From 094bd1612b657efc827a240c88378f6a1948db2d 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 f2ca391bf..2cb3e3696 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -408,6 +408,25 @@ public class Chunk implements IChunkAccess {
         if (k >= this.entitySlices.length) {
             k = this.entitySlices.length - 1;
         }
+        // Paper - remove from any old list if its in one
+        List<Entity> nextSlice = this.entitySlices[k]; // the next list to be added to
+        List<Entity> currentSlice = entity.entitySlice;
+        if (nextSlice == currentSlice) {
+            if (World.DEBUG_ENTITIES) MinecraftServer.LOGGER.warn("Entity was already in this chunk!" + entity, new Throwable());
+            return; // ??? silly plugins
+        }
+        if (currentSlice != null && currentSlice.contains(entity)) {
+            // Still in an old chunk...
+            if (World.DEBUG_ENTITIES) MinecraftServer.LOGGER.warn("Entity is still in another chunk!" + entity, new Throwable());
+            Chunk chunk = entity.getCurrentChunk();
+            if (chunk != null) {
+                chunk.removeEntity(entity);
+            } else {
+                removeEntity(entity);
+            }
+            currentSlice.remove(entity); // Just incase the above did not remove from the previous slice
+        }
+        // Paper end
 
         if (!entity.inChunk || entity.getCurrentChunk() != this) entityCounts.increment(entity.getMinecraftKeyString()); // Paper
         entity.inChunk = true;
@@ -416,6 +435,7 @@ public class Chunk implements IChunkAccess {
         entity.chunkY = k;
         entity.chunkZ = this.loc.z;
         this.entitySlices[k].add(entity);
+        entity.entitySlice = this.entitySlices[k]; // Paper
         this.markDirty(); // Paper
     }
 
@@ -439,6 +459,9 @@ public class Chunk implements IChunkAccess {
         }
         // Paper start
         if (entity.currentChunk != null && entity.currentChunk.get() == this) entity.setCurrentChunk(null);
+        if (entitySlices[i] == entity.entitySlice) {
+            entity.entitySlice = null;
+        }
         if (!this.entitySlices[i].remove(entity)) {
             return;
         }
diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
index 7f6e20e4b..a273bec56 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -67,6 +67,7 @@ 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;
-- 
2.21.0