net: calculate dns header size only once per query/response

net/dns/dns_query.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/dns/dns_query.h"
 
 #include "base/big_endian.h"
 #include "base/memory/ptr_util.h"
 #include "base/sys_byteorder.h"
 #include "net/base/io_buffer.h"
 #include "net/dns/dns_protocol.h"
 #include "net/dns/dns_util.h"
 
 namespace net {
 
+namespace {
+
+const size_t kHeaderSize = sizeof(dns_protocol::Header);
+
+}  // namespace
+
 // DNS query consists of a 12-byte header followed by a question section.
 // For details, see RFC 1035 section 4.1.1.  This header template sets RD
 // bit, which directs the name server to pursue query recursively, and sets
 // the QDCOUNT to 1, meaning the question section has a single entry.
 DnsQuery::DnsQuery(uint16_t id, const base::StringPiece& qname, uint16_t qtype)
     : qname_size_(qname.size()),
-      io_buffer_(
-          new IOBufferWithSize(sizeof(dns_protocol::Header) + question_size())),
+      io_buffer_(new IOBufferWithSize(kHeaderSize + question_size())),
       header_(reinterpret_cast<dns_protocol::Header*>(io_buffer_->data())) {
   DCHECK(!DNSDomainToString(qname).empty());
   *header_ = {};
   header_->id = base::HostToNet16(id);
   header_->flags = base::HostToNet16(dns_protocol::kFlagRD);
   header_->qdcount = base::HostToNet16(1);
 
   // Write question section after the header.
-  base::BigEndianWriter writer(
-      io_buffer_->data() + sizeof(dns_protocol::Header), question_size());
+  base::BigEndianWriter writer(io_buffer_->data() + kHeaderSize,
+                               question_size());
   writer.WriteBytes(qname.data(), qname.size());
   writer.WriteU16(qtype);
   writer.WriteU16(dns_protocol::kClassIN);
 }
 
 DnsQuery::~DnsQuery() {
 }
 
 std::unique_ptr<DnsQuery> DnsQuery::CloneWithNewId(uint16_t id) const {
   return base::WrapUnique(new DnsQuery(*this, id));
 }
 
 uint16_t DnsQuery::id() const {
   return base::NetToHost16(header_->id);
 }
 
 base::StringPiece DnsQuery::qname() const {
-  return base::StringPiece(io_buffer_->data() + sizeof(dns_protocol::Header),
-                           qname_size_);
+  return base::StringPiece(io_buffer_->data() + kHeaderSize, qname_size_);
 }
 
 uint16_t DnsQuery::qtype() const {
   uint16_t type;
-  base::ReadBigEndian<uint16_t>(
-      io_buffer_->data() + sizeof(dns_protocol::Header) + qname_size_, &type);
+  base::ReadBigEndian<uint16_t>(io_buffer_->data() + kHeaderSize + qname_size_,
+                                &type);
   return type;
 }
 
 base::StringPiece DnsQuery::question() const {
-  return base::StringPiece(io_buffer_->data() + sizeof(dns_protocol::Header),
-                           question_size());
+  return base::StringPiece(io_buffer_->data() + kHeaderSize, question_size());
 }
 
 void DnsQuery::set_flags(uint16_t flags) {
   header_->flags = flags;
 }
 
 DnsQuery::DnsQuery(const DnsQuery& orig, uint16_t id) {
   qname_size_ = orig.qname_size_;
   io_buffer_ = new IOBufferWithSize(orig.io_buffer()->size());
   memcpy(io_buffer_.get()->data(), orig.io_buffer()->data(),
          io_buffer_.get()->size());
   header_ = reinterpret_cast<dns_protocol::Header*>(io_buffer_->data());
   header_->id = base::HostToNet16(id);
 }
 
 }  // namespace net