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
+  // |hash_calculator| and stores them in |local_state|. Multiple hash stores
+  // can use the same |local_state| with distinct |hash_store_id|s.
+  //
+  // The same |hash_calculator| parameters 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 PrefHashCalculator& hash_calculator,
+                    PrefService* local_state,

[gab, 2013/12/06 17:23:38]

out-params last (this is not really an out-param, but it's not const so rule
still applies I think).

[erikwright (departed), 2013/12/09 17:59:40]

Done.

+                    const std::string& hash_store_id);
+
+  // Registers required local state preferences.
+  static void RegisterPrefs(PrefRegistrySimple* registry);
+
+  // PrefHashStore implementation.
+  virtual ValueState CheckValue(const std::string& path,
+                                const base::Value* value) OVERRIDE;
+  virtual void StoreValue(const std::string& path,
+                          const base::Value* value) OVERRIDE;
+
+ private:
+  PrefService* local_state_;
+  std::string hash_store_id_;
+  PrefHashCalculator pref_hash_calculator_;

[gab, 2013/12/06 17:23:38]

Same order here as you have in the constructor

[erikwright (departed), 2013/12/09 17:59:40]

Done.

+
+  DISALLOW_COPY_AND_ASSIGN(PrefHashStoreImpl);
+};
+
+#endif  // CHROME_BROWSER_PREFS_PREF_HASH_STORE_IMPL_H_