Several fixes and new api for experience merging/stacking (#9242)

patches/server/0908-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 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));
+     }
+