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 5f85d7f7ec57fc1b0375e62a8e8e3e8783f34193..396310a51480cf0d1ea4c0959d3f8e4ed77b99e3 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -1298,6 +1298,13 @@ 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 ecb9f6f0c9b24313b9e55b3a4c4f4bba40bc8fb3..0995a3a274df988a5c63c813de8213019a7c47c4 100644
--- a/src/main/java/net/minecraft/server/level/ServerLevel.java
+++ b/src/main/java/net/minecraft/server/level/ServerLevel.java
@@ -2202,7 +2202,7 @@ public class ServerLevel extends Level implements ServerEntityGetter, WorldGenLe
 
         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 entityplayer) {
                 ServerLevel.this.players.add(entityplayer);
                 ServerLevel.this.updateSleepingPlayerList();
@@ -2232,6 +2232,7 @@ public class ServerLevel extends Level implements ServerEntityGetter, WorldGenLe
             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());