Fix a race condition in preference metric reporting.

base/prefs/pref_filter.h

Index: base/prefs/pref_filter.h
diff --git a/base/prefs/pref_filter.h b/base/prefs/pref_filter.h
new file mode 100644
index 0000000000000000000000000000000000000000..aa560871744b1885c9a1e5b45d5ad38dd8c82631
--- /dev/null
+++ b/base/prefs/pref_filter.h
@@ -0,0 +1,31 @@
+// Copyright (c) 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 BASE_PREFS_PREF_FILTER_H_
+#define BASE_PREFS_PREF_FILTER_H_
+
+#include <string>
+#include "base/prefs/base_prefs_export.h"
+
+class PersistentPrefStore;
+
+// Filters preferences as they are loaded from disk or updated at runtime.
+// Currently supported only by JsonPrefStore.
+class BASE_PREFS_EXPORT PrefFilter {

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

I don't really like the term "filter"; to me a filter selects/discards
individual items passed through it; not verifies/modifies all items attached to
a global bucket passed to it.

On the FilterUpdate case, it kind of acts as a filter (but more as an observer),
but on the FilterOnLoad case it definitely is much more than a filter IMO.

I prefer
PrefVerificator/PrefController/PrefInterceptor/PrefVerificationDelegate.

I also don't think we need a generic interface for this delegate and a more
specific impl; in particular given there is only one and this will always be the
one (and should that change we can reconsider -- no need to be too general yet).

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

I'm happy to change it. Rather than do that now I'll let bbauer select a name.
Needed because the implementation should not be in base/.

+ public:
+  virtual ~PrefFilter() {}
+
+  // Receives notification when the pref store data has been loaded but before
+  // any other clients are notified.
+  // Changes made by a PrefFilter during FilterOnLoad do not result in
+  // notifications to |PrefStore::Observer|s.
+  virtual void FilterOnLoad(PersistentPrefStore* pref_store) = 0;
+
+  // Receives notification when a pref store value is changed, before any other
+  // clients are notified.
+  virtual void FilterUpdate(PersistentPrefStore* pref_store,
+                            const std::string& path) = 0;
+};
+
+#endif  // BASE_PREFS_PREF_FILTER_H_