Only handle NXDOMAIN as failure when nameserver is authoritive or no other nameservers are left. (#8731)

Motivation:

When using multiple nameservers and a nameserver respond with NXDOMAIN we should only fail the query if the nameserver in question is authoritive or no nameservers are left to try.

Modifications:

- Try next nameserver if NXDOMAIN was returned but the nameserver is not authoritive
- Adjust testcase to respect correct behaviour.

Result:

Fixes https://github.com/netty/netty/issues/8261

resolver-dns/src/main/java/io/netty/resolver/dns/DnsResolveContext.java

@@ -497,6 +497,29 @@ abstract class DnsResolveContext<T> {
                       queryLifecycleObserver.queryNoAnswer(code), true, promise, null);
             } else {
                 queryLifecycleObserver.queryFailed(NXDOMAIN_QUERY_FAILED_EXCEPTION);
+
+                // Try with the next server if is not authoritative for the domain.
+                //
+                // From https://tools.ietf.org/html/rfc1035 :
+                //
+                //   RCODE        Response code - this 4 bit field is set as part of
+                //                responses.  The values have the following
+                //                interpretation:
+                //
+                //                ....
+                //                ....
+                //
+                //                3               Name Error - Meaningful only for
+                //                                responses from an authoritative name
+                //                                server, this code signifies that the
+                //                                domain name referenced in the query does
+                //                                not exist.
+                //                ....
+                //                ....
+                if (!res.isAuthoritativeAnswer()) {
+                    query(nameServerAddrStream, nameServerAddrStreamIndex + 1, question,
+                            newDnsQueryLifecycleObserver(question), true, promise, null);
+                }
             }
         } finally {
             ReferenceCountUtil.safeRelease(envelope);

resolver-dns/src/test/java/io/netty/resolver/dns/DnsNameResolverTest.java

@@ -1143,7 +1143,7 @@ public class DnsNameResolverTest {
                     new TestRecursiveCacheDnsQueryLifecycleObserverFactory();
 
             DnsNameResolverBuilder builder = new DnsNameResolverBuilder(group.next())
-                    .resolvedAddressTypes(ResolvedAddressTypes.IPV6_PREFERRED)
+                    .resolvedAddressTypes(ResolvedAddressTypes.IPV4_ONLY)
                     .dnsQueryLifecycleObserverFactory(lifecycleObserverFactory)
                     .channelType(NioDatagramChannel.class)
                     .optResourceEnabled(false)
@@ -1156,18 +1156,12 @@ public class DnsNameResolverTest {
 
             TestDnsQueryLifecycleObserver observer = lifecycleObserverFactory.observers.poll();
             assertNotNull(observer);
-            assertEquals(2, lifecycleObserverFactory.observers.size());
+            assertEquals(1, lifecycleObserverFactory.observers.size());
             assertEquals(2, observer.events.size());
             QueryWrittenEvent writtenEvent = (QueryWrittenEvent) observer.events.poll();
             assertEquals(dnsServer1.localAddress(), writtenEvent.dnsServerAddress);
             QueryFailedEvent failedEvent = (QueryFailedEvent) observer.events.poll();
 
-            observer = lifecycleObserverFactory.observers.poll();
-            assertEquals(2, observer.events.size());
-            writtenEvent = (QueryWrittenEvent) observer.events.poll();
-            assertEquals(dnsServer1.localAddress(), writtenEvent.dnsServerAddress);
-            failedEvent = (QueryFailedEvent) observer.events.poll();
-
             observer = lifecycleObserverFactory.observers.poll();
             assertEquals(2, observer.events.size());
             writtenEvent = (QueryWrittenEvent) observer.events.poll();