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

net/base/network_change_notifier.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_BASE_NETWORK_CHANGE_NOTIFIER_H_
 #define NET_BASE_NETWORK_CHANGE_NOTIFIER_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/observer_list_threadsafe.h"
@@ skipping 66 matching lines @@
     DISALLOW_COPY_AND_ASSIGN(DNSObserver);
   };
 
   virtual ~NetworkChangeNotifier();
 
   // See the description of NetworkChangeNotifier::IsOffline().
   // Implementations must be thread-safe. Implementations must also be
   // 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);
 
   // Creates the process-wide, platform-specific NetworkChangeNotifier.  The
   // caller owns the returned pointer.  You may call this on any thread.  You
   // may also avoid creating this entirely (in which case nothing will be
   // monitored), but if you do create it, you must do so before any other
   // threads try to access the API below, and it must outlive all other threads
   // which might try to use it.
   static NetworkChangeNotifier* Create();
 
   // Returns true if there is currently no internet connection.
   //
   // A return value of |true| is a pretty strong indicator that the user
   // won't be able to connect to remote sites. However, a return value of
   // |false| is inconclusive; even if some link is up, it is uncertain
   // whether a particular connection attempt to a particular remote site
   // 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();
 
   // Registers |observer| to receive notifications of network changes.  The
   // thread on which this is called is the thread on which |observer| will be
   // called back with notifications.  This is safe to call if Create() has not
   // been called (as long as it doesn't race the Create() call on another
   // thread), in which case it will simply do nothing.
   static void AddIPAddressObserver(IPAddressObserver* observer);
@@ skipping 53 matching lines @@
       online_state_observer_list_;
   const scoped_refptr<ObserverListThreadSafe<DNSObserver> >
       resolver_state_observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(NetworkChangeNotifier);
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_NETWORK_CHANGE_NOTIFIER_H_