Correct job artifacts API download for expired and locked files

GitLab allows to keep artifacts around beyond their expiration, if
a pipeline is the latest on a certain REF. One can download those
files via Web UI, but the download via API was broken. This commit
corrects the expiration check and also adds a unit-test case for it.

lib/api/job_artifacts.rb

@@ -90,7 +90,7 @@ module API
         build = find_build!(params[:job_id])
         authorize_read_job_artifacts!(build)
 
-        not_found! unless build.artifacts?
+        not_found! unless build.available_artifacts?
 
         path = Gitlab::Ci::Build::Artifacts::Path
           .new(params[:artifact_path])

spec/requests/api/jobs_spec.rb

@@ -328,6 +328,17 @@ RSpec.describe API::Jobs do
             .to include('Content-Type' => 'application/json',
                         'Gitlab-Workhorse-Send-Data' => /artifacts-entry/)
         end
+
+        context 'when artifacts are locked' do
+          it 'allows access to expired artifact' do
+            pipeline.artifacts_locked!
+            job.update!(artifacts_expire_at: Time.now - 7.days)
+
+            get_artifact_file(artifact)
+
+            expect(response).to have_gitlab_http_status(:ok)
+          end
+        end
       end
     end