Move pages async when transferring projects

When moving a project across namespaces, the pages need to be moved on
disk if there are any.

This performs that move in the background if there are pages, but
skips the move if there aren't any, to avoid unnecessary jobs.

It also performs the move or the scheduling of the database
transaction.

app/services/projects/transfer_service.rb

@@ -88,15 +88,14 @@ module Projects
         # Move uploads
         move_project_uploads(project)
 
-        # Move pages
-        Gitlab::PagesTransfer.new.move_project(project.path, @old_namespace.full_path, @new_namespace.full_path)
-
         project.old_path_with_namespace = @old_path
 
         update_repository_configuration(@new_path)
 
         execute_system_hooks
       end
+
+      move_pages(project)
     rescue Exception # rubocop:disable Lint/RescueException
       rollback_side_effects
       raise
@@ -181,6 +180,19 @@ module Projects
       )
     end
 
+    def move_pages(project)
+      transfer = Gitlab::PagesTransfer.new
+
+      if Feature.enabled?(:async_pages_move_project_transfer, project)
+        # Avoid scheduling moves for directories that don't exist.
+        return unless project.pages_deployed?
+
+        transfer = transfer.async
+      end
+
+      transfer.move_project(project.path, @old_namespace.full_path, @new_namespace.full_path)
+    end
+
     def old_wiki_repo_path
       "#{old_path}#{::Gitlab::GlRepository::WIKI.path_suffix}"
     end

config/feature_flags/development/async_pages_move_project_transfer.yml

+---
+name: async_pages_move_project_transfer
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40492
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/235757
+group: team::Scalability
+type: development
+default_enabled: false

spec/services/projects/transfer_service_spec.rb

@@ -5,8 +5,8 @@ require 'spec_helper'
 RSpec.describe Projects::TransferService do
   include GitHelpers
 
-  let(:user) { create(:user) }
-  let(:group) { create(:group) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:group) { create(:group) }
   let(:project) { create(:project, :repository, :legacy_storage, namespace: user.namespace) }
 
   subject(:execute_transfer) { described_class.new(project, user).execute(group) }
@@ -489,6 +489,43 @@ RSpec.describe Projects::TransferService do
     end
   end
 
+  context 'moving pages' do
+    let_it_be(:project) { create(:project, namespace: user.namespace) }
+
+    before do
+      group.add_owner(user)
+    end
+
+    it 'schedules a job  when pages are deployed' do
+      project.mark_pages_as_deployed
+
+      expect(PagesTransferWorker).to receive(:perform_async)
+                                       .with("move_project", [project.path, user.namespace.full_path, group.full_path])
+
+      execute_transfer
+    end
+
+    it 'does not schedule a job when no pages are deployed' do
+      expect(PagesTransferWorker).not_to receive(:perform_async)
+
+      execute_transfer
+    end
+
+    context 'when async_pages_move_project_transfer is disabled' do
+      before do
+        stub_feature_flags(async_pages_move_project_transfer: false)
+      end
+
+      it 'moves pages inline' do
+        expect_next_instance_of(Gitlab::PagesTransfer) do |transfer|
+          expect(transfer).to receive(:move_project).with(project.path, user.namespace.full_path, group.full_path)
+        end
+
+        execute_transfer
+      end
+    end
+  end
+
   def rugged_config
     rugged_repo(project.repository).config
   end