From 1d8508b7ac2f243fb819e1b6b5bb88db7c60054f 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.
---
 .../ai/behavior/GateBehavior.java.patch       | 11 +++++
 .../ai/behavior/ShufflingList.java.patch      | 44 +++++++++++++++++++
 2 files changed, 55 insertions(+)
 create mode 100644 paper-server/patches/sources/net/minecraft/world/entity/ai/behavior/GateBehavior.java.patch
 create mode 100644 paper-server/patches/sources/net/minecraft/world/entity/ai/behavior/ShufflingList.java.patch

diff --git a/paper-server/patches/sources/net/minecraft/world/entity/ai/behavior/GateBehavior.java.patch b/paper-server/patches/sources/net/minecraft/world/entity/ai/behavior/GateBehavior.java.patch
new file mode 100644
index 0000000000..fdb4490626
--- /dev/null
+++ b/paper-server/patches/sources/net/minecraft/world/entity/ai/behavior/GateBehavior.java.patch
@@ -0,0 +1,11 @@
+--- a/net/minecraft/world/entity/ai/behavior/GateBehavior.java
++++ b/net/minecraft/world/entity/ai/behavior/GateBehavior.java
+@@ -18,7 +18,7 @@
+     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/paper-server/patches/sources/net/minecraft/world/entity/ai/behavior/ShufflingList.java.patch b/paper-server/patches/sources/net/minecraft/world/entity/ai/behavior/ShufflingList.java.patch
new file mode 100644
index 0000000000..3f1d97092e
--- /dev/null
+++ b/paper-server/patches/sources/net/minecraft/world/entity/ai/behavior/ShufflingList.java.patch
@@ -0,0 +1,44 @@
+--- a/net/minecraft/world/entity/ai/behavior/ShufflingList.java
++++ b/net/minecraft/world/entity/ai/behavior/ShufflingList.java
+@@ -16,12 +16,25 @@
+ 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 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() {