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 <set>
+#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 {

[gab, 2013/12/04 22:22:30]

I think it's overkill to have a public interface and an impl in this case.

I could see it being useful though if the interface was directly the delegate
interfacing with JsonPrefStore as suggested above.

[erikwright (departed), 2013/12/05 23:48:00]

The current factoring makes it very easy to test how we react to mismatches
without having to muck about with the values in local state. And it makes it
very easy to test the storage and verification of hashes in local state without
having to set up and tear down the user prefs over and over again. See for
example that the new tests are a lot simpler than the old tests.

+ 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,
+                    const std::string& hash_store_id);
+
+  // Registers required local state preferences.
+  static void RegisterPrefs(PrefRegistrySimple* registry);
+
+  // PrefHashStore implementation.
+  virtual InitializationResult InitializeTrackedValue(
+      const std::string& path, const base::Value* initial_value) OVERRIDE;
+  virtual void OnPrefValueChanged(const std::string& path,
+                                  const base::Value* new_value) OVERRIDE;
+
+ private:
+  std::set<std::string> tracked_paths_;
+  PrefService* local_state_;
+  std::string hash_store_id_;
+  PrefHashCalculator pref_hash_calculator_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrefHashStoreImpl);
+};
+
+#endif  // CHROME_BROWSER_PREFS_PREF_HASH_STORE_IMPL_H_