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>
+
+namespace base {
+class Value;
+}  // namespace base
+
+// Calculates and validates preference value hashes.
+// This class is intentionally copy-constructable.
+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);
+
+  // 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);
+
+ 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);
+
+  std::string seed_;
+  std::string device_id_;
+};
+
+#endif  // CHROME_BROWSER_PREFS_PREF_HASH_CALCULATOR_H_