DNS: Retain stale entries in HostCache and return when requested

net/dns/dns_util.h

 // 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.
 
 #ifndef NET_DNS_DNS_UTIL_H_
 #define NET_DNS_DNS_UTIL_H_
 
 #include <string>
 
 #include "base/strings/string_piece.h"
 #include "base/time/time.h"
 #include "net/base/net_export.h"
 #include "net/base/network_change_notifier.h"
 
 namespace net {
 
+class AddressList;
+
 // DNSDomainFromDot - convert a domain string to DNS format. From DJB's
 // public domain DNS library.
 //
 //   dotted: a string in dotted form: "www.google.com"
 //   out: a result in DNS form: "\x03www\x06google\x03com\x00"
 NET_EXPORT_PRIVATE bool DNSDomainFromDot(const base::StringPiece& dotted,
                                          std::string* out);
 
 // DNSDomainToString converts a domain in DNS format to a dotted string.
 // Excludes the dot at the end.
 NET_EXPORT_PRIVATE std::string DNSDomainToString(
     const base::StringPiece& domain);
 
 // Returns true if it can determine that only loopback addresses are configured.
 // i.e. if only 127.0.0.1 and ::1 are routable.
 // Also returns false if it cannot determine this.
 NET_EXPORT_PRIVATE bool HaveOnlyLoopbackAddresses();
 
 #if !defined(OS_NACL)
 NET_EXPORT_PRIVATE
 base::TimeDelta GetTimeDeltaForConnectionTypeFromFieldTrialOrDefault(
     const char* field_trial_name,
     base::TimeDelta default_delta,
     NetworkChangeNotifier::ConnectionType connection_type);
 #endif  // !defined(OS_NACL)
 
+// How similar or different two AddressLists are (see values for details).
+// Used in histograms; do not modify existing values.
+enum AddressListDeltaType {
+  // Both lists contain the same addresses in the same order.
+  DELTA_IDENTICAL = 0,
+  // Both lists contain the same addresses in a different order.
+  DELTA_REORDERED = 1,
+  // The two lists have at least one address in common, but not all of them.
+  DELTA_OVERLAP = 2,
+  // The two lists have no addresses in common.
+  DELTA_DISJOINT = 3,
+  MAX_DELTA_TYPE
+};
+
+// Compares two AddressLists to see how similar or different their addresses
+// are. (See |AddressListDeltaType| for details of exactly what's checked.)
+AddressListDeltaType FindAddressListDeltaType(const AddressList& a,
+                                              const AddressList& b);
+
 }  // namespace net
 
 #endif  // NET_DNS_DNS_UTIL_H_