Leaf/leaf-server/minecraft-patches/features/0271-Paper-PR-Fix-async-command-building-throwing-CME-in-.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: roro1506HD <roro1506mcpro@gmail.com>
Date: Fri, 24 Oct 2025 00:19:58 +0200
Subject: [PATCH] Paper PR: Fix async command building throwing CME in some
 cases

Original license: GPLv3
Original project: https://github.com/PaperMC/Paper
Paper pull request: https://github.com/PaperMC/Paper/pull/13233

The issue is from multiple places during that sendAsync process: Anything that loops through any CommandNode's children is subject to the CME.

The previous fix was only working for the root node, and wasn't even fully working given a few lines lower it's being added in an HashMap, which does .hashCode and loops through the root's children again.

This PR reverts the previous fix and fixes the concurrency issue once for all:

Changed the CommandNode's children map to a synchronized map, still backed by a LinkedHashMap
Lock everything in Commands#fillUsableCommands inside a synchronized block that uses the children map as lock object, the same lock object the synchronized map uses.
This has been tested with the test plugin published on [this branch of my fork](https://github.com/roro1506HD/Paper/tree/other/async_command_cme_fix_test_plugin)

Fixes https://github.com/PaperMC/Paper/issues/11101

diff --git a/com/mojang/brigadier/tree/CommandNode.java b/com/mojang/brigadier/tree/CommandNode.java
index f05e69aaf79e47951de1506a97f56d84a4fa9f7f..42549e8ed22f5e6265fea47d1e475710a36a7bf0 100644
--- a/com/mojang/brigadier/tree/CommandNode.java
+++ b/com/mojang/brigadier/tree/CommandNode.java
@@ -24,7 +24,7 @@ import java.util.concurrent.CompletableFuture;
 import java.util.function.Predicate;
 
 public abstract class CommandNode<S> implements Comparable<CommandNode<S>> {
-    private final Map<String, CommandNode<S>> children = new LinkedHashMap<>();
+    public final Map<String, CommandNode<S>> children = Collections.synchronizedMap(new LinkedHashMap<>()); // Paper PR - Fix async command building throwing CME in some cases - Prevent concurrent modification when sending commands; public
     private final Map<String, LiteralCommandNode<S>> literals = new LinkedHashMap<>();
     private final Map<String, ArgumentCommandNode<S, ?>> arguments = new LinkedHashMap<>();
     public Predicate<S> requirement; // Paper - public-f
diff --git a/net/minecraft/commands/Commands.java b/net/minecraft/commands/Commands.java
index 5f80c7b06411e0ad9b0f71f03bb4568daea3ea4d..f7b6ed257925f909a1d418c18538c68197210f8b 100644
--- a/net/minecraft/commands/Commands.java
+++ b/net/minecraft/commands/Commands.java
@@ -453,9 +453,7 @@ public class Commands {
         // CraftBukkit start
         // Register Vanilla commands into builtRoot as before
         // Paper start - Perf: Async command map building
-        // Copy root children to avoid concurrent modification during building
-        final java.util.Collection<CommandNode<CommandSourceStack>> commandNodes = new java.util.ArrayList<>(this.dispatcher.getRoot().getChildren());
-        COMMAND_SENDING_POOL.execute(() -> this.sendAsync(player, commandNodes));
+        COMMAND_SENDING_POOL.execute(() -> this.sendAsync(player)); // Paper PR - Fix async command building throwing CME in some cases
     }
 
     // Fixed pool, but with discard policy
@@ -469,12 +467,12 @@ public class Commands {
         new java.util.concurrent.ThreadPoolExecutor.DiscardPolicy()
     );
 
-    private void sendAsync(ServerPlayer player, java.util.Collection<CommandNode<CommandSourceStack>> dispatcherRootChildren) {
+    private void sendAsync(ServerPlayer player) { // Paper PR - Fix async command building throwing CME in some cases
         // Paper end - Perf: Async command map building
         Map<CommandNode<CommandSourceStack>, CommandNode<CommandSourceStack>> map = new HashMap<>();
         RootCommandNode<CommandSourceStack> rootCommandNode = new RootCommandNode<>();
         map.put(this.dispatcher.getRoot(), rootCommandNode);
-        fillUsableCommands(dispatcherRootChildren, rootCommandNode, player.createCommandSourceStack(), map); // Paper - Perf: Async command map building; pass copy of children
+        fillUsableCommands(this.dispatcher.getRoot(), rootCommandNode, player.createCommandSourceStack(), map); // Paper PR - Fix async command building throwing CME in some cases
 
         java.util.Collection<String> bukkit = new java.util.LinkedHashSet<>();
         for (CommandNode node : rootCommandNode.getChildren()) {
@@ -505,8 +503,11 @@ public class Commands {
         player.connection.send(new ClientboundCommandsPacket(rootCommandNode, COMMAND_NODE_INSPECTOR));
     }
 
-    private static <S> void fillUsableCommands(java.util.Collection<CommandNode<S>> children, CommandNode<S> current, S source, Map<CommandNode<S>, CommandNode<S>> output) { // Paper - Perf: Async command map building; pass copy of children
-        for (CommandNode<S> commandNode : children) {  // Paper - Perf: Async command map building; pass copy of children
+    // Paper PR start - Fix async command building throwing CME in some cases
+    private static <S> void fillUsableCommands(CommandNode<S> root, CommandNode<S> current, S source, Map<CommandNode<S>, CommandNode<S>> output) { // Paper - Perf: Async command map building; pass copy of children
+        synchronized (root.children) { // Paper - Perf: Async command map building
+            for (CommandNode<S> commandNode : root.getChildren()) {
+            // Paper PR end - Fix async command building throwing CME in some cases
             // Paper start - Brigadier API
             if (commandNode.clientNode != null) {
                 commandNode = commandNode.clientNode;
@@ -558,10 +559,11 @@ public class Commands {
                 output.put(commandNode, commandNode1);
                 current.addChild(commandNode1);
                 if (!commandNode.getChildren().isEmpty()) {
-                    fillUsableCommands(commandNode.getChildren(), commandNode1, source, output); // Paper - Perf: Async command map building; pass copy of children
+                    fillUsableCommands(commandNode, commandNode1, source, output); // Paper PR - Fix async command building throwing CME in some cases
                 }
             }
         }
+        } // Paper PR - Fix async command building throwing CME in some cases
     }
 
     public static LiteralArgumentBuilder<CommandSourceStack> literal(String name) {