Add cluster_management_project feature flag

This should be safe to enable by default.

We also have a kwarg to includee the management project or not in
Clusters::ClustersHierarchy.

app/models/clusters/clusters_hierarchy.rb

@@ -4,8 +4,9 @@ module Clusters
   class ClustersHierarchy
     DEPTH_COLUMN = :depth
 
-    def initialize(clusterable)
+    def initialize(clusterable, include_management_project: true)
       @clusterable = clusterable
+      @include_management_project = include_management_project
     end
 
     # Returns clusters in order from deepest to highest group
@@ -24,7 +25,7 @@ module Clusters
 
     private
 
-    attr_reader :clusterable
+    attr_reader :clusterable, :include_management_project
 
     def recursive_cte
       cte = Gitlab::SQL::RecursiveCTE.new(:clusters_cte)
@@ -38,7 +39,7 @@ module Clusters
                      raise ArgumentError, "unknown type for #{clusterable}"
                    end
 
-      if clusterable.is_a?(::Project)
+      if clusterable.is_a?(::Project) && include_management_project
         cte << management_clusters_query
       end
 

app/models/concerns/deployment_platform.rb

@@ -11,6 +11,10 @@ module DeploymentPlatform
 
   private
 
+  def cluster_management_project_enabled?
+    Feature.enabled?(:cluster_management_project, default_enabled: true)
+  end
+
   def find_deployment_platform(environment)
     find_platform_kubernetes_with_cte(environment) ||
       find_instance_cluster_platform_kubernetes(environment: environment)
@@ -18,7 +22,7 @@ module DeploymentPlatform
 
   # EE would override this and utilize environment argument
   def find_platform_kubernetes_with_cte(_environment)
-    Clusters::ClustersHierarchy.new(self).base_and_ancestors
+    Clusters::ClustersHierarchy.new(self, include_management_project: cluster_management_project_enabled?).base_and_ancestors
       .enabled.default_environment
       .first&.platform_kubernetes
   end

ee/app/models/concerns/ee/deployment_platform.rb

@@ -8,7 +8,8 @@ module EE
     def find_platform_kubernetes_with_cte(environment)
       return super unless environment && feature_available?(:multiple_clusters)
 
-      ::Clusters::ClustersHierarchy.new(self).base_and_ancestors
+      ::Clusters::ClustersHierarchy.new(self, include_management_project: cluster_management_project_enabled?)
+        .base_and_ancestors
         .enabled
         .on_environment(environment, relevant_only: true)
         .first&.platform_kubernetes

spec/models/clusters/clusters_hierarchy_spec.rb

@@ -4,8 +4,8 @@ require 'spec_helper'
 
 describe Clusters::ClustersHierarchy do
   describe '#base_and_ancestors' do
-    def base_and_ancestors(clusterable)
-      described_class.new(clusterable).base_and_ancestors
+    def base_and_ancestors(clusterable, include_management_project: true)
+      described_class.new(clusterable, include_management_project: include_management_project).base_and_ancestors
     end
 
     context 'project in nested group with clusters at every level' do
@@ -56,6 +56,10 @@ describe Clusters::ClustersHierarchy do
         expect(base_and_ancestors(management_project)).to eq([group_cluster])
       end
 
+      it 'returns nothing if include_management_project is false' do
+        expect(base_and_ancestors(management_project, include_management_project: false)).to be_empty
+      end
+
       it 'returns clusters for project' do
         expect(base_and_ancestors(project)).to eq([project_cluster, group_cluster])
       end

spec/models/concerns/deployment_platform_spec.rb

@@ -15,8 +15,20 @@ describe DeploymentPlatform do
     context 'when project is the cluster\'s management project ' do
       let!(:cluster_with_management_project) { create(:cluster, :provided_by_user, management_project: project) }
 
-      it 'returns the managed cluster' do
-        is_expected.to eq(cluster_with_management_project.platform_kubernetes)
+      context 'cluster_management_project feature is enabled' do
+        it 'returns the cluster with management project' do
+          is_expected.to eq(cluster_with_management_project.platform_kubernetes)
+        end
+      end
+
+      context 'cluster_management_project feature is disabled' do
+        before do
+          stub_feature_flags(cluster_management_project: false)
+        end
+
+        it 'returns nothing' do
+          is_expected.to be_nil
+        end
       end
     end
 
@@ -56,8 +68,20 @@ describe DeploymentPlatform do
       context 'when project is the cluster\'s management project ' do
         let!(:cluster_with_management_project) { create(:cluster, :provided_by_user, management_project: project) }
 
-        it 'returns the managed cluster' do
-          is_expected.to eq(cluster_with_management_project.platform_kubernetes)
+        context 'cluster_management_project feature is enabled' do
+          it 'returns the cluster with management project' do
+            is_expected.to eq(cluster_with_management_project.platform_kubernetes)
+          end
+        end
+
+        context 'cluster_management_project feature is disabled' do
+          before do
+            stub_feature_flags(cluster_management_project: false)
+          end
+
+          it 'returns the group cluster' do
+            is_expected.to eq(group_cluster.platform_kubernetes)
+          end
         end
       end