Add streaming serializer under featureflag

- Rename old Project::TreeSaver to LegacyTreeSaver
- Add feature_flag in export_service

app/services/projects/import_export/export_service.rb

@@ -54,7 +54,15 @@ module Projects
       end
 
       def project_tree_saver
-        Gitlab::ImportExport::Project::TreeSaver.new(project: project, current_user: current_user, shared: shared, params: params)
+        tree_saver_class.new(project: project, current_user: current_user, shared: shared, params: params)
+      end
+
+      def tree_saver_class
+        if ::Feature.enabled?(:streaming_serializer, project)
+          Gitlab::ImportExport::Project::TreeSaver
+        else
+          Gitlab::ImportExport::Project::LegacyTreeSaver
+        end
       end
 
       def uploads_saver

lib/gitlab/import_export/project/legacy_tree_saver.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module ImportExport
+    module Project
+      class LegacyTreeSaver
+        attr_reader :full_path
+
+        def initialize(project:, current_user:, shared:, params: {})
+          @params       = params
+          @project      = project
+          @current_user = current_user
+          @shared       = shared
+          @full_path    = File.join(@shared.export_path, ImportExport.project_filename)
+        end
+
+        def save
+          project_tree = tree_saver.serialize(@project, reader.project_tree)
+          fix_project_tree(project_tree)
+          tree_saver.save(project_tree, @shared.export_path, ImportExport.project_filename)
+
+          true
+        rescue => e
+          @shared.error(e)
+          false
+        end
+
+        private
+
+        # Aware that the resulting hash needs to be pure-hash and
+        # does not include any AR objects anymore, only objects that run `.to_json`
+        def fix_project_tree(project_tree)
+          if @params[:description].present?
+            project_tree['description'] = @params[:description]
+          end
+
+          project_tree['project_members'] += group_members_array
+        end
+
+        def reader
+          @reader ||= Gitlab::ImportExport::Reader.new(shared: @shared)
+        end
+
+        def group_members_array
+          group_members.as_json(reader.group_members_tree).each do |group_member|
+            group_member['source_type'] = 'Project' # Make group members project members of the future import
+          end
+        end
+
+        def group_members
+          return [] unless @current_user.can?(:admin_group, @project.group)
+
+          # We need `.where.not(user_id: nil)` here otherwise when a group has an
+          # invitee, it would make the following query return 0 rows since a NULL
+          # user_id would be present in the subquery
+          # See http://stackoverflow.com/questions/129077/not-in-clause-and-null-values
+          non_null_user_ids = @project.project_members.where.not(user_id: nil).select(:user_id)
+
+          GroupMembersFinder.new(@project.group).execute.where.not(user_id: non_null_user_ids)
+        end
+
+        def tree_saver
+          @tree_saver ||= RelationTreeSaver.new
+        end
+      end
+    end
+  end
+end