Load config variables from external project

Loads config variables from external project when CI config is loaded
from external project.

Changelog: fixed

app/controllers/projects/pipelines_controller.rb

@@ -191,7 +191,8 @@ class Projects::PipelinesController < Projects::ApplicationController
   def config_variables
     respond_to do |format|
       format.json do
-        result = Ci::ListConfigVariablesService.new(@project, current_user).execute(params[:sha])
+        project = @project.uses_external_project_ci_config? ? @project.ci_config_external_project : @project
+        result = Ci::ListConfigVariablesService.new(project, current_user).execute(params[:sha])
 
         result.nil? ? head(:no_content) : render(json: result)
       end

app/models/project.rb

@@ -2537,6 +2537,10 @@ class Project < ApplicationRecord
     ci_config_path.blank? || ci_config_path == Gitlab::FileDetector::PATTERNS[:gitlab_ci]
   end
 
+  def uses_external_project_ci_config?
+    !!(ci_config_path =~ %r{@.+/.+})
+  end
+
   def limited_protected_branches(limit)
     protected_branches.limit(limit)
   end
@@ -2645,6 +2649,10 @@ class Project < ApplicationRecord
     repository.gitlab_ci_yml_for(sha, ci_config_path_or_default)
   end
 
+  def ci_config_external_project
+    Project.find_by_full_path(ci_config_path.split('@', 2).last)
+  end
+
   def enabled_group_deploy_keys
     return GroupDeployKey.none unless group
 

doc/ci/pipelines/index.md

@@ -171,9 +171,6 @@ variables:
 
 You cannot set job-level variables to be pre-filled when you run a pipeline manually.
 
-Pre-filled variables do not show up when the CI/CD configuration is [external to the project](settings.md#specify-a-custom-cicd-configuration-file).
-See the [related issue](https://gitlab.com/gitlab-org/gitlab/-/issues/336184) for more details.
-
 ### Run a pipeline by using a URL query string
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/24146) in GitLab 12.5.

spec/controllers/projects/pipelines_controller_spec.rb

@@ -1292,6 +1292,38 @@ RSpec.describe Projects::PipelinesController do
       end
     end
 
+    context 'when project uses external project ci config' do
+      let(:other_project) { create(:project) }
+      let(:sha) { 'master' }
+      let(:service) { ::Ci::ListConfigVariablesService.new(other_project, user) }
+
+      let(:ci_config) do
+        {
+          variables: {
+            KEY1: { value: 'val 1', description: 'description 1' }
+          },
+          test: {
+            stage: 'test',
+            script: 'echo'
+          }
+        }
+      end
+
+      before do
+        project.update!(ci_config_path: ".gitlab-ci.yml@#{other_project.full_path}")
+        synchronous_reactive_cache(service)
+      end
+
+      it 'returns other project config variables' do
+        expect(::Ci::ListConfigVariablesService).to receive(:new).with(other_project, anything).and_return(service)
+
+        get_config_variables
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(json_response['KEY1']).to eq({ 'value' => 'val 1', 'description' => 'description 1' })
+      end
+    end
+
     private
 
     def stub_gitlab_ci_yml_for_sha(sha, result)

spec/models/project_spec.rb

@@ -2542,7 +2542,7 @@ RSpec.describe Project, factory_default: :keep do
   end
 
   describe '#uses_default_ci_config?' do
-    let(:project) { build(:project)}
+    let(:project) { build(:project) }
 
     it 'has a custom ci config path' do
       project.ci_config_path = 'something_custom'
@@ -2563,6 +2563,44 @@ RSpec.describe Project, factory_default: :keep do
     end
   end
 
+  describe '#uses_external_project_ci_config?' do
+    subject(:uses_external_project_ci_config) { project.uses_external_project_ci_config? }
+
+    let(:project) { build(:project) }
+
+    context 'when ci_config_path is configured with external project' do
+      before do
+        project.ci_config_path = '.gitlab-ci.yml@hello/world'
+      end
+
+      it { is_expected.to eq(true) }
+    end
+
+    context 'when ci_config_path is nil' do
+      before do
+        project.ci_config_path = nil
+      end
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when ci_config_path is configured with a file in the project' do
+      before do
+        project.ci_config_path = 'hello/world/gitlab-ci.yml'
+      end
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when ci_config_path is configured with remote file' do
+      before do
+        project.ci_config_path = 'https://example.org/file.yml'
+      end
+
+      it { is_expected.to eq(false) }
+    end
+  end
+
   describe '#latest_successful_build_for_ref' do
     let_it_be(:project) { create(:project, :repository) }
     let_it_be(:pipeline) { create_pipeline(project) }
@@ -7043,6 +7081,15 @@ RSpec.describe Project, factory_default: :keep do
     end
   end
 
+  describe '#ci_config_external_project' do
+    subject(:ci_config_external_project) { project.ci_config_external_project }
+
+    let(:other_project) { create(:project) }
+    let(:project) { build(:project, ci_config_path: ".gitlab-ci.yml@#{other_project.full_path}") }
+
+    it { is_expected.to eq(other_project) }
+  end
+
   describe '#enabled_group_deploy_keys' do
     let_it_be(:project) { create(:project) }