Prevent deletion a serverless domain that is in use

app/controllers/admin/serverless/domains_controller.rb

@@ -31,6 +31,12 @@ class Admin::Serverless::DomainsController < Admin::ApplicationController
   end
 
   def destroy
+    if domain.serverless_domain_clusters.count > 0
+      return redirect_to admin_serverless_domains_path,
+                         status: :conflict,
+                         notice: _('Domain cannot be deleted while associated to one or more clusters.')
+    end
+
     domain.destroy!
 
     redirect_to admin_serverless_domains_path,

locale/gitlab.pot

@@ -6880,6 +6880,9 @@ msgstr ""
 msgid "Domain"
 msgstr ""
 
+msgid "Domain cannot be deleted while associated to one or more clusters."
+msgstr ""
+
 msgid "Domain verification is an essential security measure for public GitLab sites. Users are required to demonstrate they control a domain before it is enabled"
 msgstr ""
 

spec/controllers/admin/serverless/domains_controller_spec.rb

@@ -338,6 +338,20 @@ describe Admin::Serverless::DomainsController do
             expect(flash[:notice]).to include('Domain was successfully deleted.')
           end
         end
+
+        context 'and is associated to any clusters' do
+          before do
+            create(:serverless_domain_cluster, pages_domain: domain)
+          end
+
+          it 'does not delete the domain' do
+            expect { delete :destroy, params: { id: domain.id } }
+              .not_to change { PagesDomain.count }
+
+            expect(response).to have_gitlab_http_status(:conflict)
+            expect(flash[:notice]).to include('Domain cannot be deleted while associated to one or more clusters.')
+          end
+        end
       end
 
       context 'when domain does not exist' do