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 PrefRegistrySimple;
+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 |device_id| and stores them in |local_state|. Multiple hash
+  // stores can use the same |local_state| with distinct |hash_store_id|s.
+  //
+  // The same |seed|, |device_id|, and |hash_store_id| must be used to load and
+  // validate previously stored hashes in |local_state|.
+  //
+  // |local_state| must have previously been passed to |RegisterPrefs|.
+  PrefHashStoreImpl(const std::string& hash_store_id,
+                    const std::string& seed,
+                    const std::string& device_id,
+                    PrefService* local_state);
+
+  // Registers required local state preferences.
+  static void RegisterPrefs(PrefRegistrySimple* registry);
+
+  // PrefHashStore implementation.
+  virtual ValueState CheckValue(const std::string& path,
+                                const base::Value* value) const OVERRIDE;
+  virtual void StoreHash(const std::string& path,
+                         const base::Value* value) OVERRIDE;
+
+ private:
+  std::string hash_store_id_;
+  PrefHashCalculator pref_hash_calculator_;
+  PrefService* local_state_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrefHashStoreImpl);
+};
+
+#endif  // CHROME_BROWSER_PREFS_PREF_HASH_STORE_IMPL_H_