Cache DataFixerUpper Rewrite Rules on demand

Mojang precaches every single potential rewrite rule that could ever
exist on server startup. This includes rules from all the way back to versions from 6+ years ago.

This is the source of why the server hogs every CPU core at 100% every start.

For anyone who hard resets for updates or has force upgraded their entire world, this
results in completely wasted cpu cycles.

This massive CPU usage also delays server startup time.

We improve this by making "min version to precache" that defaults to a future version
so that no rewrite rules are precached.

someone who expects to be converting a lot chunks could theoretically set
-DPaper.minPrecachedDatafixVersion=<dataVersionConvertingFrom> as a startup
parameter and only build from that point on.

However this will likely never be needed as the server will still run
the same cache logic on demand when it's actually needed. The only
cost would be some delay on the FIRST chunk conversion, but paper already
runs chunk conversions on another thread so this will likely never be
a concern for TPS.

This patch will significantly reduce CPU use on startup, reduce memory usage,
and improve server startup time.

Spigot-Server-Patches/0579-Cache-DataFixerUpper-Rewrite-Rules-on-demand.patch

@@ -0,0 +1,54 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sat, 12 Sep 2020 17:21:38 -0400
+Subject: [PATCH] Cache DataFixerUpper Rewrite Rules on demand
+
+Mojang precaches every single potential rewrite rule that could ever
+exist on server startup. This includes rules from all the way back to versions from 6+ years ago.
+
+This is the source of why the server hogs every CPU core at 100% every start.
+
+For anyone who hard resets for updates or has force upgraded their entire world, this
+results in completely wasted cpu cycles.
+
+This massive CPU usage also delays server startup time.
+
+We improve this by making "min version to precache" that defaults to a future version
+so that no rewrite rules are precached.
+
+someone who expects to be converting a lot chunks could theoretically set
+-DPaper.minPrecachedDatafixVersion=<dataVersionConvertingFrom> as a startup
+parameter and only build from that point on.
+
+However this will likely never be needed as the server will still run
+the same cache logic on demand when it's actually needed. The only
+cost would be some delay on the FIRST chunk conversion, but paper already
+runs chunk conversions on another thread so this will likely never be
+a concern for TPS.
+
+This patch will significantly reduce CPU use on startup, reduce memory usage,
+and improve server startup time.
+
+diff --git a/src/main/java/com/mojang/datafixers/DataFixerBuilder.java b/src/main/java/com/mojang/datafixers/DataFixerBuilder.java
+index edb77982d273e9492ab1a669ca1ad89da2ec3c3e..abc265b00044b14abb55c2628d454ee01fef467b 100644
+--- a/src/main/java/com/mojang/datafixers/DataFixerBuilder.java
++++ b/src/main/java/com/mojang/datafixers/DataFixerBuilder.java
+@@ -26,8 +26,10 @@ public class DataFixerBuilder {
+     private final Int2ObjectSortedMap<Schema> schemas = new Int2ObjectAVLTreeMap<>();
+     private final List<DataFix> globalList = Lists.newArrayList();
+     private final IntSortedSet fixerVersions = new IntAVLTreeSet();
++    private final int minDataFixPrecacheVersion; // Paper
+ 
+     public DataFixerBuilder(final int dataVersion) {
++        minDataFixPrecacheVersion = Integer.getInteger("Paper.minPrecachedDatafixVersion", dataVersion+1) * 10; // Paper - default to precache nothing - mojang stores versions * 10 to allow for 'sub versions'
+         this.dataVersion = dataVersion;
+     }
+ 
+@@ -65,6 +67,7 @@ public class DataFixerBuilder {
+         final IntBidirectionalIterator iterator = fixerUpper.fixerVersions().iterator();
+         while (iterator.hasNext()) {
+             final int versionKey = iterator.nextInt();
++            if (versionKey < minDataFixPrecacheVersion) continue; // Paper
+             final Schema schema = schemas.get(versionKey);
+             for (final String typeName : schema.types()) {
+                 CompletableFuture.runAsync(() -> {

scripts/importmcdev.sh

@@ -105,6 +105,7 @@ done
 
 # dont forget \ at end of each line but last
 importLibrary com.mojang authlib com/mojang/authlib yggdrasil/YggdrasilGameProfileRepository.java
+importLibrary com.mojang datafixerupper com/mojang/datafixers DataFixerBuilder.java
 importLibrary com.mojang datafixerupper com/mojang/datafixers/util Either.java
 importLibrary com.mojang datafixerupper com/mojang/serialization/codecs KeyDispatchCodec.java