From 2b1e709cee17ac542df6168116a7a4abe24dab3c 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/Entity.java b/src/main/java/net/minecraft/server/Entity.java
index 00df89d6509..0dbe2dce111 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -59,6 +59,7 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
     }
 
     // Paper start
+    boolean isQueuedForRegister = false;
     public static Random SHARED_RANDOM = new Random() {
         private boolean locked = false;
         @Override
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index 5173731dc55..3fc25183ca4 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -532,6 +532,7 @@ public class WorldServer extends World {
 
             try (co.aikar.timings.Timing ignored = this.timings.newEntities.startTiming()) { // Paper - timings
             while ((entity = (Entity) this.entitiesToAdd.poll()) != null) {
+                if (!entity.isQueuedForRegister) continue; // Paper - ignore cancelled registers
                 this.registerEntity(entity);
             }
             } // Paper - timings
@@ -1353,6 +1354,19 @@ public class WorldServer extends World {
 
     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 )
         {
@@ -1413,9 +1427,21 @@ public class WorldServer extends World {
 
     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).eo();
-- 
2.26.2