Only serve migrated pages if feature flag is enabled

It will allow us to disable all migrated deployments
if something goes wrong, as well as to gradually rollout
the migration

app/models/pages/lookup_path.rb

@@ -5,6 +5,7 @@ module Pages
     include Gitlab::Utils::StrongMemoize
 
     LegacyStorageDisabledError = Class.new(::StandardError)
+    MIGRATED_FILE_NAME = "_migrated.zip"
 
     def initialize(project, trim_prefix: nil, domain: nil)
       @project = project
@@ -54,6 +55,8 @@ module Pages
 
       return if deployment.file.file_storage? && !Feature.enabled?(:pages_serve_with_zip_file_protocol, project)
 
+      return if deployment.file.filename == MIGRATED_FILE_NAME && !Feature.enabled?(:pages_serve_from_migrated_zip, project)
+
       global_id = ::Gitlab::GlobalId.build(deployment, id: deployment.id).to_s
 
       {

config/feature_flags/development/pages_serve_from_migrated_zip.yml

+---
+name: pages_serve_from_migrated_zip
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52573
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/300021
+milestone: '13.9'
+type: development
+group: group::release
+default_enabled: false

spec/models/pages/lookup_path_spec.rb

@@ -125,6 +125,44 @@ RSpec.describe Pages::LookupPath do
         include_examples 'uses disk storage'
       end
     end
+
+    context 'when deployment were created during migration' do
+      before do
+        FileUtils.mkdir_p File.join(project.pages_path, "public")
+        File.open(File.join(project.pages_path, "public/index.html"), "w") do |f|
+          f.write("Hello!")
+        end
+
+        expect(::Pages::MigrateLegacyStorageToDeploymentService.new(project).execute[:status]).to eq(:success)
+
+        project.reload
+      end
+
+      let(:deployment) { project.pages_metadatum.pages_deployment }
+
+      it 'uses deployment from object storage' do
+        freeze_time do
+          expect(source).to(
+            eq({
+                 type: 'zip',
+                 path: deployment.file.url(expire_at: 1.day.from_now),
+                 global_id: "gid://gitlab/PagesDeployment/#{deployment.id}",
+                 sha256: deployment.file_sha256,
+                 file_size: deployment.size,
+                 file_count: deployment.file_count
+               })
+          )
+        end
+      end
+
+      context 'when pages_serve_from_migrated_zip feature flag is disabled' do
+        before do
+          stub_feature_flags(pages_serve_from_migrated_zip: false)
+        end
+
+        include_examples 'uses disk storage'
+      end
+    end
   end
 
   describe '#prefix' do