Fix a race condition in preference metric reporting.

chrome/browser/prefs/pref_hash_store_impl.h

+// Copyright 2013 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_PREFS_PREF_HASH_STORE_IMPL_H_
+#define CHROME_BROWSER_PREFS_PREF_HASH_STORE_IMPL_H_
+
+#include <string>
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "base/memory/scoped_ptr.h"
+#include "chrome/browser/prefs/pref_hash_calculator.h"
+#include "chrome/browser/prefs/pref_hash_store.h"
+
+class PrefService;
+
+namespace base {
+class Value;
+}  // namespace base
+
+// Implements PrefHashStoreImpl by storing preference hashes in a PrefService.
+class PrefHashStoreImpl : public PrefHashStore {
+ public:
+  // Constructs a PrefHashStoreImpl that calculates hashes using |seed| and
+  // stores them in |local_state|. Multiple hash stores can use the same
+  // |local_state| with distinct |hash_store_id|s.
+  //
+  // The same |seed| and |hash_store_id| must be used to  load and validate
+  // previously stored hashes in |local_state|.
+  PrefHashStoreImpl(const std::string& seed,
+                    PrefService* local_state,
+                    const std::string& hash_store_id);
+
+  // PrefHashStore implementation.
+  virtual InitializationResult InitializeTrackedValue(
+      const std::string& path, const base::Value* initial_value) OVERRIDE;
+
+ private:
+  class PrefHashTrackerImpl;

[gab, 2013/11/27 23:43:27]

Does this need to be a private class? Could it just be a standalone class in the
unnamed namespace of pref_hash_store_impl.cc?

+
+  // PrefHashStore implementation.
+  virtual scoped_ptr<PrefHashTracker> CompleteInitialization() OVERRIDE;
+
+  PrefService* local_state_;
+  std::string hash_store_id_;
+  PrefHashCalculator pref_hash_calculator_;
+  scoped_ptr<PrefHashTrackerImpl> pref_hash_tracker_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrefHashStoreImpl);
+};
+
+#endif  // CHROME_BROWSER_PREFS_PREF_HASH_STORE_IMPL_H_