[#5014] Correctly encode / decode zero-length names when encoding DnsRecords.

Motivation:

Zero-length names needs to be "prefixed" by the length as well when encoded into a ByteBuf. Also some servers not correctly prefix these so we should ensure we can workaround this and even decode in such case.

Modifications:

- Always encode the length of the name into the ByteBuf even if its zero-length.
- If there are no readable bytes for the name just asume its an empty name to workaround dns servers that not fully respect the RFC.

Result:

Correctly encode zero-length names and be able to decode empty names even when the rfc is not strictly followed.

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

@@ -107,7 +107,19 @@ public class DefaultDnsRecordDecoder implements DnsRecordDecoder {
         int position = -1;
         int checked = 0;
         final int end = in.writerIndex();
-        final StringBuilder name = new StringBuilder(in.readableBytes() << 1);
+        final int readable = in.readableBytes();
+
+        // Looking at the spec we should always have at least enough readable bytes to read a byte here but it seems
+        // some servers do not respect this for empty names. So just workaround this and return an empty name in this
+        // case.
+        //
+        // See:
+        // - https://github.com/netty/netty/issues/5014
+        // - https://www.ietf.org/rfc/rfc1035.txt , Section 3.1
+        if (readable == 0) {
+            return StringUtil.EMPTY_STRING;
+        }
+        final StringBuilder name = new StringBuilder(readable << 1);
         for (int len = in.readUnsignedByte(); in.isReadable() && len != 0; len = in.readUnsignedByte()) {
             boolean pointer = (len & 0xc0) == 0xc0;
             if (pointer) {

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

@@ -80,10 +80,14 @@ public class DefaultDnsRecordEncoder implements DnsRecordEncoder {
         String[] parts = StringUtil.split(name, '.');
         for (String part: parts) {
             final int partLen = part.length();
+            // We always need to write the length even if its 0.
+            // See:
+            // - https://github.com/netty/netty/issues/5014
+            // - https://www.ietf.org/rfc/rfc1035.txt , Section 3.1
+            buf.writeByte(partLen);
             if (partLen == 0) {
                 continue;
             }
-            buf.writeByte(partLen);
             ByteBufUtil.writeAscii(buf, part);
         }
         buf.writeByte(0); // marks end of name field

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

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2016 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.handler.codec.dns;
+
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+import io.netty.util.internal.StringUtil;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class DefaultDnsRecordDecoderTest {
+
+    @Test
+    public void testDecodeEmptyName() {
+        testDecodeEmptyName0(Unpooled.buffer().writeByte('0'));
+    }
+
+    @Test
+    public void testDecodeEmptyNameNonRFC() {
+        testDecodeEmptyName0(Unpooled.EMPTY_BUFFER);
+    }
+
+    private static void testDecodeEmptyName0(ByteBuf buffer) {
+        try {
+            DefaultDnsRecordDecoder decoder = new DefaultDnsRecordDecoder();
+            Assert.assertEquals(StringUtil.EMPTY_STRING, decoder.decodeName(buffer));
+        } finally {
+            buffer.release();
+        }
+    }
+}

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

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2016 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.handler.codec.dns;
+
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+import io.netty.util.internal.StringUtil;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class DefaultDnsRecordEncoderTest {
+
+    // Test for https://github.com/netty/netty/issues/5014
+    @Test
+    public void testEncodeEmptyName() throws Exception {
+        DefaultDnsRecordEncoder encoder = new DefaultDnsRecordEncoder();
+        ByteBuf out = Unpooled.buffer();
+        try {
+            encoder.encodeName(StringUtil.EMPTY_STRING, out);
+            assertEquals(2, out.readableBytes());
+            assertEquals(0, out.readByte());
+            assertEquals(0, out.readByte());
+        } finally {
+            out.release();
+        }
+    }
+}