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 a8b3f0e8be414c4ea92cc85c9811ecd42e5ce9c1..0e04083ff0598451c66731b1518b2eb6367ae36c 100644
--- a/src/main/java/net/minecraft/server/MinecraftServer.java
+++ b/src/main/java/net/minecraft/server/MinecraftServer.java
@@ -308,6 +308,7 @@ 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();
@@ -1475,7 +1476,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
         this.getFunctions().tick();
         MinecraftTimings.commandFunctionsTimer.stopTiming(); // Spigot // Paper
         this.profiler.popPush("levels");
-        Iterator iterator = this.getAllLevels().iterator();
+        //Iterator iterator = this.getAllLevels().iterator(); // Paper - moved down
 
         // CraftBukkit start
         // Run tasks that are waiting on processing
@@ -1507,6 +1508,8 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
         // Paper end
         MinecraftTimings.timeUpdateTimer.stopTiming(); // Spigot // Paper
 
+        this.isIteratingOverLevels = true; // Paper
+        Iterator iterator = this.getAllLevels().iterator(); // Paper - move down
         while (iterator.hasNext()) {
             ServerLevel worldserver = (ServerLevel) iterator.next();
             worldserver.hasPhysicsEvent =  org.bukkit.event.block.BlockPhysicsEvent.getHandlerList().getRegisteredListeners().length > 0; // Paper
@@ -1554,6 +1557,7 @@ 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 03cffc87cfc82f1f6f96e8cfc0e37472fc50d38b..d3b4fef3696c9999912fd37034ea7e1d6755c9ef 100644
--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
@@ -868,6 +868,11 @@ public final class CraftServer implements Server {
         return new ArrayList<World>(this.worlds.values());
     }
 
+    @Override
+    public boolean isTickingWorlds() {
+        return console.isIteratingOverLevels;
+    }
+
     public DedicatedPlayerList getHandle() {
         return this.playerList;
     }
@@ -1152,6 +1157,7 @@ public final class CraftServer implements Server {
     @Override
     public World createWorld(WorldCreator creator) {
         Preconditions.checkState(this.console.getAllLevels().iterator().hasNext(), "Cannot create additional worlds on STARTUP");
+        //Preconditions.checkState(!this.console.isIteratingOverLevels, "Cannot create a world while worlds are being ticked"); // Paper - Cat - Temp disable. We'll see how this goes.
         Validate.notNull(creator, "Creator may not be null");
 
         String name = creator.name();
@@ -1287,6 +1293,7 @@ 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 - Cat - Temp disable. We'll see how this goes.
         if (world == null) {
             return false;
         }
