From f1edc0289d2b308fd8c73f64b58c8946913b6af5 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 67e2158ca1..e510940ab3 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -712,8 +712,33 @@ public class Chunk implements IChunkAccess {
         entity.ae = this.locX;
         entity.af = k;
         entity.ag = 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();
         if (entity instanceof EntityItem) {
             itemCounts[k]++;
@@ -746,6 +771,12 @@ public class Chunk implements IChunkAccess {
         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();
         if (entity instanceof EntityItem) {
             itemCounts[i]--;
@@ -979,6 +1010,7 @@ 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 4315804ddb..785c31089e 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -64,6 +64,7 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
             }
         }
     };
+    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