Allow enabling sand duping (#10191)

Because this exploit has been widely known for years and has not been fixed by Mojang, we decided that it was worth allowing people to toggle it on/off due to how easy it is to make it configurable.

It should be noted that this decision does not promise all future exploits will be configurable.

patches/server/0860-Use-array-for-gamerule-storage.patch

@@ -0,0 +1,63 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Paul Sauve <paul@technove.co>
+Date: Sun, 9 May 2021 16:49:49 -0500
+Subject: [PATCH] Use array for gamerule storage
+
+
+diff --git a/src/main/java/net/minecraft/world/level/GameRules.java b/src/main/java/net/minecraft/world/level/GameRules.java
+index ac7a5410b01a6741e3b548d153f37ea1d8c1a4cb..4a340bd1f1859e43bb58e68aee4018fdb4ca7a5a 100644
+--- a/src/main/java/net/minecraft/world/level/GameRules.java
++++ b/src/main/java/net/minecraft/world/level/GameRules.java
+@@ -115,6 +115,7 @@ public class GameRules {
+     public static final GameRules.Key<GameRules.BooleanValue> RULE_DO_VINES_SPREAD = GameRules.register("doVinesSpread", GameRules.Category.UPDATES, GameRules.BooleanValue.create(true));
+     public static final GameRules.Key<GameRules.BooleanValue> RULE_ENDER_PEARLS_VANISH_ON_DEATH = GameRules.register("enderPearlsVanishOnDeath", GameRules.Category.PLAYER, GameRules.BooleanValue.create(true));
+     private final Map<GameRules.Key<?>, GameRules.Value<?>> rules;
++    private final GameRules.Value<?>[] gameruleArray; // Paper - Perf: Use array for gamerule storage
+ 
+     private static <T extends GameRules.Value<T>> GameRules.Key<T> register(String name, GameRules.Category category, GameRules.Type<T> type) {
+         GameRules.Key<T> gamerules_gamerulekey = new GameRules.Key<>(name, category);
+@@ -133,17 +134,30 @@ public class GameRules {
+     }
+ 
+     public GameRules() {
+-        this.rules = (Map) GameRules.GAME_RULE_TYPES.entrySet().stream().collect(ImmutableMap.toImmutableMap(Entry::getKey, (entry) -> {
++        // Paper start - Perf: Use array for gamerule storage
++        this((Map) GameRules.GAME_RULE_TYPES.entrySet().stream().collect(ImmutableMap.toImmutableMap(Entry::getKey, (entry) -> {
+             return ((GameRules.Type) entry.getValue()).createRule();
+-        }));
++        })));
++        // Paper end - Perf: Use array for gamerule storage
+     }
+ 
+     private GameRules(Map<GameRules.Key<?>, GameRules.Value<?>> rules) {
+         this.rules = rules;
++
++        // Paper start - Perf: Use array for gamerule storage
++        int arraySize = rules.keySet().stream().mapToInt(key -> key.gameRuleIndex).max().orElse(-1) + 1;
++        GameRules.Value<?>[] values = new GameRules.Value[arraySize];
++
++        for (Entry<GameRules.Key<?>, GameRules.Value<?>> entry : rules.entrySet()) {
++            values[entry.getKey().gameRuleIndex] = entry.getValue();
++        }
++
++        this.gameruleArray = values;
++        // Paper end - Perf: Use array for gamerule storage
+     }
+ 
+     public <T extends GameRules.Value<T>> T getRule(GameRules.Key<T> key) {
+-        return (T) this.rules.get(key); // CraftBukkit - decompile error
++        return key == null ? null : (T) this.gameruleArray[key.gameRuleIndex]; // Paper - Perf: Use array for gamerule storage
+     }
+ 
+     public CompoundTag createTag() {
+@@ -202,6 +216,10 @@ public class GameRules {
+     }
+ 
+     public static final class Key<T extends GameRules.Value<T>> {
++        // Paper start - Perf: Use array for gamerule storage
++        private static int lastGameRuleIndex = 0;
++        public final int gameRuleIndex = lastGameRuleIndex++;
++        // Paper end - Perf: Use array for gamerule storage
+ 
+         final String id;
+         private final GameRules.Category category;