Fix potential race condition in MRUCache

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) {
+        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() {
+        synchronized (this) {
             this.recent = null;
             this.modCount.incrementAndGet();
         }
     }
+}