Add CI_KUBERNETES_ACTIVE alternative to only:kubernetes:active

This adds a CI variable CI_KUBERNETES_ACTIVE as an alternative to
only:kubernetes:active/except:kubernetes:active for use with the rules
syntax.

This will enable us to move deployment-related Auto DevOps sub-templates
to rules syntax, see
https://gitlab.com/gitlab-org/gitlab/-/issues/213336

app/models/ci/pipeline.rb

@@ -683,6 +683,8 @@ module Ci
           variables.concat(merge_request.predefined_variables)
         end
 
+        variables.append(key: 'CI_KUBERNETES_ACTIVE', value: 'true') if has_kubernetes_active?
+
         if external_pull_request_event? && external_pull_request
           variables.concat(external_pull_request.predefined_variables)
         end

changelogs/unreleased/add-ci-kubernetes-active-pipeline-variable.yml

+---
+title: Add a CI variable CI_KUBERNETES_ACTIVE as an alternative to only:kubernetes/except:kubernetes
+  that works with the rules syntax
+merge_request: 31146
+author:
+type: added

doc/ci/variables/predefined_variables.md

@@ -64,6 +64,7 @@ future GitLab releases.**
 | `CI_JOB_TOKEN`                                | 9.0    | 1.2    | Token used for authenticating with the [GitLab Container Registry](../../user/packages/container_registry/index.md) and downloading [dependent repositories](../../user/project/new_ci_build_permissions_model.md#dependent-repositories)                                                                                                                                                                                                              |
 | `CI_JOB_JWT`                                  | 12.10  | all    | RS256 JSON web token that can be used for authenticating with third party systems that support JWT authentication, for example [HashiCorp's Vault](../examples/authenticating-with-hashicorp-vault). |
 | `CI_JOB_URL`                                  | 11.1   | 0.5    | Job details URL                                                                                                                                                                                                                                                                                                                                            |
+| `CI_KUBERNETES_ACTIVE`                        | 13.0   | all    | Included with the value `true` only if the pipeline has a Kubernetes cluster available for deployments. Not included if no cluster is availble. Can be used as an alternative to [`only:kubernetes`/`except:kubernetes`](../yaml/README.md#onlykubernetesexceptkubernetes) with [`rules:if`](../yaml/README.md#rulesif)                                    |
 | `CI_MERGE_REQUEST_ASSIGNEES`                  | 11.9   | all    | Comma-separated list of username(s) of assignee(s) for the merge request if [the pipelines are for merge requests](../merge_request_pipelines/index.md). Available only if `only: [merge_requests]` or [`rules`](../yaml/README.md#rules) syntax is used and the merge request is created.                                                                                                              |
 | `CI_MERGE_REQUEST_CHANGED_PAGE_PATHS`         | 12.9   | all    | Comma-separated list of paths of changed pages in a deployed [Review App](../review_apps/index.md) for a [Merge Request](../merge_request_pipelines/index.md). A [Route Map](../review_apps/index.md#route-maps) must be configured.                                                                                 |
 | `CI_MERGE_REQUEST_CHANGED_PAGE_URLS`          | 12.9   | all    | Comma-separated list of URLs of changed pages in a deployed [Review App](../review_apps/index.md) for a [Merge Request](../merge_request_pipelines/index.md). A [Route Map](../review_apps/index.md#route-maps) must be configured.                                                                                  |

spec/models/ci/pipeline_spec.rb

@@ -709,6 +709,28 @@ describe Ci::Pipeline, :mailer do
           )
       end
     end
+
+    describe 'variable CI_KUBERNETES_ACTIVE' do
+      context 'when pipeline.has_kubernetes_active? is true' do
+        before do
+          allow(pipeline).to receive(:has_kubernetes_active?).and_return(true)
+        end
+
+        it "is incldued with value 'true'" do
+          expect(subject.to_hash).to include('CI_KUBERNETES_ACTIVE' => 'true')
+        end
+      end
+
+      context 'when pipeline.has_kubernetes_active? is false' do
+        before do
+          allow(pipeline).to receive(:has_kubernetes_active?).and_return(false)
+        end
+
+        it 'is not included' do
+          expect(subject.to_hash).not_to have_key('CI_KUBERNETES_ACTIVE')
+        end
+      end
+    end
   end
 
   describe '#protected_ref?' do