Revamp DNS codec

Motivation:

There are various known issues in netty-codec-dns:

- Message types are not interfaces, which can make it difficult for a
  user to implement his/her own message implementation.
- Some class names and field names do not match with the terms in the
  RFC.
- The support for decoding a DNS record was limited. A user had to
  encode and decode by him/herself.
- The separation of DnsHeader from DnsMessage was unnecessary, although
  it is fine conceptually.
- Buffer leak caused by DnsMessage was difficult to analyze, because the
  leak detector tracks down the underlying ByteBuf rather than the
  DnsMessage itself.
- DnsMessage assumes DNS-over-UDP.
- To send an EDNS message, a user have to create a new DNS record class
  instance unnecessarily.

Modifications:

- Make all message types interfaces and add default implementations
- Rename some classes, properties, and constants to match the RFCs
  - DnsResource -> DnsRecord
  - DnsType -> DnsRecordType
  - and many more
- Remove DnsClass and use an integer to support EDNS better
- Add DnsRecordEncoder/DnsRecordDecoder and their default
  implementations
  - DnsRecord does not require RDATA to be ByteBuf anymore.
  - Add DnsRawRecord as the catch-all record type
- Merge DnsHeader into DnsMessage
- Make ResourceLeakDetector track AbstractDnsMessage
- Remove DnsMessage.sender/recipient properties
  - Wrap DnsMessage with AddressedEnvelope
  - Add DatagramDnsQuest and DatagramDnsResponse for ease of use
  - Rename DnsQueryEncoder to DatagramDnsQueryEncoder
  - Rename DnsResponseDecoder to DatagramDnsResponseDecoder
- Miscellaneous changes
  - Add StringUtil.TAB

Result:

- Cleaner APi
- Can support DNS-over-TCP more easily in the future
- Reduced memory footprint in the default DnsQuery/Response
  implementations
- Better leak tracking for DnsMessages
- Possibility to introduce new DnsRecord types in the future and provide
  full record encoder/decoder implementation.
- No unnecessary instantiation for an EDNS pseudo resource record

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

@@ -0,0 +1,139 @@
+/*
+ * Copyright 2015 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.util.internal.StringUtil;
+
+import static io.netty.util.internal.ObjectUtil.checkNotNull;
+
+/**
+ * A skeletal implementation of {@link DnsRecord}.
+ */
+public abstract class AbstractDnsRecord implements DnsRecord {
+
+    private final String name;
+    private final DnsRecordType type;
+    private final short dnsClass;
+    private final long timeToLive;
+    private int hashCode;
+
+    /**
+     * Creates a new {@link #CLASS_IN IN-class} record.
+     *
+     * @param name the domain name
+     * @param type the type of the record
+     * @param timeToLive the TTL value of the record
+     */
+    protected AbstractDnsRecord(String name, DnsRecordType type, long timeToLive) {
+        this(name, type, CLASS_IN, timeToLive);
+    }
+
+    /**
+     * Creates a new record.
+     *
+     * @param name the domain name
+     * @param type the type of the record
+     * @param dnsClass the class of the record, usually one of the following:
+     *                 <ul>
+     *                     <li>{@link #CLASS_IN}</li>
+     *                     <li>{@link #CLASS_CSNET}</li>
+     *                     <li>{@link #CLASS_CHAOS}</li>
+     *                     <li>{@link #CLASS_HESIOD}</li>
+     *                     <li>{@link #CLASS_NONE}</li>
+     *                     <li>{@link #CLASS_ANY}</li>
+     *                 </ul>
+     * @param timeToLive the TTL value of the record
+     */
+    protected AbstractDnsRecord(String name, DnsRecordType type, int dnsClass, long timeToLive) {
+        if (timeToLive < 0) {
+            throw new IllegalArgumentException("timeToLive: " + timeToLive + " (expected: >= 0)");
+        }
+        this.name = checkNotNull(name, "name");
+        this.type = checkNotNull(type, "type");
+        this.dnsClass = (short) dnsClass;
+        this.timeToLive = timeToLive;
+    }
+
+    @Override
+    public String name() {
+        return name;
+    }
+
+    @Override
+    public DnsRecordType type() {
+        return type;
+    }
+
+    @Override
+    public int dnsClass() {
+        return dnsClass & 0xFFFF;
+    }
+
+    @Override
+    public long timeToLive() {
+        return timeToLive;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) {
+            return true;
+        }
+
+        if (!(obj instanceof DnsRecord)) {
+            return false;
+        }
+
+        final DnsRecord that = (DnsRecord) obj;
+        final int hashCode = this.hashCode;
+        if (hashCode != 0 && hashCode != that.hashCode()) {
+            return false;
+        }
+
+        return type().intValue() == that.type().intValue() &&
+               dnsClass() == that.dnsClass() &&
+               name().equals(that.name());
+    }
+
+    @Override
+    public int hashCode() {
+        final int hashCode = this.hashCode;
+        if (hashCode != 0) {
+            return hashCode;
+        }
+
+        return this.hashCode = name.hashCode() * 31 + type().intValue() * 31 + dnsClass();
+    }
+
+    @Override
+    public String toString() {
+        StringBuilder buf = new StringBuilder(64);
+
+        buf.append(StringUtil.simpleClassName(this))
+           .append('(')
+           .append(name())
+           .append(' ')
+           .append(timeToLive())
+           .append(' ');
+
+        DnsMessageUtil.appendRecordClass(buf, dnsClass())
+                      .append(' ')
+                      .append(type().name())
+                      .append(')');
+
+        return buf.toString();
+    }
+}

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

@@ -0,0 +1,190 @@
+/*
+ * Copyright 2015 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.channel.AddressedEnvelope;
+
+import java.net.InetSocketAddress;
+import java.net.SocketAddress;
+
+/**
+ * A {@link DnsQuery} implementation for UDP/IP.
+ */
+public class DatagramDnsQuery extends DefaultDnsQuery
+        implements AddressedEnvelope<DatagramDnsQuery, InetSocketAddress> {
+
+    private final InetSocketAddress sender;
+    private final InetSocketAddress recipient;
+
+    /**
+     * Creates a new instance with the {@link DnsOpCode#QUERY} {@code opCode}.
+     *
+     * @param sender the address of the sender
+     * @param recipient the address of the recipient
+     * @param id the {@code ID} of the DNS query
+     */
+    public DatagramDnsQuery(
+            InetSocketAddress sender, InetSocketAddress recipient, int id) {
+        this(sender, recipient, id, DnsOpCode.QUERY);
+    }
+
+    /**
+     * Creates a new instance.
+     *
+     * @param sender the address of the sender
+     * @param recipient the address of the recipient
+     * @param id the {@code ID} of the DNS query
+     * @param opCode the {@code opCode} of the DNS query
+     */
+    public DatagramDnsQuery(
+            InetSocketAddress sender, InetSocketAddress recipient, int id, DnsOpCode opCode) {
+        super(id, opCode);
+
+        if (recipient == null && sender == null) {
+            throw new NullPointerException("recipient and sender");
+        }
+
+        this.sender = sender;
+        this.recipient = recipient;
+    }
+
+    @Override
+    public DatagramDnsQuery content() {
+        return this;
+    }
+
+    @Override
+    public InetSocketAddress sender() {
+        return sender;
+    }
+
+    @Override
+    public InetSocketAddress recipient() {
+        return recipient;
+    }
+
+    @Override
+    public DatagramDnsQuery setId(int id) {
+        return (DatagramDnsQuery) super.setId(id);
+    }
+
+    @Override
+    public DatagramDnsQuery setOpCode(DnsOpCode opCode) {
+        return (DatagramDnsQuery) super.setOpCode(opCode);
+    }
+
+    @Override
+    public DatagramDnsQuery setRecursionDesired(boolean recursionDesired) {
+        return (DatagramDnsQuery) super.setRecursionDesired(recursionDesired);
+    }
+
+    @Override
+    public DatagramDnsQuery setZ(int z) {
+        return (DatagramDnsQuery) super.setZ(z);
+    }
+
+    @Override
+    public DatagramDnsQuery setRecord(DnsSection section, DnsRecord record) {
+        return (DatagramDnsQuery) super.setRecord(section, record);
+    }
+
+    @Override
+    public DatagramDnsQuery addRecord(DnsSection section, DnsRecord record) {
+        return (DatagramDnsQuery) super.addRecord(section, record);
+    }
+
+    @Override
+    public DatagramDnsQuery addRecord(DnsSection section, int index, DnsRecord record) {
+        return (DatagramDnsQuery) super.addRecord(section, index, record);
+    }
+
+    @Override
+    public DatagramDnsQuery clear(DnsSection section) {
+        return (DatagramDnsQuery) super.clear(section);
+    }
+
+    @Override
+    public DatagramDnsQuery clear() {
+        return (DatagramDnsQuery) super.clear();
+    }
+
+    @Override
+    public DatagramDnsQuery touch() {
+        return (DatagramDnsQuery) super.touch();
+    }
+
+    @Override
+    public DatagramDnsQuery touch(Object hint) {
+        return (DatagramDnsQuery) super.touch(hint);
+    }
+
+    @Override
+    public DatagramDnsQuery retain() {
+        return (DatagramDnsQuery) super.retain();
+    }
+
+    @Override
+    public DatagramDnsQuery retain(int increment) {
+        return (DatagramDnsQuery) super.retain(increment);
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) {
+            return true;
+        }
+
+        if (!super.equals(obj)) {
+            return false;
+        }
+
+        if (!(obj instanceof AddressedEnvelope)) {
+            return false;
+        }
+
+        @SuppressWarnings("unchecked")
+        final AddressedEnvelope<?, SocketAddress> that = (AddressedEnvelope<?, SocketAddress>) obj;
+        if (sender() == null) {
+            if (that.sender() != null) {
+                return false;
+            }
+        } else if (!sender().equals(that.sender())) {
+            return false;
+        }
+
+        if (recipient() == null) {
+            if (that.recipient() != null) {
+                return false;
+            }
+        } else if (!recipient().equals(that.recipient())) {
+            return false;
+        }
+
+        return true;
+    }
+
+    @Override
+    public int hashCode() {
+        int hashCode = super.hashCode();
+        if (sender() != null) {
+            hashCode = hashCode * 31 + sender().hashCode();
+        }
+        if (recipient() != null) {
+            hashCode = hashCode * 31 + recipient().hashCode();
+        }
+        return hashCode;
+    }
+}