHandler.class)
+public class ForgeInternalHandlerMixin {
+
+    // Workaround for MinecraftForge#7519
+    @Redirect(method = "*", at = @At(value = "INVOKE", target = "Lnet/minecraft/world/World;addEntity(Lnet/minecraft/entity/Entity;)Z"))
+    private boolean arclight$addDuringWorldGen(World world, Entity entityIn) {
+        return world instanceof ServerWorld && ArclightCaptures.isWorldGenAdd()
+            ? ((ServerWorld) world).addEntityIfNotDuplicate(entityIn)
+            : world.addEntity(entityIn);
+    }
+}

arclight-common/src/main/java/io/izzel/arclight/common/mod/util/ArclightCaptures.java

@@ -230,6 +230,20 @@ public class ArclightCaptures {
         }
     }
 
+    private static transient boolean worldGenAdd;
+
+    public static void captureWorldGenAddEntity() {
+        worldGenAdd = true;
+    }
+
+    public static boolean isWorldGenAdd() {
+        return worldGenAdd;
+    }
+
+    public static void resetWorldGenAddEntity() {
+        worldGenAdd = false;
+    }
+
     private static void recapture(String type) {
         throw new IllegalStateException("Recapturing " + type);
     }

arclight-common/src/main/resources/mixins.arclight.forge.json

@@ -7,6 +7,7 @@
   "setSourceFile": true,
   "mixins": [
     "ForgeEventFactoryMixin",
-    "ForgeHooksMixin"
+    "ForgeHooksMixin",
+    "ForgeInternalHandlerMixin"
   ]
 }