Remove feature flag for streaming serializer

- Add chagelog for FF removal
- Remove unused specs
- Clean up Legacy code

app/services/projects/import_export/export_service.rb

@@ -58,12 +58,7 @@ module Projects
       end
 
       def tree_saver_class
-        if ::Feature.enabled?(:streaming_serializer, project, default_enabled: true)
-          Gitlab::ImportExport::Project::TreeSaver
-        else
-          # Once we remove :streaming_serializer feature flag, Project::LegacyTreeSaver should be removed as well
-          Gitlab::ImportExport::Project::LegacyTreeSaver
-        end
+        Gitlab::ImportExport::Project::TreeSaver
       end
 
       def uploads_saver

changelogs/unreleased/211816-remove-streaming-serializer-feature-flag.yml

+---
+title: Use faster streaming serializer for project exports
+merge_request: 28925
+author:
+type: performance

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 ||= Gitlab::ImportExport::LegacyRelationTreeSaver.new
-        end
-      end
-    end
-  end
-end

spec/features/projects/import_export/export_file_spec.rb

@@ -38,7 +38,11 @@ describe 'Import/Export - project export integration test', :js do
       sign_in(user)
     end
 
-    shared_examples 'export file without sensitive words' do
+    context "with streaming serializer" do
+      before do
+        stub_feature_flags(project_export_as_ndjson: false)
+      end
+
       it 'exports a project successfully', :sidekiq_inline do
         export_project_and_download_file(page, project)
 
@@ -59,27 +63,8 @@ describe 'Import/Export - project export integration test', :js do
       end
     end
 
-    context "with legacy export" do
-      before do
-        stub_feature_flags(streaming_serializer: false)
-        stub_feature_flags(project_export_as_ndjson: false)
-      end
-
-      it_behaves_like "export file without sensitive words"
-    end
-
-    context "with streaming serializer" do
-      before do
-        stub_feature_flags(streaming_serializer: true)
-        stub_feature_flags(project_export_as_ndjson: false)
-      end
-
-      it_behaves_like "export file without sensitive words"
-    end
-
     context "with ndjson" do
       before do
-        stub_feature_flags(streaming_serializer: true)
         stub_feature_flags(project_export_as_ndjson: true)
       end
 

spec/services/projects/import_export/export_service_spec.rb

@@ -26,28 +26,10 @@ describe Projects::ImportExport::ExportService do
       service.execute
     end
 
-    context 'when :streaming_serializer feature is enabled' do
-      before do
-        stub_feature_flags(streaming_serializer: true)
-      end
-
-      it 'saves the models' do
-        expect(Gitlab::ImportExport::Project::TreeSaver).to receive(:new).and_call_original
-
-        service.execute
-      end
-    end
+    it 'saves the models' do
+      expect(Gitlab::ImportExport::Project::TreeSaver).to receive(:new).and_call_original
 
-    context 'when :streaming_serializer feature is disabled' do
-      before do
-        stub_feature_flags(streaming_serializer: false)
-      end
-
-      it 'saves the models' do
-        expect(Gitlab::ImportExport::Project::LegacyTreeSaver).to receive(:new).and_call_original
-
-        service.execute
-      end
+      service.execute
     end
 
     it 'saves the uploads' do