Properly remove entities on dimension teleport

To teleport an entity between dimensions, the server makes a copy
and puts the copy in the new location, and marks the old one dead.

If this method got called for the same world in the same tick,
the entity would not have been removed from the UUID map, and the
world readd would fail.

This can be triggered even with a plugin if the entity is teleported
twice in the same tick, from world A to B, then back from B to A.

The re-add to A will fail to add the entity to the world. It will
actually be there, but it will not be visible on the client until
the server is restarted to re-try the add to world process again.

This bug was unlikely to be seen by many due to the double teleport
requirement, but plugins (such as my own) use this method to
trigger a "reload" of the entity on the client.

Spigot-Server-Patches/0305-Properly-remove-entities-on-dimension-teleport.patch

@@ -0,0 +1,51 @@
+From 95da0e7cef89c1762b8d624855186175c60cfcee Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sun, 10 Jun 2018 20:04:42 -0400
+Subject: [PATCH] Properly remove entities on dimension teleport
+
+To teleport an entity between dimensions, the server makes a copy
+and puts the copy in the new location, and marks the old one dead.
+
+If this method got called for the same world in the same tick,
+the entity would not have been removed from the UUID map, and the
+world readd would fail.
+
+This can be triggered even with a plugin if the entity is teleported
+twice in the same tick, from world A to B, then back from B to A.
+
+The re-add to A will fail to add the entity to the world. It will
+actually be there, but it will not be visible on the client until
+the server is restarted to re-try the add to world process again.
+
+This bug was unlikely to be seen by many due to the double teleport
+requirement, but plugins (such as my own) use this method to
+trigger a "reload" of the entity on the client.
+
+diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
+index 58fe1aa3e..4431f6e99 100644
+--- a/src/main/java/net/minecraft/server/Entity.java
++++ b/src/main/java/net/minecraft/server/Entity.java
+@@ -2405,7 +2405,7 @@ public abstract class Entity implements ICommandListener {
+             }
+             // CraftBukkit end */
+ 
+-            this.world.kill(this);
++            this.world.removeEntity(this); // Paper - Fully remove entity, can't have dupes in the UUID map
+             this.dead = false;
+             this.world.methodProfiler.a("reposition");
+             /* CraftBukkit start - Handled in calculateTarget
+diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
+index 49019d54d..9fe5c4406 100644
+--- a/src/main/java/net/minecraft/server/WorldServer.java
++++ b/src/main/java/net/minecraft/server/WorldServer.java
+@@ -1205,6 +1205,7 @@ public class WorldServer extends World implements IAsyncTaskHandler {
+     }
+ 
+     protected void c(Entity entity) {
++        if (!entity.valid) return; // Paper - Already removed, dont fire twice - this looks like it can happen even without our changes
+         super.c(entity);
+         this.entitiesById.d(entity.getId());
+         this.entitiesByUUID.remove(entity.getUniqueID());
+-- 
+2.17.1
+