From be6b2c15dde0d4d8b87768e2d56809bacc5cf54a 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 c7f67bce6c..a7cbd9b201 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -710,8 +710,34 @@ 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);
+        List<Entity> currentSlice = entity.entitySlice;
+        if ((currentSlice != null && currentSlice.contains(entity)) || inThis) {
+            if (currentSlice == entitySlice || inThis) {
+                LogManager.getLogger().warn(entity + " was already in this chunk section! Report this to https://github.com/PaperMC/Paper/issues/1302");
+                new Throwable().printStackTrace();
+                return;
+            } else {
+                LogManager.getLogger().warn(entity + " is still in another ChunkSection! Report this to https://github.com/PaperMC/Paper/issues/1302");
+
+                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]++;
@@ -741,9 +767,13 @@ 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;
+        } else {
+            LogManager.getLogger().warn(entity + " was removed from a entitySlice we did not expect. Report this to https://github.com/PaperMC/Paper/issues/1302");
+            new Throwable().printStackTrace();
         }
+        if (!this.entitySlices[i].remove(entity)) { return; }
         this.markDirty();
         if (entity instanceof EntityItem) {
             itemCounts[i]--;
@@ -1017,6 +1047,7 @@ 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 0381f8e658..965cd592f3 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
             }
         }
     };
+    List<Entity> entitySlice = null;
     // Paper end
     static boolean isLevelAtLeast(NBTTagCompound tag, int level) {
         return tag.hasKey("Bukkit.updateLevel") && tag.getInt("Bukkit.updateLevel") >= level;
-- 
2.19.0