From 1c7a3f36379778527ce9725cbeb9f5cab5e1475f Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Thu, 26 Jul 2018 00:11:12 -0400
Subject: [PATCH] Prevent Saving Bad entities to chunks

See https://github.com/PaperMC/Paper/issues/1223

Minecraft is saving invalid entities to the chunk files.

Avoid saving bad data, and also make improvements to handle
loading these chunks. Any invalid entity will be instant killed,
so lets avoid adding it to the world...

This lets us be safer about the dupe UUID resolver too, as now
we can ignore instant killed entities and avoid risk of duplicating
an invalid entity.

This should reduce log occurrences of dupe uuid messages.

diff --git a/src/main/java/net/minecraft/server/ChunkRegionLoader.java b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
index a97e024ec4..bd52bf6561 100644
--- a/src/main/java/net/minecraft/server/ChunkRegionLoader.java
+++ b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
@@ -561,11 +561,22 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
 
         Iterator iterator;
 
+        java.util.List<Entity> toUpdate = new java.util.ArrayList<>(); // Paper
         for (int j = 0; j < chunk.getEntitySlices().length; ++j) {
             iterator = chunk.getEntitySlices()[j].iterator();
 
             while (iterator.hasNext()) {
                 Entity entity = (Entity) iterator.next();
+                // Paper start
+                if ((int)Math.floor(entity.locX) >> 4 != chunk.locX || (int)Math.floor(entity.locZ) >> 4 != chunk.locZ) {
+                    LogManager.getLogger().warn(entity + " is not in this chunk, skipping save. This a bug fix to a vanilla bug. Do not report this to PaperMC please.");
+                    toUpdate.add(entity);
+                    continue;
+                }
+                if (entity.dead) {
+                    continue;
+                }
+                // Paper end
                 NBTTagCompound nbttagcompound1 = new NBTTagCompound();
 
                 if (entity.d(nbttagcompound1)) {
@@ -574,6 +585,11 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
                 }
             }
         }
+        // Paper start - move entities to the correct chunk
+        for (Entity entity : toUpdate) {
+            world.entityJoinedWorld(entity, false);
+        }
+        // Paper end
 
         nbttagcompound.set("Entities", nbttaglist1);
         NBTTagList nbttaglist2 = new NBTTagList();
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index 3c9d9f1ab0..0f2dadfa39 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -1066,7 +1066,7 @@ public abstract class World implements GeneratorAccess, IIBlockAccess, AutoClose
             }
 
             this.getChunkAt(i, j).a(entity);
-            this.entityList.add(entity);
+            if (!entity.dead) this.entityList.add(entity); // Paper - don't add dead entities, chunk registration may of killed it
             this.b(entity);
             return true;
         }
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index 7a9f28421b..b57e1ff364 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -992,7 +992,7 @@ public class WorldServer extends World implements IAsyncTaskHandler {
             if (this.entitiesByUUID.containsKey(uuid)) {
                 Entity entity1 = (Entity) this.entitiesByUUID.get(uuid);
 
-                if (this.g.contains(entity1)) {
+                if (this.g.contains(entity1) || entity1.dead) { // Paper - overwrite the current dead one
                     this.g.remove(entity1);
                 } else {
                     if (!(entity instanceof EntityHuman)) {
-- 
2.18.0