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diff --git a/chrome/browser/metrics/histogram_synchronizer.h b/chrome/browser/metrics/histogram_synchronizer.h
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+// 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 CHROME_BROWSER_METRICS_HISTOGRAM_SYNCHRONIZER_H_
+#define CHROME_BROWSER_METRICS_HISTOGRAM_SYNCHRONIZER_H_
+
+#include <string>
+#include <vector>
+
+#include "base/basictypes.h"
+#include "base/condition_variable.h"
+#include "base/lock.h"
+#include "base/ref_counted.h"
+#include "base/time.h"
+
+class MessageLoop;
+class Task;
+
+class HistogramSynchronizer : public
+    base::RefCountedThreadSafe<HistogramSynchronizer> {
+ public:
+
+  enum RendererHistogramRequester {
+    ASYNC_HISTOGRAMS,
+    SYNCHRONOUS_HISTOGRAMS
+  };
+
+  HistogramSynchronizer();
+
+  ~HistogramSynchronizer();
+
+  // Return pointer to the singleton instance, which is allocated and
+  // deallocated on the main UI thread (during system startup and teardown).
+  static HistogramSynchronizer* CurrentSynchronizer();
+
+  // Contact all renderers, and get them to upload to the browser any/all
+  // changes to histograms.  Return when all changes have been acquired, or when
+  // the wait time expires (whichever is sooner). This method is called on the
+  // main UI thread from about:histograms.
+  void FetchRendererHistogramsSynchronously(base::TimeDelta wait_time);
+
+  // Contact all renderers, and get them to upload to the browser any/all
+  // changes to histograms.  When all changes have been acquired, or when the
+  // wait time expires (whichever is sooner), post the callback_task to the UI
+  // thread. Note the callback_task is posted exactly once. This method is
+  // called on the IO thread from UMA via PostMessage.
+  static void FetchRendererHistogramsAsynchronously(
+      MessageLoop* callback_thread, Task* callback_task, int wait_time);
+
+  // This method is called on the IO thread. Desrializes the histograms and
+  // records that we have received histograms from a renderer process.
+  static void DeserializeHistogramList(
+      int sequence_number, const std::vector<std::string>& histograms);
+
+ private:
+  // Records that we have received the histograms from a renderer for the given
+  // sequence number. If we have received a response from all histograms, either
+  // signal the waiting process or call the callback function. Returns true when
+  // we receive histograms from the last of N renderers that were contacted for
+  // an update. This is called on IO Thread.
+  bool RecordRendererHistogram(int sequence_number);
+
+  void SetCallbackTaskToCallAfterGettingHistograms(
+      MessageLoop* callback_thread, Task* callback_task);
+
+  void ForceHistogramSynchronizationDoneCallback(int sequence_number);
+
+  // Calls the callback task, if there is a callback_task.
+  void CallCallbackTaskAndResetData();
+
+  // Gets a new sequence number to be sent to renderers from browser process.
+  // This will also reset the current pending renderers for the given type.
+  int GetNextAvaibleSequenceNumber(RendererHistogramRequester requster);
+
+  // Increments the count of the renderers we're waiting for for the request
+  // of the given type.
+  void IncrementPendingRenderers(RendererHistogramRequester requester);
+
+  // For use ONLY in a DCHECK. This method initializes io_message_loop_ in its
+  // first call and then compares io_message_loop_ with MessageLoop::current()
+  // in subsequent calls. This method guarantees we're consistently on the
+  // singular IO thread and we don't need to worry about locks.
+  bool IsOnIoThread();
+
+  // This lock_ protects access to sequence number and
+  // synchronous_renderers_pending_.
+  Lock lock_;
+
+  // This condition variable is used to block caller of the synchronous request
+  // to update histograms, and to signal that thread when updates are completed.
+  ConditionVariable received_all_renderer_historgrams_;
+
+  // When a request is made to asynchronously update the histograms, we store
+  // the task and thread we use to post a completion notification in
+  // callback_task_ and callback_thread_.
+  Task* callback_task_;
+  MessageLoop* callback_thread_;
+
+  // For use ONLY in a DCHECK and is used in IsOnIoThread(). io_message_loop_ is
+  // initialized during the first call to IsOnIoThread(), and then compares
+  // MessageLoop::current() against io_message_loop_ in subsequent calls for
+  // consistency.
+  MessageLoop* io_message_loop_;
+
+  // We don't track the actual renderers that are contacted for an update, only
+  // the count of the number of renderers, and we can sometimes time-out and
+  // give up on a "slow to respond" renderer.  We use a sequence_number to be
+  // sure a response from a renderer is associated with the current round of
+  // requests (and not merely a VERY belated prior response).
+  // next_available_sequence_number_ is the next available number (used to
+  // avoid reuse for a long time).
+  int next_available_sequence_number_;
+
+  // The sequence number used by the most recent asynchronous update request to
+  // contact all renderers.
+  int async_sequence_number_;
+
+  // The number of renderers that have not yet responded to requests (as part of
+  // an asynchronous update).
+  int async_renderers_pending_;
+
+  // The time when we were told to start the fetch histograms asynchronously
+  // from renderers.
+  base::TimeTicks async_callback_start_time_;
+
+  // The sequence number used by the most recent synchronous update request to
+  // contact all renderers.
+  int synchronous_sequence_number_;
+
+  // The number of renderers that have not yet responded to requests (as part of
+  // a synchronous update).
+  int synchronous_renderers_pending_;
+
+  // This singleton instance should be started during the single threaded
+  // portion of main(). It initializes globals to provide support for all future
+  // calls. This object is created on the UI thread, and it is destroyed after
+  // all the other threads have gone away. As a result, it is ok to call it
+  // from the UI thread (for UMA uploads), or for about:histograms.
+  static HistogramSynchronizer* histogram_synchronizer_;
+
+  DISALLOW_COPY_AND_ASSIGN(HistogramSynchronizer);
+};
+
+#endif  // CHROME_BROWSER_METRICS_HISTOGRAM_SYNCHRONIZER_H_