From b2869ded2f63a5b8a6f043551781ef4415968f57 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 4af5a230ba..6371f2f5b1 100644
--- a/src/main/java/net/minecraft/server/ChunkRegionLoader.java
+++ b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
@@ -300,6 +300,7 @@ public class ChunkRegionLoader {
         nbttagcompound1.set("TileEntities", nbttaglist1);
         NBTTagList nbttaglist2 = new NBTTagList();
 
+        java.util.List<Entity> toUpdate = new java.util.ArrayList<>(); // Paper
         if (ichunkaccess.getChunkStatus().getType() == ChunkStatus.Type.LEVELCHUNK) {
             Chunk chunk = (Chunk) ichunkaccess;
 
@@ -311,13 +312,29 @@ public class ChunkRegionLoader {
                 while (iterator1.hasNext()) {
                     Entity entity = (Entity) iterator1.next();
                     NBTTagCompound nbttagcompound4 = new NBTTagCompound();
-
+                    // Paper start
+                    if ((int) Math.floor(entity.locX()) >> 4 != chunk.getPos().x || (int) Math.floor(entity.locZ()) >> 4 != chunk.getPos().z) {
+                        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
                     if (entity.d(nbttagcompound4)) {
                         chunk.d(true);
                         nbttaglist2.add(nbttagcompound4);
                     }
                 }
             }
+
+            // Paper start - move entities to the correct chunk
+            for (Entity entity : toUpdate) {
+                worldserver.entityJoinedWorld(entity);
+            }
+            // Paper end
+
         } else {
             ProtoChunk protochunk = (ProtoChunk) ichunkaccess;
 
-- 
2.24.1