From 3e0934988343ec1ba1f5950d8302636c306ebc0d Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Mon, 23 Jul 2018 22:44:23 -0400
Subject: [PATCH] Monitor and detect illegal Entity slice state

If we detect unexpected state, try to recover

This should hopefully avoid duplicate entities ever being created
if the entity was to end up in 2 different chunk slices

diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
index e5c900a85..e4d5e65c8 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -681,8 +681,27 @@ public class Chunk {
         entity.ab = this.locX;
         entity.ac = k;
         entity.ad = this.locZ;
-        this.entitySlices[k].add(entity);
+
         // Paper start
+        List<Entity> entitySlice = this.entitySlices[k];
+        boolean inThis = entitySlice.contains(entity);
+        List<Entity> currentSlice = entity.entitySlice;
+        if (inThis || (currentSlice != null && currentSlice.contains(entity))) {
+            if (currentSlice == entitySlice || inThis) {
+                return;
+            } else {
+                Chunk chunk = entity.getCurrentChunk();
+                if (chunk != null) {
+                    chunk.removeEntity(entity);
+                } else {
+                    removeEntity(entity);
+                }
+                currentSlice.remove(entity); // Just incase the above did not remove from this target slice
+            }
+        }
+        entity.entitySlice = entitySlice;
+        entitySlice.add(entity);
+
         this.markDirty();
         entity.setCurrentChunk(this);
         entityCounts.increment(entity.getMinecraftKeyString());
@@ -725,6 +744,9 @@ public class Chunk {
         }
 
         // Paper start
+        if (entity.entitySlice == null || !entity.entitySlice.contains(entity) || entitySlices[i] == entity.entitySlice) {
+            entity.entitySlice = null;
+        }
         if (!this.entitySlices[i].remove(entity)) { return; }
         this.markDirty();
         entity.setCurrentChunk(null);
@@ -955,6 +977,7 @@ public class Chunk {
                 }
                 // Spigot End
                 entity.setCurrentChunk(null); // Paper
+                entity.entitySlice = null; // Paper
 
                 // Do not pass along players, as doing so can get them stuck outside of time.
                 // (which for example disables inventory icon updates and prevents block breaking)
diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
index b761524f0..a2654690f 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -59,6 +59,7 @@ public abstract class Entity implements ICommandListener, KeyedObject { // Paper
             }
         }
     };
+    List<Entity> entitySlice = null;
     // Paper end
     static boolean isLevelAtLeast(NBTTagCompound tag, int level) {
         return tag.hasKey("Bukkit.updateLevel") && tag.getInt("Bukkit.updateLevel") >= level;
-- 
2.21.0