[net] Add DNS-related signals and NetLog to NetworkChangeNotifier.

net/base/network_change_notifier.h

Index: net/base/network_change_notifier.h
diff --git a/net/base/network_change_notifier.h b/net/base/network_change_notifier.h
index f4875015e48532b15d71bd94da6c8d00d9f36da7..23a4e858b04799b228c48bdc3345faca8d85aef0 100644
--- a/net/base/network_change_notifier.h
+++ b/net/base/network_change_notifier.h
@@ -84,6 +84,12 @@ class NET_EXPORT NetworkChangeNotifier {
   // cheap as this could be called (repeatedly) from the IO thread.
   virtual bool IsCurrentlyOffline() const = 0;
 
+  // Returns true if DNS watchers are operational.
+  // Otherwise, OnDNSChanged might not be issued for future changes.
+  // TODO(szym): This is a temporary interface, consider restarting them.
+  //             http://crbug.com/116139
+  virtual bool IsCurrentlyWatchingDNS() const;

[cbentzel, 2012/02/29 21:51:58]

Which consumers are using this? 

[cbentzel, 2012/02/29 21:51:58]

why not pure virtual? should also include similar notes as IsCurrentlyOffline
about expected behaviors.

[szym, 2012/02/29 22:03:49]

Any DNSObserver that must know if it can expect to receive OnDNSChanged signals
in the future. For example HostResolverImpl must not use
DnsConfig/DnsTransaction if it cannot reliably expect future OnDNSChanged
signals.

This requirement is very strict but the aim is to err on the side of correctness
for now.

I have considered adding a CHANGE_DNS_FAIL flag to |detail| in OnDNSChanged, but
a DNSObserver added after this notification is sent will miss it.

[szym, 2012/02/29 22:03:49]

Not pure virtual, because this is not an essential functionality and if some
platform (e.g. CrOS) implements its own NetworkChangeNotifier without DNS
signals, it will return false by default.

+
   // Replaces the default class factory instance of NetworkChangeNotifier class.
   // The method will take over the ownership of |factory| object.
   static void SetFactory(NetworkChangeNotifierFactory* factory);
@@ -105,6 +111,9 @@ class NET_EXPORT NetworkChangeNotifier {
   // will be successfully.
   static bool IsOffline();
 
+  // Returns true if DNS watchers are operational.
+  static bool IsWatchingDNS();
+
   // Like Create(), but for use in tests.  The mock object doesn't monitor any
   // events, it merely rebroadcasts notifications when requested.
   static NetworkChangeNotifier* CreateMock();