From abf1bf94c272bcaf8daf2d6e5252e54a426bf936 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..1c9e2cb1ce 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 ((currentSlice != null && currentSlice.contains(entity)) || inThis) {
+            if (currentSlice == entitySlice || inThis) {
+                return;
+            } else {
+                Chunk chunk = entity.getCurrentChunk();
+                if (chunk != null) {
+                    chunk.removeEntity(entity);
+                } else {
+                    removeEntity(entity);
+                }
+                new Throwable().printStackTrace();
+            }
+        }
+        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