From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sun, 29 Mar 2020 18:26:14 -0400
Subject: [PATCH] Ensure Entity is never double registered

If something calls register twice, and the world is ticking, it could be
enqueued to add twice.

Vs behavior of non ticking of just overwriting state.

We will now simply log a warning when this happens instead of crashing the server.

diff --git a/src/main/java/net/minecraft/server/level/WorldServer.java b/src/main/java/net/minecraft/server/level/WorldServer.java
index 12be2c5406ece16a21abaebb3ca89171304cb929..7dbb7e75c10d443fe9c90eadec8e9cdf01024548 100644
--- a/src/main/java/net/minecraft/server/level/WorldServer.java
+++ b/src/main/java/net/minecraft/server/level/WorldServer.java
@@ -649,6 +649,7 @@ public class WorldServer extends World implements GeneratorAccessSeed {
             Entity entity2;
 
             while ((entity2 = (Entity) this.entitiesToAdd.poll()) != null) {
+                if (!entity2.isQueuedForRegister) continue; // Paper - ignore cancelled registers
                 this.registerEntity(entity2);
             }
 
@@ -1406,6 +1407,19 @@ public class WorldServer extends World implements GeneratorAccessSeed {
 
     public void unregisterEntity(Entity entity) {
         org.spigotmc.AsyncCatcher.catchOp("entity unregister"); // Spigot
+        // Paper start - fix entity registration issues
+        if (entity instanceof EntityComplexPart) {
+            // Usually this is a no-op for complex parts, and ID's should be removed, but go ahead and remove it anyways
+            // Dragon parts are handled special in register. they don't receive a valid = true or register by UUID etc.
+            this.entitiesById.remove(entity.getId(), entity);
+            return;
+        }
+        if (!entity.valid) {
+            // Someone called remove before we ever got added, cancel the add.
+            entity.isQueuedForRegister = false;
+            return;
+        }
+        // Paper end
         // Spigot start
         if ( entity instanceof EntityHuman )
         {
@@ -1472,9 +1486,21 @@ public class WorldServer extends World implements GeneratorAccessSeed {
 
     private void registerEntity(Entity entity) {
         org.spigotmc.AsyncCatcher.catchOp("entity register"); // Spigot
+        // Paper start - don't double enqueue entity registration
+        //noinspection ObjectEquality
+        if (this.entitiesById.get(entity.getId()) == entity) {
+            LOGGER.error(entity + " was already registered!");
+            new Throwable().printStackTrace();
+            return;
+        }
+        // Paper end
         if (this.tickingEntities) {
-            this.entitiesToAdd.add(entity);
+            if (!entity.isQueuedForRegister) { // Paper
+                this.entitiesToAdd.add(entity);
+                entity.isQueuedForRegister = true; // Paper
+            }
         } else {
+            entity.isQueuedForRegister = false; // Paper
             this.entitiesById.put(entity.getId(), entity);
             if (entity instanceof EntityEnderDragon) {
                 EntityComplexPart[] aentitycomplexpart = ((EntityEnderDragon) entity).eJ();
diff --git a/src/main/java/net/minecraft/world/entity/Entity.java b/src/main/java/net/minecraft/world/entity/Entity.java
index 68fdb01c3f11c3b060d3d621099d67f6b29431d6..f95aa9b4cc53c1e3258b7b32249ec1c3ef4ae2f1 100644
--- a/src/main/java/net/minecraft/world/entity/Entity.java
+++ b/src/main/java/net/minecraft/world/entity/Entity.java
@@ -148,6 +148,7 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, ne
     }
 
     // Paper start
+    public boolean isQueuedForRegister = false;
     public static Random SHARED_RANDOM = new Random() {
         private boolean locked = false;
         @Override