[Sync] Implements the loopback sync server.

components/sync/engine_impl/loopback_server/persistent_permanent_entity.h

+// Copyright 2014 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_SYNC_ENGINE_IMPL_LOOPBACK_SERVER_PERSISTENT_PERMANENT_ENTITY_H_
+#define COMPONENTS_SYNC_ENGINE_IMPL_LOOPBACK_SERVER_PERSISTENT_PERMANENT_ENTITY_H_
+
+#include <memory>
+#include <string>
+
+#include "components/sync/base/model_type.h"
+#include "components/sync/engine_impl/loopback_server/loopback_server_entity.h"
+#include "components/sync/protocol/sync.pb.h"
+
+namespace syncer {
+
+// A server-created, permanent entity.
+class PersistentPermanentEntity : public LoopbackServerEntity {
+ public:
+  PersistentPermanentEntity(const std::string& id,
+                            int64_t version,
+                            const syncer::ModelType& model_type,
+                            const std::string& name,
+                            const std::string& parent_id,
+                            const std::string& server_defined_unique_tag,
+                            const sync_pb::EntitySpecifics& entity_specifics);
+
+  ~PersistentPermanentEntity() override;
+
+  // Factory function for PersistentPermanentEntity. |server_tag| should be a
+  // globally

[pavely, 2016/10/06 23:48:46]

Could you reformat line wraps in comments.

[pastarmovj, 2016/10/13 14:13:40]

Done.

+  // unique identifier.
+  static std::unique_ptr<LoopbackServerEntity> Create(
+      const syncer::ModelType& model_type,
+      const std::string& server_tag,
+      const std::string& name,
+      const std::string& parent_server_tag);
+
+  // Factory function for a top level PersistentPermanentEntity. Top level means
+  // that the
+  // entity's parent is the root entity (no PersistentPermanentEntity exists for
+  // root).
+  static std::unique_ptr<LoopbackServerEntity> CreateTopLevel(
+      const syncer::ModelType& model_type);
+
+  // Factory function for creating an updated version of a
+  // PersistentPermanentEntity.
+  // This function should only be called for the Nigori entity.
+  static std::unique_ptr<LoopbackServerEntity> CreateUpdatedNigoriEntity(
+      const sync_pb::SyncEntity& client_entity,
+      const LoopbackServerEntity& current_server_entity);
+
+  // LoopbackServerEntity implementation.
+  bool RequiresParentId() const override;
+  std::string GetParentId() const override;
+  void SerializeAsProto(sync_pb::SyncEntity* proto) const override;
+  bool IsFolder() const override;
+  bool IsPermanent() const override;
+
+ private:
+  // All member values have equivalent fields in SyncEntity.
+  std::string server_defined_unique_tag_;
+  std::string parent_id_;
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_ENGINE_IMPL_LOOPBACK_SERVER_PERSISTENT_PERMANENT_ENTITY_H_