Avoid doing concurrent DNS resolves of the same hostname

net/http/http_network_layer.h

 // Copyright (c) 2006-2008 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_HTTP_HTTP_NETWORK_LAYER_H_
 #define NET_HTTP_HTTP_NETWORK_LAYER_H_
 
 #include "base/ref_counted.h"
 #include "base/scoped_ptr.h"
 #include "net/http/http_transaction_factory.h"
 
 namespace net {
 
+class HostResolver;
 class HttpNetworkSession;
 class ProxyInfo;
 class ProxyService;
 
 class HttpNetworkLayer : public HttpTransactionFactory {
  public:
-  // |proxy_service| must remain valid for the lifetime of HttpNetworkLayer.
-  explicit HttpNetworkLayer(ProxyService* proxy_service);
+  // |proxy_service| and |host_resolver| must remain valid for the lifetime of
+  // HttpNetworkLayer.
+  HttpNetworkLayer(HostResolver* host_resolver, ProxyService* proxy_service);
   // Construct a HttpNetworkLayer with an existing HttpNetworkSession which
   // contains a valid ProxyService.
   explicit HttpNetworkLayer(HttpNetworkSession* session);
   ~HttpNetworkLayer();
 
   // This function hides the details of how a network layer gets instantiated
   // and allows other implementations to be substituted.
-  static HttpTransactionFactory* CreateFactory(ProxyService* proxy_service);
+  static HttpTransactionFactory* CreateFactory(HostResolver* host_resolver,
+                                               ProxyService* proxy_service);
   // Create a transaction factory that instantiate a network layer over an
   // existing network session. Network session contains some valuable
   // information (e.g. authentication data) that we want to share across
   // multiple network layers. This method exposes the implementation details
   // of a network layer, use this method with an existing network layer only
   // when network session is shared.
   static HttpTransactionFactory* CreateFactory(HttpNetworkSession* session);
 
   // HttpTransactionFactory methods:
   virtual HttpTransaction* CreateTransaction();
   virtual HttpCache* GetCache();
   virtual void Suspend(bool suspend);
 
   HttpNetworkSession* GetSession();
 
  private:
+  // The host resolver being used for the session.
+  HostResolver* host_resolver_;
+
   // The proxy service being used for the session.
   ProxyService* proxy_service_;
 
   scoped_refptr<HttpNetworkSession> session_;
   bool suspended_;
 };
 
 }  // namespace net
 
 #endif  // NET_HTTP_HTTP_NETWORK_LAYER_H_