Cronet: Use stale DNS cache entries experimentally.

net/dns/host_resolver.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_HOST_RESOLVER_H_
 #define NET_DNS_HOST_RESOLVER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 10 matching lines @@
 #include "net/dns/host_cache.h"
 
 namespace base {
 class Value;
 }
 
 namespace net {
 
 class AddressList;
 class BoundNetLog;
+class HostResolverImpl;
 class HostResolverProc;
 class NetLog;
 
 // This class represents the task of resolving hostnames (or IP address
 // literal) to an AddressList object.
 //
 // HostResolver can handle multiple requests at a time, so when cancelling a
 // request the RequestHandle that was returned by Resolve() needs to be
 // given.  A simpler alternative for consumers that only have 1 outstanding
 // request at a time is to create a SingleRequestHostResolver wrapper around
@@ skipping 144 matching lines @@
   // Returns the current DNS configuration |this| is using, as a Value, or
   // nullptr if it's configured to always use the system host resolver.
   virtual std::unique_ptr<base::Value> GetDnsConfigAsValue() const;
 
   // Creates a HostResolver implementation that queries the underlying system.
   // (Except if a unit-test has changed the global HostResolverProc using
   // ScopedHostResolverProc to intercept requests to the system).
   static std::unique_ptr<HostResolver> CreateSystemResolver(
       const Options& options,
       NetLog* net_log);
+  // Same, but explicitly returns the HostResolverImpl. Only used by
+  // StaleHostResolver in cronet.
+  static std::unique_ptr<HostResolverImpl> CreateSystemResolverImpl(

[xunjieli, 2016/07/25 17:18:15]

Is this preferred over doing a runtime cast of HostResolver to HostResolverImpl?

[Julia Tuttle, 2016/07/25 19:29:16]

They're both ugly; I think this is slightly less ugly, because it means someone
who wants to change what CreateSystemResolver returns will have to think about
it instead of just breaking things.

[xunjieli, 2016/07/25 21:09:14]

Acknowledged.

+      const Options& options,
+      NetLog* net_log);
 
   // As above, but uses default parameters.
   static std::unique_ptr<HostResolver> CreateDefaultResolver(NetLog* net_log);
+  // Same, but explicitly returns the HostResolverImpl. Only used by
+  // StaleHostResolver in cronet.
+  static std::unique_ptr<HostResolverImpl> CreateDefaultResolverImpl(
+      NetLog* net_log);
 
  protected:
   HostResolver();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(HostResolver);
 };
 
 }  // namespace net
 
 #endif  // NET_DNS_HOST_RESOLVER_H_