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 0b9f4541cc0d0f27e811c1a6798d6758a2687b0c..2101c0a3b000a60733ceada248c202e2c4783af5 100644
--- a/src/main/java/net/minecraft/server/MinecraftServer.java
+++ b/src/main/java/net/minecraft/server/MinecraftServer.java
@@ -328,6 +328,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
     public final double[] recentTps = new double[ 3 ];
     // Spigot end
     public final io.papermc.paper.configuration.PaperConfigurations paperConfigurations; // Paper - add paper configuration files
+    public boolean isIteratingOverLevels = false; // Paper - Throw exception on world create while being ticked
 
     public static <S extends MinecraftServer> S spin(Function<Thread, S> serverFactory) {
         AtomicReference<S> atomicreference = new AtomicReference();
@@ -1658,7 +1659,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
         this.getFunctions().tick();
         MinecraftTimings.commandFunctionsTimer.stopTiming(); // Spigot // Paper
         gameprofilerfiller.popPush("levels");
-        Iterator iterator = this.getAllLevels().iterator();
+        //Iterator iterator = this.getAllLevels().iterator(); // Paper - Throw exception on world create while being ticked; moved down
 
         // CraftBukkit start
         // Run tasks that are waiting on processing
@@ -1690,6 +1691,8 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
         // Paper end - Perf: Optimize time updates
         MinecraftTimings.timeUpdateTimer.stopTiming(); // Spigot // Paper
 
+        this.isIteratingOverLevels = true; // Paper - Throw exception on world create while being ticked
+        Iterator iterator = this.getAllLevels().iterator(); // Paper - Throw exception on world create while being ticked; move down
         while (iterator.hasNext()) {
             ServerLevel worldserver = (ServerLevel) iterator.next();
             worldserver.hasPhysicsEvent = org.bukkit.event.block.BlockPhysicsEvent.getHandlerList().getRegisteredListeners().length > 0; // Paper - BlockPhysicsEvent
@@ -1724,6 +1727,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
             gameprofilerfiller.pop();
             gameprofilerfiller.pop();
         }
+        this.isIteratingOverLevels = false; // Paper - Throw exception on world create while being ticked
 
         gameprofilerfiller.popPush("connection");
         MinecraftTimings.connectionTimer.startTiming(); // Spigot // Paper
diff --git a/src/main/java/org/bukkit/craftbukkit/CraftServer.java b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
index b31f3013ff63bb6ece80a9d10e51641a7e93f3df..9921c1e03a9e4eb2153fd88d2932f7f4867e0a03 100644
--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
@@ -920,6 +920,11 @@ 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;
     }
@@ -1183,6 +1188,7 @@ 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.
         Preconditions.checkArgument(creator != null, "WorldCreator cannot be null");
 
         String name = creator.name();
@@ -1359,6 +1365,7 @@ 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;
         }