Fix a race condition in preference metric reporting.

chrome/browser/prefs/pref_hash_calculator.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_CALCULATOR_H_
+#define CHROME_BROWSER_PREFS_PREF_HASH_CALCULATOR_H_
+
+#include <string>
+
+#include "base/basictypes.h"
+
+namespace base {
+class Value;
+}  // namespace base
+
+// Calculates and validates preference value hashes.
+class PrefHashCalculator {
+ public:
+  enum ValidationResult {
+    INVALID,
+    VALID,
+    VALID_LEGACY,
+  };
+
+  // Constructs a PrefHashCalculator using |seed| and |device_id|. The same
+  // parameters must be used in order to successfully validate generated hashes.
+  // |device_id| may be empty.
+  PrefHashCalculator(const std::string& seed, const std::string& device_id);
+
+  // Calculates a hash value for the supplied preference |path| and |value|.
+  // |value| may be null if the preference has no value.
+  std::string Calculate(const std::string& path, const base::Value* value)
+      const;
+
+  // Validates the provided preference hash using current and legacy hashing
+  // algorithms.
+  ValidationResult Validate(const std::string& path,
+                            const base::Value* value,
+                            const std::string& hash) const;
+
+ private:
+  // Calculate a hash using a deprecated hash algorithm. For validating old
+  // hashes during migration.
+  std::string CalculateLegacyHash(const std::string& path,
+                                  const base::Value* value) const;
+
+  std::string seed_;

[Alexei Svitkine (slow), 2013/12/17 16:01:21]

Can you document |seed_| somewhere in this header file? I only see "Constructs a
PrefHashCalculator using |seed|", which doesn't tell me what it is.

+  std::string device_id_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrefHashCalculator);
+};
+
+#endif  // CHROME_BROWSER_PREFS_PREF_HASH_CALCULATOR_H_