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.

Spigot-Server-Patches/0456-Prevent-Double-PlayerChunkMap-adds-crashing-server.patch

@@ -0,0 +1,51 @@
+From dac1fdedb69c18999aa0764714f86e00e30916b9 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 9f0b0b4270..bfa62e13e9 100644
+--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
++++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
+@@ -1484,6 +1484,14 @@ 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 941eee838b..c7ec8cbc11 100644
+--- a/src/main/java/net/minecraft/server/WorldServer.java
++++ b/src/main/java/net/minecraft/server/WorldServer.java
+@@ -1449,7 +1449,7 @@ 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);
+@@ -1460,6 +1460,7 @@ 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();
+-- 
+2.25.1
+