Fix a race condition in preference metric reporting.

chrome/browser/prefs/pref_hash_store.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_PREFERENCE_HASH_STORE_H_
+#define CHROME_BROWSER_PREFS_PREFERENCE_HASH_STORE_H_
+
+#include <string>
+
+#include "base/memory/scoped_ptr.h"
+
+class PrefHashTracker;
+
+namespace base {
+class Value;
+}  // namespace base
+
+// Stores hashes of and verifies preference values. To use, first call
+// |InitializeTrackedValue| with each preference that should be tracked. Then
+// use |CreateTracker| to build an object that can be used to track changes to
+// the configured preferences.
+class PrefHashStore {
+ public:
+  virtual ~PrefHashStore() {}
+
+  enum InitializationResult {
+    // The preference value corresponds to its stored hash.
+    UNCHANGED,
+    // The preference has been cleared since the last hash.
+    CLEARED,
+    // The preference value corresponds to its stored hash, which was calculated
+    // using a legacy hash algorithm.
+    MIGRATED,
+    // The preference value has been changed since the last hash.
+    CHANGED,
+    // No stored hash exists for the preference value.
+    INITIALIZED

[gab, 2013/11/27 23:43:27]

nit: Add trailing comma (no trailing comma usually indicates that no new values
should be appended after that one (i.e., the should be added before if
anything); e.g. for enum values like FOO_COUNT).

+  };
+
+  // Configures tracking of the preference named by |path|. Checks
+  // |initial_value| against the existing stored hashes and returns the result.
+  // If the result is not UNCHANGED a new hash will be calculated and stored.
+  virtual InitializationResult InitializeTrackedValue(

[gab, 2013/11/27 23:43:27]

This may eventually do more than just "initialize" and "report", but I don't
really have a better name to propose so feel free to ignore this :)!

+      const std::string& path, const base::Value* initial_value) = 0;
+
+  // Returns a PrefHashTracker that can be used to maintain the hash store as
+  // tracked preferences are changed. The returned tracker will ignore changes
+  // to preferences not previously initialized by calling
+  // InitializeTrackedValue.
+  static scoped_ptr<PrefHashTracker> CreateTracker(
+      scoped_ptr<PrefHashStore> builder);

[robertshield, 2013/11/28 02:54:26]

Since this accepts an instance of the class it is defined in, why not make it an
instance method rather than a static method?

+
+ private:
+  // Returns an implementation specific PrefHashTracker. The PrefHashStore will
+  // be destroyed immediately after this method is called.

[gab, 2013/11/27 23:43:27]

Lifetime overly complex?

[robertshield, 2013/11/28 02:54:26]

I agree, it's not clear to me how the current instance of PrefHashStore would be
destroyed when this returns.

+  virtual scoped_ptr<PrefHashTracker> CompleteInitialization() = 0;
+};
+
+#endif  // CHROME_BROWSER_PREFS_PREFERENCE_HASH_STORE_H_