From c694fb42c86f64e9cda92745dd020ac0121d470a Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Mon, 23 Jul 2018 22:44:23 -0400
Subject: [PATCH] Add some Debug to Chunk Entity slices

If we detect unexpected state, log and 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 42b76b2122..7dd59ee031 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -714,8 +714,27 @@ public class Chunk implements IChunkAccess {
         entity.chunkX = this.locX;
         entity.chunkY = k;
         entity.chunkZ = 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();
         if (entity instanceof EntityItem) {
             itemCounts[k]++;
@@ -745,9 +764,10 @@ public class Chunk implements IChunkAccess {
             i = this.entitySlices.length - 1;
         }
         // Paper start
-        if (!this.entitySlices[i].remove(entity)) {
-            return;
+        if (entity.entitySlice == null || !entity.entitySlice.contains(entity) || entitySlices[i] == entity.entitySlice) {
+            entity.entitySlice = null;
         }
+        if (!this.entitySlices[i].remove(entity)) { return; }
         this.markDirty();
         if (entity instanceof EntityItem) {
             itemCounts[i]--;
@@ -1028,6 +1048,7 @@ public class Chunk implements IChunkAccess {
                 }
                 // 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 61a547a40d..15a81d1eb9 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -62,6 +62,7 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
             }
         }
     };
+    List<Entity> entitySlice = null;
     // Paper end
     static boolean isLevelAtLeast(NBTTagCompound tag, int level) {
         return tag.hasKey("Bukkit.updateLevel") && tag.getInt("Bukkit.updateLevel") >= level;
-- 
2.20.1