DnsAddressResolverGroup should respect configured EventLoop (#10479)

Motivation:

DnsAddressResolverGroup allows to be constructed with a DnsNameResolverBuilder and so should respect its configured EventLoop.

Modifications:

- Correctly respect the configured EventLoop
- Ensure there are no thread-issues by calling copy()
- Add unit tests

Result:

Fixes https://github.com/netty/netty/issues/10460

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

@@ -51,14 +51,14 @@ public class DnsAddressResolverGroup extends AddressResolverGroup<InetSocketAddr
     public DnsAddressResolverGroup(
             Class<? extends DatagramChannel> channelType,
             DnsServerAddressStreamProvider nameServerProvider) {
-        this(new DnsNameResolverBuilder());
+        this.dnsResolverBuilder = new DnsNameResolverBuilder();
         dnsResolverBuilder.channelType(channelType).nameServerProvider(nameServerProvider);
     }
 
     public DnsAddressResolverGroup(
             ChannelFactory<? extends DatagramChannel> channelFactory,
             DnsServerAddressStreamProvider nameServerProvider) {
-        this(new DnsNameResolverBuilder());
+        this.dnsResolverBuilder = new DnsNameResolverBuilder();
         dnsResolverBuilder.channelFactory(channelFactory).nameServerProvider(nameServerProvider);
     }
 
@@ -73,7 +73,8 @@ public class DnsAddressResolverGroup extends AddressResolverGroup<InetSocketAddr
 
         // we don't really need to pass channelFactory and nameServerProvider separately,
         // but still keep this to ensure backward compatibility with (potentially) override methods
-        return newResolver((EventLoop) executor,
+        EventLoop loop = dnsResolverBuilder.eventLoop;
+        return newResolver(loop == null ? (EventLoop) executor : loop,
                 dnsResolverBuilder.channelFactory(),
                 dnsResolverBuilder.nameServerProvider());
     }
@@ -103,9 +104,11 @@ public class DnsAddressResolverGroup extends AddressResolverGroup<InetSocketAddr
                                                         ChannelFactory<? extends DatagramChannel> channelFactory,
                                                         DnsServerAddressStreamProvider nameServerProvider)
             throws Exception {
+        DnsNameResolverBuilder builder = dnsResolverBuilder.copy();
+
         // once again, channelFactory and nameServerProvider are most probably set in builder already,
         // but I do reassign them again to avoid corner cases with override methods
-        return dnsResolverBuilder.eventLoop(eventLoop)
+        return builder.eventLoop(eventLoop)
                 .channelFactory(channelFactory)
                 .nameServerProvider(nameServerProvider)
                 .build();

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

@@ -36,7 +36,7 @@ import static io.netty.util.internal.ObjectUtil.intValue;
  * A {@link DnsNameResolver} builder.
  */
 public final class DnsNameResolverBuilder {
-    private EventLoop eventLoop;
+    volatile EventLoop eventLoop;
     private ChannelFactory<? extends DatagramChannel> channelFactory;
     private ChannelFactory<? extends SocketChannel> socketChannelFactory;
     private DnsCache resolveCache;

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

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2020 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package io.netty.resolver.dns;
+
+import io.netty.channel.DefaultEventLoopGroup;
+import io.netty.channel.EventLoop;
+import io.netty.channel.nio.NioEventLoopGroup;
+import io.netty.channel.socket.nio.NioDatagramChannel;
+import io.netty.resolver.AddressResolver;
+import io.netty.util.concurrent.Future;
+import io.netty.util.concurrent.FutureListener;
+import io.netty.util.concurrent.Promise;
+import org.hamcrest.Matchers;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.net.SocketAddress;
+import java.nio.channels.UnsupportedAddressTypeException;
+
+public class DnsAddressResolverGroupTest {
+    @Test
+    public void testUseConfiguredEventLoop() throws InterruptedException {
+        NioEventLoopGroup group = new NioEventLoopGroup(1);
+        final EventLoop loop = group.next();
+        DefaultEventLoopGroup defaultEventLoopGroup = new DefaultEventLoopGroup(1);
+        DnsNameResolverBuilder builder = new DnsNameResolverBuilder()
+                .eventLoop(loop).channelType(NioDatagramChannel.class);
+        DnsAddressResolverGroup resolverGroup = new DnsAddressResolverGroup(builder);
+        try {
+            final Promise<?> promise = loop.newPromise();
+            AddressResolver<?> resolver = resolverGroup.getResolver(defaultEventLoopGroup.next());
+            resolver.resolve(new SocketAddress() {
+                private static final long serialVersionUID = 3169703458729818468L;
+            }).addListener(new FutureListener<Object>() {
+                @Override
+                public void operationComplete(Future<Object> future) {
+                    try {
+                        Assert.assertThat(future.cause(), Matchers.instanceOf(UnsupportedAddressTypeException.class));
+                        Assert.assertTrue(loop.inEventLoop());
+                        promise.setSuccess(null);
+                    } catch (Throwable cause) {
+                        promise.setFailure(cause);
+                    }
+                }
+            }).await();
+            promise.sync();
+        } finally {
+            resolverGroup.close();
+            group.shutdownGracefully();
+            defaultEventLoopGroup.shutdownGracefully();
+        }
+    }
+}