Make the discovery body more accurate

The response for `/apis/extensions/v1beta1` does not have secrets,
serviceaccounts, services, not pods. It instead has deployments and
ingresses.

Verified from a real Kubernetes cluster. Run `kubectl proxy`, then
navigate to http://127.0.0.1:8001/apis/extensions/v1beta1

changelogs/unreleased/get_deployments_for_kubernetes_1-16.yml

+---
+title: Stop NoMethodError happening for 1.16+ Kubernetes clusters
+merge_request: 23149
+author:
+type: fixed

lib/gitlab/kubernetes/kube_client.rb

@@ -16,6 +16,7 @@ module Gitlab
       SUPPORTED_API_GROUPS = {
         core: { group: 'api', version: 'v1' },
         rbac: { group: 'apis/rbac.authorization.k8s.io', version: 'v1' },
+        apps: { group: 'apis/apps', version: 'v1' },
         extensions: { group: 'apis/extensions', version: 'v1beta1' },
         istio: { group: 'apis/networking.istio.io', version: 'v1alpha3' },
         knative: { group: 'apis/serving.knative.dev', version: 'v1alpha1' }
@@ -74,10 +75,6 @@ module Gitlab
         :update_role_binding,
         to: :rbac_client
 
-      # Deployments resource is currently on the apis/extensions api group
-      delegate :get_deployments,
-        to: :extensions_client
-
       # non-entity methods that can only work with the core client
       # as it uses the pods/log resource
       delegate :get_pod_log,
@@ -103,6 +100,21 @@ module Gitlab
         validate_url!
       end
 
+      # Deployments resource is currently on the apis/extensions api group
+      # until Kubernetes 1.15. Kubernetest 1.16+ has deployments resources in
+      # the apis/apps api group.
+      #
+      # As we still support Kubernetes 1.12+, we will need to support both.
+      def get_deployments(**args)
+        extensions_client.discover unless extensions_client.discovered
+
+        if extensions_client.respond_to?(:get_deployments)
+          extensions_client.get_deployments(**args)
+        else
+          apps_client.get_deployments(**args)
+        end
+      end
+
       def create_or_update_cluster_role_binding(resource)
         if cluster_role_binding_exists?(resource)
           update_cluster_role_binding(resource)

spec/lib/gitlab/kubernetes/kube_client_spec.rb

@@ -229,20 +229,30 @@ describe Gitlab::Kubernetes::KubeClient do
     end
   end
 
-  describe 'extensions API group' do
-    let(:api_groups) { ['apis/extensions'] }
+  describe '#get_deployments' do
     let(:extensions_client) { client.extensions_client }
+    let(:apps_client) { client.apps_client }
 
-    describe '#get_deployments' do
-      include_examples 'redirection not allowed', 'get_deployments'
-      include_examples 'dns rebinding not allowed', 'get_deployments'
+    include_examples 'redirection not allowed', 'get_deployments'
+    include_examples 'dns rebinding not allowed', 'get_deployments'
 
-      it 'delegates to the extensions client' do
-        expect(client).to delegate_method(:get_deployments).to(:extensions_client)
+    it 'delegates to the extensions client' do
+      expect(extensions_client).to receive(:get_deployments)
+
+      client.get_deployments
+    end
+
+    context 'extensions does not have deployments for Kubernetes 1.16+ clusters' do
+      before do
+        WebMock
+          .stub_request(:get, api_url + '/apis/extensions/v1beta1')
+          .to_return(kube_response(kube_1_16_extensions_v1beta1_discovery_body))
       end
 
-      it 'responds to the method' do
-        expect(client).to respond_to :get_deployments
+      it 'delegates to the apps client' do
+        expect(apps_client).to receive(:get_deployments)
+
+        client.get_deployments
       end
     end
   end

spec/support/helpers/kubernetes_helpers.rb

@@ -27,7 +27,10 @@ module KubernetesHelpers
     WebMock.stub_request(:get, api_url + '/api/v1').to_return(kube_response(kube_v1_discovery_body))
     WebMock
       .stub_request(:get, api_url + '/apis/extensions/v1beta1')
-      .to_return(kube_response(kube_v1beta1_discovery_body))
+      .to_return(kube_response(kube_extensions_v1beta1_discovery_body))
+    WebMock
+      .stub_request(:get, api_url + '/apis/apps/v1')
+      .to_return(kube_response(kube_apps_v1_discovery_body))
     WebMock
       .stub_request(:get, api_url + '/apis/rbac.authorization.k8s.io/v1')
       .to_return(kube_response(kube_v1_rbac_authorization_discovery_body))
@@ -275,15 +278,33 @@ module KubernetesHelpers
     }
   end
 
-  def kube_v1beta1_discovery_body
+  # From Kubernetes 1.16+ Deployments are no longer served from apis/extensions
+  def kube_1_16_extensions_v1beta1_discovery_body
+    {
+      "kind" => "APIResourceList",
+      "resources" => [
+        { "name" => "ingresses", "namespaced" => true, "kind" => "Deployment" }
+      ]
+    }
+  end
+
+  def kube_extensions_v1beta1_discovery_body
     {
       "kind" => "APIResourceList",
       "resources" => [
-        { "name" => "pods", "namespaced" => true, "kind" => "Pod" },
         { "name" => "deployments", "namespaced" => true, "kind" => "Deployment" },
-        { "name" => "secrets", "namespaced" => true, "kind" => "Secret" },
-        { "name" => "serviceaccounts", "namespaced" => true, "kind" => "ServiceAccount" },
-        { "name" => "services", "namespaced" => true, "kind" => "Service" }
+        { "name" => "ingresses", "namespaced" => true, "kind" => "Ingress" }
+      ]
+    }
+  end
+
+  # Yes, deployments are defined in both apis/extensions/v1beta1 and apis/v1
+  # (for Kubernetes < 1.16). This matches what Kubenetes API server returns.
+  def kube_apps_v1_discovery_body
+    {
+      "kind" => "APIResourceList",
+      "resources" => [
+        { "name" => "deployments", "namespaced" => true, "kind" => "Deployment" }
       ]
     }
   end