EE: Refactor: model errors for multi cluster validation

The current approach requires catching exceptions to handle these errors
and callers are already handling model validations so it seems more
appropriate.  Also it seemed to convoluted to add this logic directly to
the model since the model needs to check too many possible associations
to determine whether or not there are more than one cluster since the
model doesn't know what it's being created on. Additionally we only
wanted to validate during create to avoid the risk of existing models
becoming invalid by many different edge cases.

ee/app/policies/ee/clusterable_actions.rb

-# frozen_string_literal: true
-
-module EE
-  module ClusterableActions
-    extend ::Gitlab::Utils::Override
-
-    private
-
-    override :multiple_clusters_available?
-    def multiple_clusters_available?
-      subject.feature_available?(:multiple_clusters)
-    end
-  end
-end

ee/app/presenters/ee/clusterable_presenter.rb

@@ -2,8 +2,17 @@
 
 module EE
   module ClusterablePresenter
+    extend ::Gitlab::Utils::Override
+
     def metrics_cluster_path(cluster, params = {})
       raise NotImplementedError
     end
+
+    private
+
+    override :multiple_clusters_available?
+    def multiple_clusters_available?
+      clusterable.feature_available?(:multiple_clusters)
+    end
   end
 end

ee/spec/policies/clusters/instance_policy_spec.rb

@@ -7,40 +7,22 @@ describe Clusters::InstancePolicy do
   let(:policy) { described_class.new(user, Clusters::Instance.new) }
 
   describe 'rules' do
-    context 'multiple clusters allowed' do
-      before do
-        stub_feature_flags(multiple_clusters: true)
-      end
-
-      context 'no existing instance level cluster' do
+    context 'when admin' do
+      it { expect(policy).to be_allowed :read_cluster }
       it { expect(policy).to be_allowed :add_cluster }
+      it { expect(policy).to be_allowed :create_cluster }
+      it { expect(policy).to be_allowed :update_cluster }
+      it { expect(policy).to be_allowed :admin_cluster }
     end
 
-      context 'with an existing instance level cluster' do
-        before do
-          create(:cluster, :instance)
-        end
-
-        it { expect(policy).to be_allowed :add_cluster }
-      end
-    end
-
-    context 'multiple clusters disallowed' do
-      before do
-        stub_feature_flags(multiple_clusters: false)
-      end
-
-      context 'no existing instance level cluster' do
-        it { expect(policy).to be_allowed :add_cluster }
-      end
-
-      context 'with an existing instance level cluster' do
-        before do
-          create(:cluster, :instance)
-        end
+    context 'when not admin' do
+      let(:user) { create(:user) }
 
+      it { expect(policy).to be_disallowed :read_cluster }
       it { expect(policy).to be_disallowed :add_cluster }
-      end
+      it { expect(policy).to be_disallowed :create_cluster }
+      it { expect(policy).to be_disallowed :update_cluster }
+      it { expect(policy).to be_disallowed :admin_cluster }
     end
   end
 end

ee/spec/policies/group_policy_spec.rb

@@ -374,14 +374,4 @@ describe GroupPolicy do
       end
     end
   end
-
-  it_behaves_like 'ee clusterable policies' do
-    let(:clusterable) { create(:group) }
-    let(:cluster) do
-      create(:cluster,
-             :provided_by_gcp,
-             :group,
-             groups: [clusterable])
-    end
-  end
 end

ee/spec/policies/project_policy_spec.rb

@@ -737,17 +737,6 @@ describe ProjectPolicy do
     end
   end
 
-  it_behaves_like 'ee clusterable policies' do
-    let(:clusterable) { create(:project, :repository) }
-
-    let(:cluster) do
-      create(:cluster,
-             :provided_by_gcp,
-             :project,
-             projects: [clusterable])
-    end
-  end
-
   context 'alert bot' do
     let(:current_user) { User.alert_bot }
 

ee/spec/support/shared_examples/clusterable_shared_examples.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-shared_examples 'ee clusterable policies' do
-  describe '#can_add_cluster?' do
-    let(:current_user) { create(:user) }
-
-    subject { described_class.new(current_user, clusterable) }
-
-    before do
-      clusterable.add_maintainer(current_user)
-    end
-
-    context 'when multiple_clusters feature is available' do
-      before do
-        stub_licensed_features(multiple_clusters: true)
-      end
-
-      context 'when clusterable has clusters' do
-        before do
-          cluster
-        end
-
-        it { is_expected.to be_allowed(:add_cluster) }
-      end
-
-      context 'when clusterable does not have clusters' do
-        it { is_expected.to be_allowed(:add_cluster) }
-      end
-    end
-
-    context 'when multiple_clusters feature is not available' do
-      before do
-        stub_licensed_features(multiple_clusters: false)
-      end
-
-      context 'when clusterable has clusters' do
-        before do
-          cluster
-        end
-
-        it { is_expected.to be_disallowed(:add_cluster) }
-      end
-
-      context 'when clusterable does not have clusters' do
-        it { is_expected.to be_allowed(:add_cluster) }
-      end
-    end
-  end
-end