Avoid potential place where the world map could be modified after its…

… iterator is created (#8315)

patches/server/0900-Throw-exception-on-world-create-unload-while-being-t.patch

@@ -0,0 +1,65 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Jake Potrebic <[email protected]>
+Date: Tue, 22 Mar 2022 12:44:30 -0700
+Subject: [PATCH] Throw exception on world create/unload while being ticked
+
+There are no plans to support creating worlds while worlds are
+being ticked themselves.
+
+diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
+index 4d920031300a9801debc2eb39a4d3cb9d8fbb330..db3d34f0ad2baa2b8685c039a1de162c1b5ec212 100644
+--- a/src/main/java/net/minecraft/server/MinecraftServer.java
++++ b/src/main/java/net/minecraft/server/MinecraftServer.java
+@@ -297,6 +297,7 @@ 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();
+@@ -1495,7 +1496,6 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
+         this.getFunctions().tick();
+         MinecraftTimings.commandFunctionsTimer.stopTiming(); // Spigot // Paper
+         this.profiler.popPush("levels");
+-        Iterator iterator = this.getAllLevels().iterator();
+
+         // CraftBukkit start
+         // Run tasks that are waiting on processing
+@@ -1527,6 +1527,8 @@ 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
+@@ -1574,6 +1576,7 @@ 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 81eedf4812b654bd068a00e7f0e63ae6388dfe31..b867d19113862300968a54cb3d6926bef2d04e69 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+@@ -1139,6 +1139,7 @@ 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
+         Validate.notNull(creator, "Creator may not be null");
+
+         String name = creator.name();
+@@ -1263,6 +1264,7 @@ 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;
+         }