Create a strict order in ReadingListSpecifics

components/reading_list/ios/reading_list_store.h

Index: components/reading_list/ios/reading_list_store.h
diff --git a/components/reading_list/ios/reading_list_store.h b/components/reading_list/ios/reading_list_store.h
index ab90ca43d584cee4962e813e9dbc97d72adf9a81..82a6cf5bc3714238acbc08bb1b6cb9f9992a3cb7 100644
--- a/components/reading_list/ios/reading_list_store.h
+++ b/components/reading_list/ios/reading_list_store.h
@@ -72,6 +72,18 @@ class ReadingListStore : public syncer::ModelTypeSyncBridge,
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityChangeList entity_changes) override;
 
+  // Returns if 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.
+  // The order is defined as follow:
+  // - Primary key: update time
+  // - Secondary key: state with UNSEEN < READ && UNSEEN < UNREAD.
+  // If order cannot be determine strictly, the entry should not be sent to
+  // sync.
+  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;