Paper/patches/server/0874-Fix-concurrenct-access-to-lookups-field-in-RegistryO.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Mon, 15 May 2023 00:20:59 -0700
Subject: [PATCH] Fix concurrenct access to lookups field in RegistryOps

The concurrent access occurs on the Netty IO threads when
serializing packets. Thus, it seems it was an oversight of
the implementator of this function as there are typically
more than one Netty IO thread.

Fixes https://github.com/PaperMC/Folia/issues/11

diff --git a/src/main/java/net/minecraft/resources/RegistryOps.java b/src/main/java/net/minecraft/resources/RegistryOps.java
index 7709eeac907c4895a264cec0a3d453aa8b194c18..c7cfc3ca58f9439142fe5828117f6d576d7df10e 100644
--- a/src/main/java/net/minecraft/resources/RegistryOps.java
+++ b/src/main/java/net/minecraft/resources/RegistryOps.java
@@ -19,11 +19,14 @@ public class RegistryOps<T> extends DelegatingOps<T> {
 
     private static RegistryOps.RegistryInfoLookup memoizeLookup(final RegistryOps.RegistryInfoLookup registryInfoGetter) {
         return new RegistryOps.RegistryInfoLookup() {
-            private final Map<ResourceKey<? extends Registry<?>>, Optional<? extends RegistryOps.RegistryInfo<?>>> lookups = new HashMap<>();
+            // The concurrent access occurs on the Netty IO threads when serializing packets.
+            // Thus, it seems it was an oversight of the implementator of this function as there
+            // are typically more than one Netty IO thread.
+            private final Map<ResourceKey<? extends Registry<?>>, Optional<? extends RegistryOps.RegistryInfo<?>>> lookups = new java.util.concurrent.ConcurrentHashMap<>(); // Paper - fix concurrent access to lookups field
 
             @Override
             public <T> Optional<RegistryOps.RegistryInfo<T>> lookup(ResourceKey<? extends Registry<? extends T>> registryRef) {
-                return this.lookups.computeIfAbsent(registryRef, registryInfoGetter::lookup);
+                return (Optional<RegistryOps.RegistryInfo<T>>)this.lookups.computeIfAbsent(registryRef, registryInfoGetter::lookup); // Paper - fix concurrent access to lookups field
             }
         };
     }