From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Thu, 2 Apr 2020 01:42:39 -0400
Subject: [PATCH] Prevent Double PlayerChunkMap adds crashing server

Suspected case would be around the technique used in .stopRiding
Stack will identify any causer of this and warn instead of crashing.

diff --git a/src/main/java/net/minecraft/server/level/ChunkMap.java b/src/main/java/net/minecraft/server/level/ChunkMap.java
index fcdd8a8570d07e8bd3339275c7b6999be3f726b3..b2e95249ee5aa37d5e41679a7cb94e01c1b41494 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -1030,6 +1030,13 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
 
     public void addEntity(Entity entity) {
         org.spigotmc.AsyncCatcher.catchOp("entity track"); // Spigot
+        // Paper start - ignore and warn about illegal addEntity calls instead of crashing server
+        if (!entity.valid || entity.level != this.level || this.entityMap.containsKey(entity.getId())) {
+            LOGGER.error("Illegal ChunkMap::addEntity for world " + this.level.getWorld().getName()
+                + ": " + entity  + (this.entityMap.containsKey(entity.getId()) ? " ALREADY CONTAINED (This would have crashed your server)" : ""), new Throwable());
+            return;
+        }
+        // Paper end
         if (!(entity instanceof EnderDragonPart)) {
             EntityType<?> entitytypes = entity.getType();
             int i = entitytypes.clientTrackingRange() * 16;
diff --git a/src/main/java/net/minecraft/server/level/ServerLevel.java b/src/main/java/net/minecraft/server/level/ServerLevel.java
index 52a514b657485246827bf9c153303b8b5229bf5b..62f1f568bba5fbcb68e49640adb79c9384619bc6 100644
--- a/src/main/java/net/minecraft/server/level/ServerLevel.java
+++ b/src/main/java/net/minecraft/server/level/ServerLevel.java
@@ -2325,7 +2325,7 @@ public class ServerLevel extends Level implements WorldGenLevel {
 
         public void onTrackingStart(Entity entity) {
             org.spigotmc.AsyncCatcher.catchOp("entity register"); // Spigot
-            ServerLevel.this.getChunkSource().addEntity(entity);
+            // ServerLevel.this.getChunkSource().addEntity(entity); // Paper - moved down below valid=true
             if (entity instanceof ServerPlayer) {
                 ServerPlayer entityplayer = (ServerPlayer) entity;
 
@@ -2359,6 +2359,7 @@ public class ServerLevel extends Level implements WorldGenLevel {
 
             entity.updateDynamicGameEventListener(DynamicGameEventListener::add);
             entity.valid = true; // CraftBukkit
+            ServerLevel.this.getChunkSource().addEntity(entity);
             // Paper start - Set origin location when the entity is being added to the world
             if (entity.getOriginVector() == null) {
                 entity.setOrigin(entity.getBukkitEntity().getLocation());