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/PlayerChunkMap.java b/src/main/java/net/minecraft/server/PlayerChunkMap.java
index 4ba661c5a89bebe29c8802387bc93c10094b7606..6a2cf0c6f649e7a74b58fc292f57a08c0663b0a4 100644
--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
+++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
@@ -1411,6 +1411,14 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
 
     protected 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.world != this.world || this.trackedEntities.containsKey(entity.getId())) {
+            new Throwable("[ERROR] Illegal PlayerChunkMap::addEntity for world " + this.world.getWorld().getName()
+                + ": " + entity  + (this.trackedEntities.containsKey(entity.getId()) ? " ALREADY CONTAINED (This would have crashed your server)" : ""))
+                .printStackTrace();
+            return;
+        }
+        // Paper end
         if (!(entity instanceof EntityComplexPart)) {
             if (!(entity instanceof EntityLightning)) {
                 EntityTypes<?> entitytypes = entity.getEntityType();
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index 68c7c243a6d12d5c2f1045dfc5dd36f455dfbffa..dfc9f4d5738b01e3f80af8f3d25aec51b6e9d8d9 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -1463,7 +1463,7 @@ public class WorldServer extends World {
                 }
             }
 
-            this.getChunkProvider().addEntity(entity);
+            // this.getChunkProvider().addEntity(entity); // Paper - moved down below valid=true
             // CraftBukkit start - SPIGOT-5278
             if (entity instanceof EntityDrowned) {
                 this.navigators.add(((EntityDrowned) entity).navigationWater);
@@ -1474,6 +1474,7 @@ public class WorldServer extends World {
                 this.navigators.add(((EntityInsentient) entity).getNavigation());
             }
             entity.valid = true; // CraftBukkit
+            this.getChunkProvider().addEntity(entity); // Paper - from above to be below valid=true
             // Paper start - Set origin location when the entity is being added to the world
             if (entity.origin == null) {
                 entity.origin = entity.getBukkitEntity().getLocation();