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 e1498a5a8e664b922c77a8524ed2ea38c91834ce..71d5c298e14869d9d44226aeb6ffe3f051827d1a 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -1467,6 +1467,14 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
 
     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.level != this.level || this.entityMap.containsKey(entity.getId())) {
+            new Throwable("[ERROR] Illegal PlayerChunkMap::addEntity for world " + this.level.getWorld().getName()
+                + ": " + entity  + (this.entityMap.containsKey(entity.getId()) ? " ALREADY CONTAINED (This would have crashed your server)" : ""))
+                .printStackTrace();
+            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 03939f43ade3859a53ccf90e7fc221555ce6e97f..7340a25e49af30ecbb4b212365b9941e24e221d2 100644
--- a/src/main/java/net/minecraft/server/level/ServerLevel.java
+++ b/src/main/java/net/minecraft/server/level/ServerLevel.java
@@ -2120,7 +2120,7 @@ public class ServerLevel extends net.minecraft.world.level.Level implements Worl
 
         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) {
                 ServerLevel.this.players.add((ServerPlayer) entity);
                 ServerLevel.this.updateSleepingPlayerList();
@@ -2142,6 +2142,7 @@ public class ServerLevel extends net.minecraft.world.level.Level implements Worl
             }
 
             entity.valid = true; // CraftBukkit
+            ServerLevel.this.getChunkSource().addEntity(entity);
         }
 
         public void onTrackingEnd(Entity entity) {