From 21ae1dbac000fc5936b33e6367e8454ab035e0b1 Mon Sep 17 00:00:00 2001
From: Luck <git@lucko.me>
Date: Sun, 21 Jun 2020 22:07:53 +0100
Subject: [PATCH] Fix potential race condition in MRUCache

---
 .../lucko/luckperms/common/cache/MRUCache.java   | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/common/src/main/java/me/lucko/luckperms/common/cache/MRUCache.java b/common/src/main/java/me/lucko/luckperms/common/cache/MRUCache.java
index 0e4e60c96..e9cb4ce4d 100644
--- a/common/src/main/java/me/lucko/luckperms/common/cache/MRUCache.java
+++ b/common/src/main/java/me/lucko/luckperms/common/cache/MRUCache.java
@@ -46,13 +46,21 @@ public abstract class MRUCache<T> {
     }
 
     protected void offerRecent(int validAt, T offer) {
-        if (validAt == this.modCount.get()) {
-            this.recent = offer;
+        synchronized (this) {
+            if (validAt == this.modCount.get()) {
+                this.recent = offer;
+            }
         }
     }
 
+    /**
+     * Calling clearRecent effectively resets the instance. any other threads going to call
+     * offerRecent in the future with the previous mod count will be ignored.
+     */
     protected void clearRecent() {
-        this.recent = null;
-        this.modCount.incrementAndGet();
+        synchronized (this) {
+            this.recent = null;
+            this.modCount.incrementAndGet();
+        }
     }
 }