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 064b008c3869346315f7a9fb2ce7a111f5adf729..b2bc8869c454eba5c55a45d5671e2e406a0d12a7 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -1461,6 +1461,13 @@ 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 - ignore and warn about illegal addEntity calls instead of crashing server
         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 dd7071404555548c7f102737456f8773860b597b..ecc716ce94f40ab8f85e5f57fecad68c08d4e85e 100644
--- a/src/main/java/net/minecraft/server/level/ServerLevel.java
+++ b/src/main/java/net/minecraft/server/level/ServerLevel.java
@@ -2224,7 +2224,7 @@ 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 - ignore and warn about illegal addEntity calls instead of crashing server; moved down below valid=true
             if (entity instanceof ServerPlayer) {
                 ServerPlayer entityplayer = (ServerPlayer) entity;
 
@@ -2259,6 +2259,7 @@ public class ServerLevel extends Level implements WorldGenLevel {
             entity.updateDynamicGameEventListener(DynamicGameEventListener::add);
             entity.inWorld = true; // CraftBukkit - Mark entity as in world
             entity.valid = true; // CraftBukkit
+            ServerLevel.this.getChunkSource().addEntity(entity); // Paper - ignore and warn about illegal addEntity calls instead of crashing server
             // Paper start - Entity origin API
             if (entity.getOriginVector() == null) {
                 entity.setOrigin(entity.getBukkitEntity().getLocation());