From ec29810bd4cedf7fa9c2690520c27bff8a989749 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 3d8f0b5786..db5c6e0f74 100644
--- a/src/main/java/net/minecraft/server/ChunkRegionLoader.java
+++ b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
@@ -563,11 +563,22 @@ 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)) {
@@ -576,6 +587,11 @@ 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();
-- 
2.19.0