From 9dc4ad8cef4b26e7d210716d9c96fb367193b6b6 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Fri, 3 Aug 2018 22:47:46 -0400
Subject: [PATCH] Entity add to world fixes

1) Chunk Registration might kill an entity, don't add it to the world if it did!

2) By default, entities are added to the world per slice iteration.
This opens risk of the slices being manipulated during chunk add if an
EntityAddToWorldEvent spawns an entity into this chunk.
Fix this by differing entity add to world for all entities at the same time

3) If a duplicate entity is attempted to add to the world of an entity, and
the original entity is dead, overwrite it as the logic does for unloaod queued entities.

diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
index 7b6b8f4824..eed37198b0 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -898,6 +898,7 @@ public class Chunk implements IChunkAccess {
         this.world.a(this.tileEntities.values());
         List[] aentityslice = this.entitySlices; // Spigot
         int i = aentityslice.length;
+        List<Entity> toAdd = new java.util.ArrayList<>(32); // Paper
 
         for (int j = 0; j < i; ++j) {
             List entityslice = aentityslice[j]; // Spigot
@@ -945,12 +946,11 @@ public class Chunk implements IChunkAccess {
                     thisChunk.put(entity.uniqueID, entity);
                 }
             }
-            // Paper end
 
-            this.world.a(entityslice.stream().filter((entity) -> {
-                return !(entity instanceof EntityHuman);
-            }));
+            toAdd.addAll(entityslice);
+            // Paper end
         }
+        this.world.addChunkEntities(toAdd.stream().filter((entity) -> !(entity instanceof EntityHuman))); // Paper - add all at same time to avoid entities adding to world modifying slice state
 
         // CraftBukkit start
         org.bukkit.Server server = this.world.getServer();
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index 26007862ab..4ea31bc201 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -1089,6 +1089,7 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
             }
 
             this.getChunkAt(i, j).a(entity);
+            if (entity.dead) return false; // Paper - don't add dead entities, chunk registration may of killed it
             this.entityList.add(entity);
             this.b(entity);
             return true;
@@ -2494,9 +2495,13 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
         return j;
     }
 
+    public void addChunkEntities(Stream<Entity> collection) { a(collection); } // Paper - OBFHELPER
     public void a(Stream<Entity> stream) {
         org.spigotmc.AsyncCatcher.catchOp( "entity world add"); // Spigot
         stream.forEach((entity) -> {
+            if (entity == null || entity.dead || entity.valid) { // Paper - prevent adding already added or dead entities
+                return;
+            }
             this.entityList.add(entity);
             this.b(entity);
         });
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index 6579387623..788ab40d0e 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -987,7 +987,7 @@ public class WorldServer extends World implements IAsyncTaskHandler {
             if (this.entitiesByUUID.containsKey(uuid)) {
                 Entity entity1 = (Entity) this.entitiesByUUID.get(uuid);
 
-                if (this.g.contains(entity1)) {
+                if (this.g.contains(entity1) || entity1.dead) { // Paper - if dupe is dead, overwrite
                     this.g.remove(entity1);
                 } else {
                     if (!(entity instanceof EntityHuman)) {
-- 
2.18.0