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+// Copyright 2008, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//    * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//    * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//    * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef NET_HTTP_WINHTTP_REQUEST_THROTTLE_H_
+#define NET_HTTP_WINHTTP_REQUEST_THROTTLE_H_
+
+#include <windows.h>
+#include <winhttp.h>
+
+#include <map>
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/linked_ptr.h"
+#include "testing/gtest/include/gtest/gtest_prod.h"
+
+namespace net {
+
+// The WinHttpRequestThrottle class regulates the rate at which we call
+// WinHttpSendRequest, ensuring that at any time there are at most 6 WinHTTP
+// requests in progress for each server or proxy.
+//
+// The throttling is intended to cause WinHTTP to maintain at most 6
+// persistent HTTP connections with each server or proxy.  This works well in
+// most cases, except when making HTTPS requests via a proxy, in which case
+// WinHTTP may open much more than 6 connections to the proxy in spite of our
+// rate limiting.
+//
+// Because we identify a server by its hostname rather than its IP address,
+// we also can't distinguish between two different hostnames that resolve to
+// the same IP address.
+//
+// Although WinHTTP has the WINHTTP_OPTION_MAX_CONNS_PER_SERVER option to
+// limit the number of connections allowed per server, we can't use it
+// because it has two serious bugs:
+// 1. It causes WinHTTP to not close idle persistent connections, leaving
+//    many connections in the CLOSE_WAIT state.  This may cause system
+//    crashes (Blue Screen of Death) when VPN is used.
+// 2. It causes WinHTTP to crash intermittently in
+//    HTTP_REQUEST_HANDLE_OBJECT::OpenProxyTunnel_Fsm() if a proxy is used.
+// Therefore, we have to resort to throttling our WinHTTP requests to achieve
+// the same effect.
+//
+// Note on thread safety: The WinHttpRequestThrottle class is only used by
+// the IO thread, so it doesn't need to be protected with a lock.  The
+// drawback is that the time we mark a request done is only approximate.
+// We do that in the HttpTransactionWinHttp destructor, rather than in the
+// WinHTTP status callback upon receiving HANDLE_CLOSING.
+class WinHttpRequestThrottle {
+ public:
+  WinHttpRequestThrottle() {}
+
+  virtual ~WinHttpRequestThrottle();
+
+  // Intended to be a near drop-in replacement of WinHttpSendRequest.
+  BOOL SubmitRequest(const std::string& server,
+                     HINTERNET request_handle,
+                     DWORD total_size,
+                     DWORD_PTR context);
+
+  // Called when a request failed or completed successfully.
+  void NotifyRequestDone(const std::string& server);
+
+  // Called from the HttpTransactionWinHttp destructor.
+  void RemoveRequest(const std::string& server,
+                     HINTERNET request_handle);
+
+ protected:
+  // Unit tests can stub out this method in a derived class.
+  virtual BOOL SendRequest(HINTERNET request_handle,
+                           DWORD total_size,
+                           DWORD_PTR context,
+                           bool report_async_error);
+
+ private:
+  FRIEND_TEST(WinHttpRequestThrottleTest, GarbageCollect);
+
+  class RequestQueue;
+
+  struct PerServerThrottle {
+    PerServerThrottle();
+    ~PerServerThrottle();
+
+    int num_requests;  // Number of requests in progress
+    linked_ptr<RequestQueue> request_queue;  // Requests waiting to be sent
+  };
+
+  typedef std::map<std::string, PerServerThrottle> ThrottleMap;
+
+  static const int kMaxConnectionsPerServer;
+  static const int kGarbageCollectionThreshold;
+
+  void GarbageCollect();
+
+  ThrottleMap throttles_;
+
+  DISALLOW_EVIL_CONSTRUCTORS(WinHttpRequestThrottle);
+};
+
+}  // namespace net
+
+#endif  // NET_HTTP_WINHTTP_REQUEST_THROTTLE_H_