Log errors for failed pipeline creation in PostReceive

When a pipeline fails to create in `PostReceive`, the error is silently
discarded, making it difficult to understand why a pipeline was not
created. We now add a Sidekiq warning message for this. Adding a Sentry
exception when this happens would generate a lot of noise for invalid CI
files.

Relates to https://gitlab.com/gitlab-org/gitlab-ee/issues/14720

app/services/git/base_hooks_service.rb

@@ -57,7 +57,9 @@ module Git
 
       Ci::CreatePipelineService
         .new(project, current_user, pipeline_params)
-        .execute(:push, pipeline_options)
+        .execute!(:push, pipeline_options)
+    rescue Ci::CreatePipelineService::CreateError => ex
+      log_pipeline_errors(ex)
     end
 
     def execute_project_hooks
@@ -125,5 +127,29 @@ module Git
       project.mark_stuck_remote_mirrors_as_failed!
       project.update_remote_mirrors
     end
+
+    def log_pipeline_errors(exception)
+      data = {
+        class: self.class.name,
+        correlation_id: Labkit::Correlation::CorrelationId.current_id.to_s,
+        project_id: project.id,
+        project_path: project.full_path,
+        message: "Error creating pipeline",
+        errors: exception.to_s,
+        pipeline_params: pipeline_params
+      }
+
+      logger.warn(data)
+    end
+
+    def logger
+      if Sidekiq.server?
+        Sidekiq.logger
+      else
+        # This service runs in Sidekiq, so this shouldn't ever be
+        # called, but this is included just in case.
+        Gitlab::ProjectServiceLogger
+      end
+    end
   end
 end

changelogs/unreleased/sh-add-sidekiq-logging-for-bad-ci.yml

+---
+title: Log errors for failed pipeline creation in PostReceive
+merge_request: 32633
+author:
+type: other

spec/services/git/branch_push_service_spec.rb

@@ -99,6 +99,20 @@ describe Git::BranchPushService, services: true do
       expect(pipeline).to be_push
       expect(Gitlab::Git::BRANCH_REF_PREFIX + pipeline.ref).to eq(ref)
     end
+
+    context 'when pipeline has errors' do
+      before do
+        config = YAML.dump({ test: { script: 'ls', only: ['feature'] } })
+        stub_ci_pipeline_yaml_file(config)
+      end
+
+      it 'reports an error' do
+        allow(Sidekiq).to receive(:server?).and_return(true)
+        expect(Sidekiq.logger).to receive(:warn)
+
+        expect { subject }.not_to change { Ci::Pipeline.count }
+      end
+    end
   end
 
   describe "Updates merge requests" do