From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Mon, 6 Jul 2020 18:36:41 -0400
Subject: [PATCH] Fix Concurrency issue in ShufflingList

if multiple threads from worldgen sort at same time, it will crash.
So make a copy of the list for sorting purposes.

diff --git a/src/main/java/net/minecraft/world/entity/ai/behavior/GateBehavior.java b/src/main/java/net/minecraft/world/entity/ai/behavior/GateBehavior.java
index c1f22c5e17418f91736237af1495a8a9910a61d5..5a5d454b5987bb01d03f91c15b7a6bff46f1de71 100644
--- a/src/main/java/net/minecraft/world/entity/ai/behavior/GateBehavior.java
+++ b/src/main/java/net/minecraft/world/entity/ai/behavior/GateBehavior.java
@@ -16,7 +16,7 @@ public class GateBehavior<E extends LivingEntity> extends Behavior<E> {
     private final Set<MemoryModuleType<?>> exitErasedMemories;
     private final GateBehavior.OrderPolicy orderPolicy;
     private final GateBehavior.RunningPolicy runningPolicy;
-    private final ShufflingList<Behavior<? super E>> behaviors = new ShufflingList<>();
+    private final ShufflingList<Behavior<? super E>> behaviors = new ShufflingList<>(false); // Paper - don't use a clone
 
     public GateBehavior(Map<MemoryModuleType<?>, MemoryStatus> requiredMemoryState, Set<MemoryModuleType<?>> memoriesToForgetWhenStopped, GateBehavior.OrderPolicy order, GateBehavior.RunningPolicy runMode, List<Pair<Behavior<? super E>, Integer>> tasks) {
         super(requiredMemoryState);
diff --git a/src/main/java/net/minecraft/world/entity/ai/behavior/ShufflingList.java b/src/main/java/net/minecraft/world/entity/ai/behavior/ShufflingList.java
index 2af08836c60ddf0e1df7c53316eae6e329b5747f..090bba46b6ecd7d0e1415feb678b9b23264fe5e9 100644
--- a/src/main/java/net/minecraft/world/entity/ai/behavior/ShufflingList.java
+++ b/src/main/java/net/minecraft/world/entity/ai/behavior/ShufflingList.java
@@ -14,12 +14,25 @@ import net.minecraft.util.RandomSource;
 public class ShufflingList<U> {
     protected final List<ShufflingList.WeightedEntry<U>> entries;
     private final RandomSource random = RandomSource.create();
+    private final boolean isUnsafe; // Paper
 
     public ShufflingList() {
+        // Paper start
+        this(true);
+    }
+    public ShufflingList(boolean isUnsafe) {
+        this.isUnsafe = isUnsafe;
+        // Paper end
         this.entries = Lists.newArrayList();
     }
 
     private ShufflingList(List<ShufflingList.WeightedEntry<U>> list) {
+        // Paper start
+        this(list, true);
+    }
+    private ShufflingList(List<ShufflingList.WeightedEntry<U>> list, boolean isUnsafe) {
+        this.isUnsafe = isUnsafe;
+        // Paper end
         this.entries = Lists.newArrayList(list);
     }
 
@@ -35,11 +48,12 @@ public class ShufflingList<U> {
     }
 
     public ShufflingList<U> shuffle() {
-        this.entries.forEach((entry) -> {
-            entry.setRandom(this.random.nextFloat());
-        });
-        this.entries.sort(Comparator.comparingDouble(ShufflingList.WeightedEntry::getRandWeight));
-        return this;
+        // Paper start - make concurrent safe, work off a clone of the list
+        List<ShufflingList.WeightedEntry<U>> list = this.isUnsafe ? Lists.newArrayList(this.entries) : this.entries;
+        list.forEach(entry -> entry.setRandom(this.random.nextFloat()));
+        list.sort(Comparator.comparingDouble(ShufflingList.WeightedEntry::getRandWeight));
+        return this.isUnsafe ? new ShufflingList<>(list, this.isUnsafe) : this;
+        // Paper end
     }
 
     public Stream<U> stream() {