From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Jake Potrebic <jake.m.potrebic@gmail.com>
Date: Tue, 22 Mar 2022 12:44:30 -0700
Subject: [PATCH] Throw exception on world create while being ticked

There are no plans to support creating worlds while worlds are
being ticked themselvess.

diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
index ed799a59411d637d0f50a62c73e2b23053fbcb0e..77cd45f616e2ff38ad6a648b8b865a99e544f3ec 100644
--- a/src/main/java/net/minecraft/server/MinecraftServer.java
+++ b/src/main/java/net/minecraft/server/MinecraftServer.java
@@ -294,6 +294,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
 
     public volatile Thread shutdownThread; // Paper
     public volatile boolean abnormalExit = false; // Paper
+    public boolean isIteratingOverLevels = false; // Paper
 
     public static <S extends MinecraftServer> S spin(Function<Thread, S> serverFactory) {
         AtomicReference<S> atomicreference = new AtomicReference();
@@ -1525,6 +1526,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
         // Paper end
         MinecraftTimings.timeUpdateTimer.stopTiming(); // Spigot // Paper
 
+        this.isIteratingOverLevels = true; // Paper
         while (iterator.hasNext()) {
             ServerLevel worldserver = (ServerLevel) iterator.next();
             worldserver.hasPhysicsEvent =  org.bukkit.event.block.BlockPhysicsEvent.getHandlerList().getRegisteredListeners().length > 0; // Paper
@@ -1572,6 +1574,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
             this.profiler.pop();
             worldserver.explosionDensityCache.clear(); // Paper - Optimize explosions
         }
+        this.isIteratingOverLevels = false; // Paper
 
         this.profiler.popPush("connection");
         MinecraftTimings.connectionTimer.startTiming(); // Spigot
diff --git a/src/main/java/org/bukkit/craftbukkit/CraftServer.java b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
index a8ed075021088d86753e0340ee23698191ee07e0..57da844bc8993d7eb8850df3696a8870600a8603 100644
--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
@@ -1135,6 +1135,7 @@ public final class CraftServer implements Server {
     @Override
     public World createWorld(WorldCreator creator) {
         Preconditions.checkState(!console.levels.isEmpty(), "Cannot create additional worlds on STARTUP");
+        Preconditions.checkState(!this.console.isIteratingOverLevels, "Cannot create a world while worlds are being ticked"); // Paper
         Validate.notNull(creator, "Creator may not be null");
 
         String name = creator.name();
@@ -1259,6 +1260,7 @@ public final class CraftServer implements Server {
 
     @Override
     public boolean unloadWorld(World world, boolean save) {
+        Preconditions.checkState(!this.console.isIteratingOverLevels, "Cannot unload a world while worlds are being ticked"); // Paper
         if (world == null) {
             return false;
         }