Implement displaying downstream pipeline error details

Sometimes downstream pipelines can not be created.
We need to show why it happens to users.
In this implementation, we store downstream pipeline error message
in options attribute of bridge. And we display it.

app/services/ci/create_cross_project_pipeline_service.rb

@@ -47,6 +47,7 @@ module Ci
           # and update the status when the downstream pipeline completes.
           subject.success! unless subject.dependent?
         else
+          subject.options[:downstream_errors] = pipeline.errors.full_messages
           subject.drop!(:downstream_pipeline_creation_failed)
         end
       end

changelogs/unreleased/200016-display-downstream-pipeline-errors.yml

+---
+title: Implement displaying downstream pipeline error details
+merge_request: 32844
+author:
+type: fixed

lib/gitlab/ci/status/bridge/failed.rb

@@ -5,6 +5,14 @@ module Gitlab
     module Status
       module Bridge
         class Failed < Status::Build::Failed
+          private
+
+          def failure_reason_message
+            [
+              self.class.reasons.fetch(subject.failure_reason.to_sym),
+              subject.options[:downstream_errors]
+            ].flatten.compact.join(', ')
+          end
         end
       end
     end

spec/lib/gitlab/ci/status/bridge/factory_spec.rb

@@ -59,12 +59,13 @@ describe Gitlab::Ci::Status::Bridge::Factory do
 
     context 'failed with downstream_pipeline_creation_failed' do
       before do
+        bridge.options = { downstream_errors: ['No stages / jobs for this pipeline.', 'other error'] }
         bridge.failure_reason = 'downstream_pipeline_creation_failed'
       end
 
       it 'fabricates correct status_tooltip' do
         expect(status.status_tooltip).to eq(
-          "#{s_('CiStatusText|failed')} - (downstream pipeline can not be created)"
+          "#{s_('CiStatusText|failed')} - (downstream pipeline can not be created, No stages / jobs for this pipeline., other error)"
         )
       end
     end

spec/services/ci/create_cross_project_pipeline_service_spec.rb

@@ -487,10 +487,11 @@ describe Ci::CreateCrossProjectPipelineService, '#execute' do
       end
 
       it 'does not create a pipeline and drops the bridge' do
-        service.execute(bridge)
+        expect { service.execute(bridge) }.not_to change(downstream_project.ci_pipelines, :count)
 
         expect(bridge.reload).to be_failed
         expect(bridge.failure_reason).to eq('downstream_pipeline_creation_failed')
+        expect(bridge.options[:downstream_errors]).to eq(['Reference not found'])
       end
     end
 
@@ -509,10 +510,35 @@ describe Ci::CreateCrossProjectPipelineService, '#execute' do
       end
 
       it 'does not create a pipeline and drops the bridge' do
-        service.execute(bridge)
+        expect { service.execute(bridge) }.not_to change(downstream_project.ci_pipelines, :count)
+
+        expect(bridge.reload).to be_failed
+        expect(bridge.failure_reason).to eq('downstream_pipeline_creation_failed')
+        expect(bridge.options[:downstream_errors]).to eq(['No stages / jobs for this pipeline.'])
+      end
+    end
+
+    context 'when downstream pipeline has invalid YAML' do
+      before do
+        stub_ci_pipeline_yaml_file(config)
+      end
+
+      let(:config) do
+        <<-EOY
+          test:
+            stage: testx
+            script: echo 1
+        EOY
+      end
+
+      it 'creates the pipeline but drops the bridge' do
+        expect { service.execute(bridge) }.to change(downstream_project.ci_pipelines, :count).by(1)
 
         expect(bridge.reload).to be_failed
         expect(bridge.failure_reason).to eq('downstream_pipeline_creation_failed')
+        expect(bridge.options[:downstream_errors]).to eq(
+          ['test job: chosen stage does not exist; available stages are .pre, build, test, deploy, .post']
+        )
       end
     end
   end