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().

patches/server/1030-Fix-race-condition-on-UpgradeData.BlockFixers-class-.patch

@@ -0,0 +1,27 @@
+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 62709ed72faf823e18ad77477eb2f5dbf9c98660..27fcb816f151527029fb1fd02d8247863ccc3d32 100644
+--- a/src/main/java/net/minecraft/world/level/chunk/UpgradeData.java
++++ b/src/main/java/net/minecraft/world/level/chunk/UpgradeData.java
+@@ -139,6 +139,7 @@ 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);
+         });