Create a strict order in ReadingListSpecifics

components/reading_list/ios/reading_list_store.h

 // Copyright 2016 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 COMPONENTS_READING_LIST_IOS_READING_LIST_STORE_H_
 #define COMPONENTS_READING_LIST_IOS_READING_LIST_STORE_H_
 
 #include "base/threading/non_thread_safe.h"
 #include "components/reading_list/ios/reading_list_model_storage.h"
 #include "components/reading_list/ios/reading_list_store_delegate.h"
@@ skipping 54 matching lines @@
 
   // Apply changes from the sync server locally.
   // Please note that |entity_changes| might have fewer entries than
   // |metadata_change_list| in case when some of the data changes are filtered
   // out, or even be empty in case when a commit confirmation is processed and
   // only the metadata needs to persisted.
   syncer::SyncError ApplySyncChanges(
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityChangeList entity_changes) override;
 
+  // Returns whether entries respect a strict order for sync and if |rhs| can be
+  // submitted to sync after |lhs| has been received.
+  // The order should ensure that there is no sync loop in sync and should be
+  // submitted to sync in strictly increasing order.
+  // Entries are in increasing order if all the fields respect increasing order.
+  // - URL must be the same.
+  // - title must verify rhs.title.compare(lhs.title) >= 0
+  // - rhs.creation_time_us >= lhs.creation_time_us
+  // - rhs.update_time_us >= lhs.update_time_us
+  // - if rhs.update_time_us > lhs.update_time_us, rhs.state can be anything.
+  // - if rhs.update_time_us == lhs.update_time_us, rhs.state >= lhs.state in
+  //          the order UNSEEN, UNREAD, READ.
+  static bool CompareEntriesForSync(const sync_pb::ReadingListSpecifics& lhs,
+                                    const sync_pb::ReadingListSpecifics& rhs);
+
   // Asynchronously retrieve the corresponding sync data for |storage_keys|.
   void GetData(StorageKeyList storage_keys, DataCallback callback) override;
 
   // Asynchronously retrieve all of the local sync data.
   void GetAllData(DataCallback callback) override;
 
   // Get or generate a client tag for |entity_data|. This must be the same tag
   // that was/would have been generated in the SyncableService/Directory world
   // for backward compatibility with pre-USS clients. The only time this
   // theoretically needs to be called is on the creation of local data, however
@@ skipping 43 matching lines @@
   std::unique_ptr<syncer::ModelTypeStore> store_;
   ReadingListModel* model_;
   ReadingListStoreDelegate* delegate_;
   StoreFactoryFunction create_store_callback_;
 
   int pending_transaction_count_;
   std::unique_ptr<syncer::ModelTypeStore::WriteBatch> batch_;
 };
 
 #endif  // COMPONENTS_READING_LIST_IOS_READING_LIST_STORE_H_