From 80a062ecc95b8346f6a9161e375e41d5f5df3882 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 6de053781..be0b411e5 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -681,8 +681,33 @@ 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);
+        if (entity.entitySlice != null || inThis) {
+            if (entity.entitySlice == entitySlice || inThis) {
+                LogManager.getLogger().warn(entity + " was already in this chunk section! Report this to https://github.com/PaperMC/Paper/issues/1223");
+                new Throwable().printStackTrace();
+                return;
+            } else {
+                LogManager.getLogger().warn(entity + " is still in another ChunkSection! Report this to https://github.com/PaperMC/Paper/issues/1223");
+
+                Chunk chunk = entity.getCurrentChunk();
+                if (chunk != null) {
+                    if (chunk != this) {
+                        LogManager.getLogger().warn(entity + " was in another chunk at that! " + chunk.locX + "," + chunk.locZ);
+                    }
+                    chunk.removeEntity(entity);
+                } else {
+                    removeEntity(entity);
+                }
+                new Throwable().printStackTrace();
+            }
+        }
+        entity.entitySlice = entitySlice;
+        entitySlice.add(entity);
+
         this.markDirty();
         entity.setCurrentChunk(this);
         entityCounts.increment(entity.entityKeyString);
@@ -726,6 +751,12 @@ public class Chunk {
 
         // Paper start
         if (!this.entitySlices[i].remove(entity)) { return; }
+        if (entitySlices[i] == entity.entitySlice) {
+            entity.entitySlice = null;
+        } else {
+            LogManager.getLogger().warn(entity + " was removed from a entitySlice we did not expect. Report this to https://github.com/PaperMC/Paper/issues/1223");
+            new Throwable().printStackTrace();
+        }
         this.markDirty();
         entity.setCurrentChunk(null);
         entityCounts.decrement(entity.entityKeyString);
diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
index 7188d0c99..b3120d7cc 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
             }
         }
     };
+    Object entitySlice = null;
     // Paper end
     static boolean isLevelAtLeast(NBTTagCompound tag, int level) {
         return tag.hasKey("Bukkit.updateLevel") && tag.getInt("Bukkit.updateLevel") >= level;
-- 
2.18.0