Fix buffer leaks in DnsResponseDecoder and DnsResponseDecoderTest

Motivation:

There were two buffer leaks in the codec-dns.

Modifications:

- Fix buffer leak in DnsResponseTest.readResponseTest()
- Correctly release DnsResources on Exception

Result:

No more buffer leaks in the codec-dns module.

codec-dns/src/main/java/io/netty/handler/codec/dns/DnsResponseDecoder.java

@@ -64,6 +64,8 @@ public class DnsResponseDecoder extends MessageToMessageDecoder<DatagramPacket>
             out.add(response);
             return;
         }
+        boolean release = true;
+        try {
             for (int i = 0; i < answers; i++) {
                 response.addAnswer(decodeResource(buf));
             }
@@ -74,6 +76,23 @@ public class DnsResponseDecoder extends MessageToMessageDecoder<DatagramPacket>
                 response.addAdditionalResource(decodeResource(buf));
             }
             out.add(response);
+            release = false;
+        } finally {
+            if (release) {
+                // We need to release te DnsResources in case of an Exception as we called retain() on the buffer.
+                releaseDnsResources(response.answers());
+                releaseDnsResources(response.authorityResources());
+                releaseDnsResources(response.additionalResources());
+            }
+        }
+    }
+
+    private static void releaseDnsResources(List<DnsResource> resources) {
+        int size = resources.size();
+        for (int i = 0; i < size; i++) {
+            DnsResource resource = resources.get(i);
+            resource.release();
+        }
     }
 
     /**

codec-dns/src/test/java/io/netty/handler/codec/dns/DnsResponseTest.java

@@ -74,7 +74,6 @@ public class DnsResponseTest {
             ByteBuf packet = embedder.alloc().buffer(512).writeBytes(p);
             embedder.writeInbound(new DatagramPacket(packet, null, new InetSocketAddress(0)));
             DnsResponse decoded = embedder.readInbound();
-            packet.retain().readerIndex(0);
             ByteBuf raw = Unpooled.wrappedBuffer(p);
             Assert.assertEquals("Invalid id, expected: " + raw.getUnsignedShort(0) + ", actual: "
                     + decoded.header().id(), raw.getUnsignedShort(0), decoded.header().id());