Paper/patches/server/0909-Fix-race-condition-on-UpgradeData.BlockFixers-class-.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Tue, 8 Aug 2023 17:29:33 -0700
Subject: [PATCH] Fix race condition on UpgradeData.BlockFixers class init

The CHUNKY_FIXERS field is modified during the constructors
of the BlockFixers, but the code that uses CHUNKY_FIXERS does
not properly ensure that BlockFixers has been initialised before
using it, leading to a possible race condition where instances of
BlockFixers are accessed before they have initialised correctly.

We can force the class to initialise fully before accessing the
field by calling any method on the class, and for convenience
we use values().

diff --git a/src/main/java/net/minecraft/world/level/chunk/UpgradeData.java b/src/main/java/net/minecraft/world/level/chunk/UpgradeData.java
index 22b6d0851a51da180cd8fbbe6554c5370f5ac5bd..cd9b65f278a750a0177a3252271015d43172b2e9 100644
--- a/src/main/java/net/minecraft/world/level/chunk/UpgradeData.java
+++ b/src/main/java/net/minecraft/world/level/chunk/UpgradeData.java
@@ -140,6 +140,7 @@ public class UpgradeData {
             Fluid fluid = tick.type() == Fluids.EMPTY ? level.getFluidState(tick.pos()).getType() : tick.type();
             level.scheduleTick(tick.pos(), fluid, tick.delay(), tick.priority());
         });
+        UpgradeData.BlockFixers.values(); // Paper - force the class init so that we don't access CHUNKY_FIXERS before all BlockFixers are initialised
         CHUNKY_FIXERS.forEach(logic -> logic.processChunk(level));
     }