From c2f6872c419c0bab2865724c87b1badc9ee59ef5 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 bd31a7dcab..8b80830933 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -901,6 +901,7 @@ public class Chunk implements IChunkAccess {
         this.world.b(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
@@ -948,10 +949,12 @@ public class Chunk implements IChunkAccess {
                     thisChunk.put(entity.uniqueID, entity);
                 }
             }
-            // Paper end
 
-            this.world.a((Collection) entityslice);
+            //this.world.a((Collection) entityslice); // Move down, add all entities at same time
+            toAdd.addAll(entityslice);
+            // Paper end
         }
+        this.world.addChunkEntities(toAdd); // 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 b0053e5e63..004c3ec474 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -1069,6 +1069,7 @@ public abstract class World implements GeneratorAccess, IIBlockAccess, AutoClose
             }
 
             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;
@@ -2463,6 +2464,7 @@ public abstract class World implements GeneratorAccess, IIBlockAccess, AutoClose
         return i;
     }
 
+    public void addChunkEntities(Collection<Entity> collection) { a(collection); } // Paper - OBFHELPER
     public void a(Collection<Entity> collection) {
         org.spigotmc.AsyncCatcher.catchOp( "entity world add"); // Spigot
         // CraftBukkit start
@@ -2472,7 +2474,7 @@ public abstract class World implements GeneratorAccess, IIBlockAccess, AutoClose
         while (iterator.hasNext()) {
             Entity entity = (Entity) iterator.next();
 
-            if (entity == null) {
+            if (entity == null || entity.dead || entity.valid) { // Paper - prevent adding already added or dead entities
                 continue;
             }
             this.entityList.add(entity);
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index 6b5b2c8258..f5911fbf08 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -992,7 +992,7 @@ 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