Update pages config async when updating domain

This makes the update to the pages config file async when updating a
pages domain.

This also skips the update if pages aren't deployed, since in that
case, there would be no config file on disk to update.

app/models/pages_domain.rb

@@ -3,6 +3,7 @@
 class PagesDomain < ApplicationRecord
   include Presentable
   include FromUnion
+  include AfterCommitQueue
 
   VERIFICATION_KEY = 'gitlab-pages-verification-code'
   VERIFICATION_THRESHOLD = 3.days.freeze
@@ -222,6 +223,8 @@ class PagesDomain < ApplicationRecord
   private
 
   def pages_deployed?
+    return false unless project
+
     # TODO: remove once `pages_metadatum` is migrated
     # https://gitlab.com/gitlab-org/gitlab/issues/33106
     unless project.pages_metadatum
@@ -244,8 +247,13 @@ class PagesDomain < ApplicationRecord
   # rubocop: disable CodeReuse/ServiceClass
   def update_daemon
     return if usage_serverless?
+    return unless pages_deployed?
 
-    ::Projects::UpdatePagesConfigurationService.new(project).execute
+    if Feature.enabled?(:async_update_pages_config, project)
+      run_after_commit { PagesUpdateConfigurationWorker.perform_async(project_id) }
+    else
+      Projects::UpdatePagesConfigurationService.new(project).execute
+    end
   end
   # rubocop: enable CodeReuse/ServiceClass
 

spec/models/pages_domain_spec.rb

@@ -328,9 +328,11 @@ RSpec.describe PagesDomain do
   end
 
   describe '#update_daemon' do
+    let_it_be(:project) { create(:project).tap(&:mark_pages_as_deployed) }
+
     context 'when usage is serverless' do
       it 'does not call the UpdatePagesConfigurationService' do
-        expect(Projects::UpdatePagesConfigurationService).not_to receive(:new)
+        expect(PagesUpdateConfigurationWorker).not_to receive(:perform_async)
 
         create(:pages_domain, usage: :serverless)
       end
@@ -352,12 +354,30 @@ RSpec.describe PagesDomain do
       domain.destroy!
     end
 
-    it 'delegates to Projects::UpdatePagesConfigurationService' do
+    it 'delegates to Projects::UpdatePagesConfigurationService when not running async' do
+      stub_feature_flags(async_update_pages_config: false)
+
       service = instance_double('Projects::UpdatePagesConfigurationService')
       expect(Projects::UpdatePagesConfigurationService).to receive(:new) { service }
       expect(service).to receive(:execute)
 
-      create(:pages_domain)
+      create(:pages_domain, project: project)
+    end
+
+    it "schedules a PagesUpdateConfigurationWorker" do
+      expect(PagesUpdateConfigurationWorker).to receive(:perform_async).with(project.id)
+
+      create(:pages_domain, project: project)
+    end
+
+    context "when the pages aren't deployed" do
+      let_it_be(:project) { create(:project).tap(&:mark_pages_as_not_deployed) }
+
+      it "does not schedule a PagesUpdateConfigurationWorker" do
+        expect(PagesUpdateConfigurationWorker).not_to receive(:perform_async).with(project.id)
+
+        create(:pages_domain, project: project)
+      end
     end
 
     context 'configuration updates when attributes change' do