[Activity log] Make database writes in counting policy more robust

chrome/browser/extensions/activity_log/counting_policy.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_EXTENSIONS_ACTIVITY_LOG_COUNTING_POLICY_H_
 #define CHROME_BROWSER_EXTENSIONS_ACTIVITY_LOG_COUNTING_POLICY_H_
 
 #include <string>
 
 #include "base/containers/hash_tables.h"
 #include "chrome/browser/extensions/activity_log/activity_database.h"
 #include "chrome/browser/extensions/activity_log/activity_log_policy.h"
 #include "chrome/browser/extensions/activity_log/database_string_table.h"
 
 namespace extensions {
 
-// A policy for logging the stream of actions, but without arguments.
+// A policy for logging the stream of actions, but with most arguments stripped
+// out (to improve privacy and reduce database size) and with multiple
+// identical rows combined together using a count column to track the total
+// number of repetitions.  Identical rows within the same day are merged, but
+// actions on separate days are kept distinct.  Data is kept for up to a few
+// days then deleted.
 class CountingPolicy : public ActivityLogDatabasePolicy {
  public:
   explicit CountingPolicy(Profile* profile);
   virtual ~CountingPolicy();
 
   virtual void ProcessAction(scoped_refptr<Action> action) OVERRIDE;
 
   virtual void ReadData(
       const std::string& extension_id,
       const int day,
@@ skipping 108 matching lines @@
 
   friend class CountingPolicyTest;
   FRIEND_TEST_ALL_PREFIXES(CountingPolicyTest, EarlyFlush);
   FRIEND_TEST_ALL_PREFIXES(CountingPolicyTest, MergingAndExpiring);
   FRIEND_TEST_ALL_PREFIXES(CountingPolicyTest, StringTableCleaning);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_ACTIVITY_LOG_COUNTING_POLICY_H_