Gracefully handle missing latest CI pipeline

If a user attempted to retrieve the latest CI pipeline but no such
pipeline existed, the user would see a 500 error.

We should handle this case gracefully with a 404.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/212552

app/controllers/projects/pipelines_controller.rb

@@ -13,6 +13,7 @@ class Projects::PipelinesController < Projects::ApplicationController
   before_action do
     push_frontend_feature_flag(:junit_pipeline_view)
   end
+  before_action :ensure_pipeline, only: [:show]
 
   around_action :allow_gitaly_ref_name_caching, only: [:index, :show]
 
@@ -214,6 +215,10 @@ class Projects::PipelinesController < Projects::ApplicationController
     params.require(:pipeline).permit(:ref, variables_attributes: %i[key variable_type secret_value])
   end
 
+  def ensure_pipeline
+    render_404 unless pipeline
+  end
+
   # rubocop: disable CodeReuse/ActiveRecord
   def pipeline
     @pipeline ||= if params[:id].blank? && params[:latest]

changelogs/unreleased/sh-handle-no-latest-pipeline.yml

+---
+title: Gracefully handle missing latest CI pipeline
+merge_request: 28263
+author:
+type: fixed

spec/controllers/projects/pipelines_controller_spec.rb

@@ -923,10 +923,18 @@ describe Projects::PipelinesController do
     end
 
     context 'ref provided' do
+      render_views
+
       before do
         create(:ci_pipeline, ref: 'master', project: project)
       end
 
+      it 'shows a 404 if no pipeline exists' do
+        get :show, params: { namespace_id: project.namespace, project_id: project, latest: true, ref: 'non-existence' }
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+
       it 'shows the latest pipeline for the provided ref' do
         get :show, params: { namespace_id: project.namespace, project_id: project, latest: true, ref: branch_secondary.name }