Add build prerequisite for Kubernetes namespaces

Builds that have deployments require Kubernetes resources
to be created before the build can be deployed. These
resources are no longer created when the cluster is
created, which allows us to only create the resources
required by each specific build.

changelogs/unreleased/57115-just-in-time-k8s-resource-creation.yml

+---
+title: Create Kubernetes resources for projects when their deployment jobs run.
+merge_request: 25586
+author:
+type: changed

lib/gitlab/ci/build/prerequisite/factory.rb

@@ -8,7 +8,7 @@ module Gitlab
           attr_reader :build
 
           def self.prerequisites
-            []
+            [KubernetesNamespace]
           end
 
           def initialize(build)

lib/gitlab/ci/build/prerequisite/kubernetes_namespace.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Ci
+    module Build
+      module Prerequisite
+        class KubernetesNamespace < Base
+          ##
+          # Cluster settings may have changed since the last deploy,
+          # so we must always ensure the namespace is up to date.
+          #
+          def unmet?
+            build.has_deployment? && clusters_missing_namespaces.present?
+          end
+
+          def complete!
+            return unless unmet?
+
+            clusters_missing_namespaces.each do |cluster|
+              create_or_update_namespace(cluster)
+            end
+          end
+
+          private
+
+          def project
+            build.project
+          end
+
+          def create_or_update_namespace(cluster)
+            kubernetes_namespace = cluster.find_or_initialize_kubernetes_namespace_for_project(project)
+
+            Clusters::Gcp::Kubernetes::CreateOrUpdateNamespaceService.new(
+              cluster: cluster,
+              kubernetes_namespace: kubernetes_namespace
+            ).execute
+          end
+
+          def clusters_missing_namespaces
+            strong_memoize(:clusters_missing_namespaces) do
+              project.all_clusters.missing_kubernetes_namespace(project.kubernetes_namespaces).to_a
+            end
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/build/prerequisite/kubernetes_namespace_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Ci::Build::Prerequisite::KubernetesNamespace do
+  let(:build) { create(:ci_build) }
+
+  describe '#unmet?' do
+    subject { described_class.new(build).unmet? }
+
+    context 'build has no deployment' do
+      before do
+        expect(build.deployment).to be_nil
+      end
+
+      it { is_expected.to be_falsey }
+    end
+
+    context 'build has a deployment, and no existing kubernetes namespace' do
+      let!(:deployment) { create(:deployment, deployable: build) }
+      let!(:cluster) { create(:cluster, projects: [build.project]) }
+
+      before do
+        expect(build.project.kubernetes_namespaces).to be_empty
+      end
+
+      it { is_expected.to be_truthy }
+    end
+
+    context 'build has a deployment and kubernetes namespaces' do
+      let!(:deployment) { create(:deployment, deployable: build) }
+      let!(:cluster) { create(:cluster, projects: [build.project]) }
+      let!(:kubernetes_namespace) { create(:cluster_kubernetes_namespace, cluster: cluster) }
+
+      it { is_expected.to be_falsey }
+    end
+  end
+
+  describe '#complete!' do
+    let(:cluster) { create(:cluster, projects: [build.project]) }
+    let(:service) { double(execute: true) }
+
+    subject { described_class.new(build).complete! }
+
+    context 'completion is required' do
+      let!(:deployment) { create(:deployment, deployable: build) }
+
+      it 'creates a kubernetes namespace' do
+        expect(Clusters::Gcp::Kubernetes::CreateOrUpdateNamespaceService)
+          .to receive(:new)
+          .with(cluster: cluster, kubernetes_namespace: instance_of(Clusters::KubernetesNamespace))
+          .and_return(service)
+
+        expect(service).to receive(:execute).once
+
+        subject
+      end
+    end
+
+    context 'completion is not required' do
+      before do
+        expect(build.deployment).to be_nil
+      end
+
+      it 'does not create a namespace' do
+        expect(Clusters::Gcp::Kubernetes::CreateOrUpdateNamespaceService).not_to receive(:new)
+
+        subject
+      end
+    end
+  end
+end