From 76b94c274fe9775efcfd51c676d80c88a4f7fdb9 Mon Sep 17 00:00:00 2001
From: Erik Krogen <ekrogen@linkedin.com>
Date: Fri, 7 Jun 2019 14:20:44 -0700
Subject: [PATCH] HADOOP-16345. Fix a potential NPE when instantiating
 FairCallQueue metrics. Contributed by Erik Krogen.

---
 .../org/apache/hadoop/ipc/FairCallQueue.java     | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java
index 380426fe5b..b4e953948c 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java
@@ -377,9 +377,21 @@ public void setDelegate(FairCallQueue<? extends Schedulable> obj) {
       this.revisionNumber++;
     }
 
+    /**
+     * Fetch the current call queue from the weak reference delegate. If there
+     * is no delegate, or the delegate is empty, this will return null.
+     */
+    private FairCallQueue<? extends Schedulable> getCallQueue() {
+      WeakReference<FairCallQueue<? extends Schedulable>> ref = this.delegate;
+      if (ref == null) {
+        return null;
+      }
+      return ref.get();
+    }
+
     @Override
     public int[] getQueueSizes() {
-      FairCallQueue<? extends Schedulable> obj = this.delegate.get();
+      FairCallQueue<? extends Schedulable> obj = getCallQueue();
       if (obj == null) {
         return new int[]{};
       }
@@ -389,7 +401,7 @@ public int[] getQueueSizes() {
 
     @Override
     public long[] getOverflowedCalls() {
-      FairCallQueue<? extends Schedulable> obj = this.delegate.get();
+      FairCallQueue<? extends Schedulable> obj = getCallQueue();
       if (obj == null) {
         return new long[]{};
       }