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 334962f1bf5988059fec506f0d3aaf4302205220..6a270c9adb044a6e0b1c8e09b9106d51989fd761 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
@@ -18,7 +18,7 @@ public class GateBehavior<E extends LivingEntity> implements BehaviorControl<E>
     private final Set<MemoryModuleType<?>> exitErasedMemories;
     private final GateBehavior.OrderPolicy orderPolicy;
     private final GateBehavior.RunningPolicy runningPolicy;
-    private final ShufflingList<BehaviorControl<? super E>> behaviors = new ShufflingList<>();
+    private final ShufflingList<BehaviorControl<? super E>> behaviors = new ShufflingList<>(false); // Paper - Fix Concurrency issue in ShufflingList during worldgen
     private Behavior.Status status = Behavior.Status.STOPPED;
 
     public GateBehavior(
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 195eea55fabc7a9a665e0a8f04934a3aaf9f8b71..3fac11bf02652b5f51f30f99bdf516504467d0d2 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
@@ -16,12 +16,25 @@ import net.minecraft.util.RandomSource;
 public class ShufflingList<U> implements Iterable<U> {
     protected final List<ShufflingList.WeightedEntry<U>> entries;
     private final RandomSource random = RandomSource.create();
+    private final boolean isUnsafe; // Paper - Fix Concurrency issue in ShufflingList during worldgen
 
     public ShufflingList() {
+        // Paper start - Fix Concurrency issue in ShufflingList during worldgen
+        this(true);
+    }
+    public ShufflingList(boolean isUnsafe) {
+        this.isUnsafe = isUnsafe;
+        // Paper end - Fix Concurrency issue in ShufflingList during worldgen
         this.entries = Lists.newArrayList();
     }
 
     private ShufflingList(List<ShufflingList.WeightedEntry<U>> list) {
+        // Paper start - Fix Concurrency issue in ShufflingList during worldgen
+        this(list, true);
+    }
+    private ShufflingList(List<ShufflingList.WeightedEntry<U>> list, boolean isUnsafe) {
+        this.isUnsafe = isUnsafe;
+        // Paper end - Fix Concurrency issue in ShufflingList during worldgen
         this.entries = Lists.newArrayList(list);
     }
 
@@ -35,9 +48,12 @@ public class ShufflingList<U> implements Iterable<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 - Fix Concurrency issue in ShufflingList during worldgen
+        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 - Fix Concurrency issue in ShufflingList during worldgen
     }
 
     public Stream<U> stream() {