throw exception if worlds are created while being ticked (#7653)

patches/server/0912-Throw-exception-on-world-create-while-being-ticked.patch

@@ -0,0 +1,48 @@
+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 c8d56947305c981a3268ce4ae3e975db350ceff2..da15a224b8b974e78b9d8d5f514229b3b2a5a63e 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();
+@@ -1567,6 +1568,7 @@ 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
+@@ -1614,6 +1616,7 @@ 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 c3f15ee7505e82418d9659ba3cee4f807546e0db..8a248e34fce1e04de4460f98b7627df495c66af1 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();