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.

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

The only reason for this I believe is because it's copied in the constructor.

I actually think a better design would be for the PrefHashStoreImpl to take the
seed/device_id as parameters and construct the PrefHashCalculator itself in its
constructor.

The calculator is really an implementation detail of the impl and it doesn't
make sense to force users of the API to know about it and build one for us.

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

Done.

+class PrefHashCalculator {
+ public:
+  enum ValidationResult {
+    INVALID,
+    VALID,
+    VALID_LEGACY

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

nit: add trailing comma

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

Done.

+  };
+
+  // Constructs a PrefHashCalculator using |seed| and |device_id|. The same
+  // parameters must be used in order to successfully validate generated hashes.
+  //

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

nit: I don't think an empty line is required here.

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

Done.

+  // |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|

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

Put path and value in ||.

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

Done.

+  // may be null if the preference has no value.
+  std::string Calculate(const std::string& path, const base::Value* value);

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

Make this method and other methods of this class const.

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

Done.

+
+  // 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_