From f3c6da32d79b1053ca4667191fa7a660a457b44e Mon Sep 17 00:00:00 2001
From: Norman Maurer <norman_maurer@apple.com>
Date: Mon, 15 Jan 2018 14:17:52 +0100
Subject: [PATCH] Fix concurrency issue in DnsNameResolver when DefaultDnsCache
 is used.

Motivation:

We need to ensure we only call List.* methods in the synchronized block as the returned List may not be thread-safe.

Modifications:

Do not call isEmpty() outside of the synchronized block.

Result:

Fixes [#7583]
---
 .../java/io/netty/resolver/dns/DnsNameResolver.java  | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/resolver-dns/src/main/java/io/netty/resolver/dns/DnsNameResolver.java b/resolver-dns/src/main/java/io/netty/resolver/dns/DnsNameResolver.java
index 8510488c68..1a928f8c23 100644
--- a/resolver-dns/src/main/java/io/netty/resolver/dns/DnsNameResolver.java
+++ b/resolver-dns/src/main/java/io/netty/resolver/dns/DnsNameResolver.java
@@ -600,7 +600,7 @@ public class DnsNameResolver extends InetNameResolver {
                                     Promise<InetAddress> promise,
                                     DnsCache resolveCache) {
         final List<? extends DnsCacheEntry> cachedEntries = resolveCache.get(hostname, additionals);
-        if (cachedEntries == null || cachedEntries.isEmpty()) {
+        if (cachedEntries == null) {
             return false;
         }
 
@@ -608,7 +608,9 @@ public class DnsNameResolver extends InetNameResolver {
         Throwable cause = null;
         synchronized (cachedEntries) {
             final int numEntries = cachedEntries.size();
-            assert numEntries > 0;
+            if (numEntries == 0) {
+                return false;
+            }
 
             if (cachedEntries.get(0).cause() != null) {
                 cause = cachedEntries.get(0).cause();
@@ -730,7 +732,7 @@ public class DnsNameResolver extends InetNameResolver {
                                        Promise<List<InetAddress>> promise,
                                        DnsCache resolveCache) {
         final List<? extends DnsCacheEntry> cachedEntries = resolveCache.get(hostname, additionals);
-        if (cachedEntries == null || cachedEntries.isEmpty()) {
+        if (cachedEntries == null) {
             return false;
         }
 
@@ -738,7 +740,9 @@ public class DnsNameResolver extends InetNameResolver {
         Throwable cause = null;
         synchronized (cachedEntries) {
             final int numEntries = cachedEntries.size();
-            assert numEntries > 0;
+            if (numEntries == 0) {
+                return false;
+            }
 
             if (cachedEntries.get(0).cause() != null) {
                 cause = cachedEntries.get(0).cause();