Add project config path as a predefined project variable

The value is the defined project's CI config path,
defaulting to .gitlab-ci.yml if no override is provided.

app/models/project.rb

@@ -1989,6 +1989,7 @@ class Project < ApplicationRecord
       .append(key: 'CI_PROJECT_VISIBILITY', value: Gitlab::VisibilityLevel.string_level(visibility_level))
       .append(key: 'CI_PROJECT_REPOSITORY_LANGUAGES', value: repository_languages.map(&:name).join(',').downcase)
       .append(key: 'CI_DEFAULT_BRANCH', value: default_branch)
+      .append(key: 'CI_PROJECT_CONFIG_PATH', value: ci_config_path_or_default)
   end
 
   def predefined_ci_server_variables

changelogs/unreleased/ci-project-config-path-variable.yml

+---
+title: Add project config path as a predefined project variable
+merge_request: 50301
+author:
+type: added

doc/ci/variables/predefined_variables.md

@@ -108,6 +108,7 @@ Kubernetes-specific environment variables are detailed in the
 | `CI_PIPELINE_SOURCE`                          | 10.0   | all    | Indicates how the pipeline was triggered. Possible options are: `push`, `web`, `schedule`, `api`, `external`, `chat`, `webide`, `merge_request_event`, `external_pull_request_event`, `parent_pipeline`, [`trigger`, or `pipeline`](../triggers/README.md#authentication-tokens). For pipelines created before GitLab 9.5, this is displayed as `unknown`. |
 | `CI_PIPELINE_TRIGGERED`                       | all    | all    | The flag to indicate that job was [triggered](../triggers/README.md)                                                                                                                                                                                                                                                                                       |
 | `CI_PIPELINE_URL`                             | 11.1   | 0.5    | Pipeline details URL                                                                                                                                                                                                                                                                                                                                       |
+| `CI_PROJECT_CONFIG_PATH`                      | 13.8   | all    | The CI configuration path for the project
 | `CI_PROJECT_DIR`                              | all    | all    | The full path where the repository is cloned and where the job is run. If the GitLab Runner `builds_dir` parameter is set, this variable is set relative to the value of `builds_dir`. For more information, see [Advanced configuration](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runners-section) for GitLab Runner. |
 | `CI_PROJECT_ID`                               | all    | all    | The unique ID of the current project that GitLab CI/CD uses internally                                                                                                                                                                                                                                                                                        |
 | `CI_PROJECT_NAME`                             | 8.10   | 0.5    | The name of the directory for the project that is currently being built. For example, if the project URL is `gitlab.example.com/group-name/project-1`, the `CI_PROJECT_NAME` would be `project-1`.                                                                                                                                                         |

spec/models/ci/build_spec.rb

@@ -2472,6 +2472,7 @@ RSpec.describe Ci::Build do
           { key: 'CI_PROJECT_VISIBILITY', value: 'private', public: true, masked: false },
           { key: 'CI_PROJECT_REPOSITORY_LANGUAGES', value: project.repository_languages.map(&:name).join(',').downcase, public: true, masked: false },
           { key: 'CI_DEFAULT_BRANCH', value: project.default_branch, public: true, masked: false },
+          { key: 'CI_PROJECT_CONFIG_PATH', value: project.ci_config_path_or_default, public: true, masked: false },
           { key: 'CI_PAGES_DOMAIN', value: Gitlab.config.pages.host, public: true, masked: false },
           { key: 'CI_PAGES_URL', value: project.pages_url, public: true, masked: false },
           { key: 'CI_DEPENDENCY_PROXY_SERVER', value: "#{Gitlab.config.gitlab.host}:#{Gitlab.config.gitlab.port}", public: true, masked: false },
@@ -3002,7 +3003,7 @@ RSpec.describe Ci::Build do
       let(:ci_config_path) { { key: 'CI_CONFIG_PATH', value: 'custom', public: true, masked: false } }
 
       before do
-        expect_any_instance_of(Project).to receive(:ci_config_path) { 'custom' }
+        project.update!(ci_config_path: 'custom')
       end
 
       it { is_expected.to include(ci_config_path) }

spec/models/project_spec.rb

@@ -4548,6 +4548,24 @@ RSpec.describe Project, factory_default: :keep do
     end
   end
 
+  describe '#predefined_project_variables' do
+    let_it_be(:project) { create(:project, :repository) }
+
+    subject { project.predefined_project_variables.to_runner_variables }
+
+    specify do
+      expect(subject).to include({ key: 'CI_PROJECT_CONFIG_PATH', value: Ci::Pipeline::DEFAULT_CONFIG_PATH, public: true, masked: false })
+    end
+
+    context 'when ci config path is overridden' do
+      before do
+        project.update!(ci_config_path: 'random.yml')
+      end
+
+      it { expect(subject).to include({ key: 'CI_PROJECT_CONFIG_PATH', value: 'random.yml', public: true, masked: false }) }
+    end
+  end
+
   describe '#auto_devops_enabled?' do
     before do
       Feature.enable_percentage_of_actors(:force_autodevops_on_by_default, 0)