From a2b85a306df8b02ea2e6f222f56f75355660582b Mon Sep 17 00:00:00 2001
From: Norman Maurer <norman_maurer@apple.com>
Date: Mon, 1 Apr 2019 19:44:28 +0200
Subject: [PATCH] Fix NPE that was encounter by debugger (will never happen in
 real code). (#8992)

Motivation:

We synchronize on the chunk.arena when produce the String returned by PoolSubpage.toString() which may raise a NPE when chunk == null. Chunk == null for the head of the linked-list and so a NPE may raised by a debugger. This NPE can never happen in real code tho as we never access toString() of the head.

Modifications:

Add null checks and so fix the possible NPE

Result:

No NPE when using a debugger and inspect the PooledByteBufAllocator.
---
 .../java/io/netty/buffer/PoolSubpage.java     | 43 ++++++++++++++-----
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/buffer/src/main/java/io/netty/buffer/PoolSubpage.java b/buffer/src/main/java/io/netty/buffer/PoolSubpage.java
index f897eeeb48..328aae646d 100644
--- a/buffer/src/main/java/io/netty/buffer/PoolSubpage.java
+++ b/buffer/src/main/java/io/netty/buffer/PoolSubpage.java
@@ -204,16 +204,24 @@ final class PoolSubpage<T> implements PoolSubpageMetric {
         final int maxNumElems;
         final int numAvail;
         final int elemSize;
-        synchronized (chunk.arena) {
-            if (!this.doNotDestroy) {
-                doNotDestroy = false;
-                // Not used for creating the String.
-                maxNumElems = numAvail = elemSize = -1;
-            } else {
-                doNotDestroy = true;
-                maxNumElems = this.maxNumElems;
-                numAvail = this.numAvail;
-                elemSize = this.elemSize;
+        if (chunk == null) {
+            // This is the head so there is no need to synchronize at all as these never change.
+            doNotDestroy = true;
+            maxNumElems = 0;
+            numAvail = 0;
+            elemSize = -1;
+        } else {
+            synchronized (chunk.arena) {
+                if (!this.doNotDestroy) {
+                    doNotDestroy = false;
+                    // Not used for creating the String.
+                    maxNumElems = numAvail = elemSize = -1;
+                } else {
+                    doNotDestroy = true;
+                    maxNumElems = this.maxNumElems;
+                    numAvail = this.numAvail;
+                    elemSize = this.elemSize;
+                }
             }
         }
 
@@ -227,6 +235,11 @@ final class PoolSubpage<T> implements PoolSubpageMetric {
 
     @Override
     public int maxNumElements() {
+        if (chunk == null) {
+            // It's the head.
+            return 0;
+        }
+
         synchronized (chunk.arena) {
             return maxNumElems;
         }
@@ -234,6 +247,11 @@ final class PoolSubpage<T> implements PoolSubpageMetric {
 
     @Override
     public int numAvailable() {
+        if (chunk == null) {
+            // It's the head.
+            return 0;
+        }
+
         synchronized (chunk.arena) {
             return numAvail;
         }
@@ -241,6 +259,11 @@ final class PoolSubpage<T> implements PoolSubpageMetric {
 
     @Override
     public int elementSize() {
+        if (chunk == null) {
+            // It's the head.
+            return -1;
+        }
+
         synchronized (chunk.arena) {
             return elemSize;
         }