Read the AutoDevOps instance domain in ProjectAutoDevOps

app/models/project.rb

@@ -1609,13 +1609,7 @@ class Project < ActiveRecord::Base
   def auto_devops_variables
     return [] unless auto_devops_enabled?
 
-    auto_devops&.variables || if current_application_settings.auto_devops_domain.present?
-                                [{ key: 'AUTO_DEVOPS_DOMAIN',
-                                   value: current_application_settings.auto_devops_domain,
-                                   public: true }]
-                              else
-                                []
-                              end
+    (auto_devops || ProjectAutoDevops.new)&.variables
   end
 
   def append_or_update_attribute(name, value)

app/models/project_auto_devops.rb

 class ProjectAutoDevops < ActiveRecord::Base
+  include Gitlab::CurrentSettings
+
   belongs_to :project
 
   scope :enabled, -> { where(enabled: true) }
@@ -6,13 +8,17 @@ class ProjectAutoDevops < ActiveRecord::Base
 
   validates :domain, allow_blank: true, hostname: { allow_numeric_hostname: true }
 
+  def instance_domain
+    current_application_settings.auto_devops_domain
+  end
+
   def has_domain?
-    domain.present?
+    domain.present? || instance_domain.present?
   end
 
   def variables
     variables = []
-    variables << { key: 'AUTO_DEVOPS_DOMAIN', value: domain, public: true } if domain.present?
+    variables << { key: 'AUTO_DEVOPS_DOMAIN', value: domain || instance_domain, public: true } if has_domain?
     variables
   end
 end

spec/models/project_auto_devops_spec.rb

@@ -18,7 +18,21 @@ describe ProjectAutoDevops do
     context 'when domain is empty' do
       let(:auto_devops) { build_stubbed(:project_auto_devops, project: project, domain: '') }
 
-      it { expect(auto_devops).not_to have_domain }
+      context 'when there is an instance domain specified' do
+        before do
+          stub_application_setting(auto_devops_domain: 'example.com')
+        end
+
+        it { expect(auto_devops).to have_domain }
+      end
+
+      context 'when there is no instance domain specified' do
+        before do
+          stub_application_setting(auto_devops_domain: nil)
+        end
+
+        it { expect(auto_devops).not_to have_domain }
+      end
     end
   end
 
@@ -29,9 +43,32 @@ describe ProjectAutoDevops do
       let(:domain) { 'example.com' }
 
       it 'returns AUTO_DEVOPS_DOMAIN' do
-        expect(auto_devops.variables).to include(
-          { key: 'AUTO_DEVOPS_DOMAIN', value: 'example.com', public: true })
+        expect(auto_devops.variables).to include(domain_variable)
       end
     end
+
+    context 'when domain is not defined' do
+      let(:domain) { nil }
+
+      context 'when there is an instance domain specified' do
+        before do
+          stub_application_setting(auto_devops_domain: 'example.com')
+        end
+
+        it { expect(auto_devops.variables).to include(domain_variable) }
+      end
+
+      context 'when there is no instance domain specified' do
+        before do
+          stub_application_setting(auto_devops_domain: nil)
+        end
+
+        it { expect(auto_devops.variables).not_to include(domain_variable) }
+      end
+    end
+
+    def domain_variable
+      { key: 'AUTO_DEVOPS_DOMAIN', value: 'example.com', public: true }
+    end
   end
 end