Extension Settings API: make it so that when changes received from Sync fail to

chrome/browser/extensions/settings/syncable_settings_storage.h

 // Copyright (c) 2011 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_SETTINGS_SYNCABLE_SETTINGS_STORAGE_H_
 #define CHROME_BROWSER_EXTENSIONS_SETTINGS_SYNCABLE_SETTINGS_STORAGE_H_
 #pragma once
 
 #include "base/compiler_specific.h"
 #include "base/memory/weak_ptr.h"
@@ skipping 23 matching lines @@
   virtual ReadResult Get(const std::string& key) OVERRIDE;
   virtual ReadResult Get(const std::vector<std::string>& keys) OVERRIDE;
   virtual ReadResult Get() OVERRIDE;
   virtual WriteResult Set(const std::string& key, const Value& value) OVERRIDE;
   virtual WriteResult Set(const DictionaryValue& settings) OVERRIDE;
   virtual WriteResult Remove(const std::string& key) OVERRIDE;
   virtual WriteResult Remove(const std::vector<std::string>& keys) OVERRIDE;
   virtual WriteResult Clear() OVERRIDE;
 
   // Sync-related methods, analogous to those on SyncableService (handled by
-  // ExtensionSettings).
+  // ExtensionSettings), but with looser guarantees about when the methods
+  // can be called.
+
+  // Must only be called if sync isn't already active; not idempotent since
+  // behaviour would be undefined given differerent values of |sync_state|.
   SyncError StartSyncing(
       // Either SETTINGS or APP_SETTINGS.
       syncable::ModelType type,
       const DictionaryValue& sync_state,
       // Must NOT be NULL. Ownership NOT taken.
       SyncChangeProcessor* sync_processor);
+
+  // May be called at any time (idempotent).
   void StopSyncing();
-  std::vector<SyncError> ProcessSyncChanges(
-      const SettingSyncDataList& sync_changes);
+
+  // May be called at any time; changes will be ignored if sync isn't active.
+  SyncError ProcessSyncChanges(const SettingSyncDataList& sync_changes);
 
  private:
   // Propagates some changes to sync by sending an ADD/UPDATE/DELETE depending
   // on the change and the current state of sync.
   void SendChangesToSync(const SettingChangeList& changes);
 
   // Sends all local settings to sync (synced settings assumed to be empty).
   SyncError SendLocalSettingsToSync(
       const DictionaryValue& settings);
 
@@ skipping 38 matching lines @@
   // Keys of the settings that are currently being synced.  Only used to decide
   // whether an ACTION_ADD or ACTION_UPDATE should be sent to sync on a Set().
   std::set<std::string> synced_keys_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncableSettingsStorage);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_SETTINGS_SYNCABLE_SETTINGS_STORAGE_H_