Persist `needs:` validation as config error

In case when `needs:` is missing, but when requested by service,
we would not save the pipeline with config_error.

This makes it explicit that we want to persist the error
as `config_error` failure reason.

changelogs/unreleased/persist-needs-error.yml

+---
+title: Persist `needs:` validation as config error
+merge_request:
+author:
+type: fixed

lib/gitlab/ci/pipeline/chain/helpers.rb

@@ -5,7 +5,12 @@ module Gitlab
     module Pipeline
       module Chain
         module Helpers
-          def error(message)
+          def error(message, config_error: false)
+            if config_error && command.save_incompleted
+              pipeline.yaml_errors = message
+              pipeline.drop!(:config_error)
+            end
+
             pipeline.errors.add(:base, message)
           end
         end

lib/gitlab/ci/pipeline/chain/populate.rb

@@ -26,7 +26,7 @@ module Gitlab
             # Gather all runtime build/stage errors
             #
             if seeds_errors = pipeline.stage_seeds.flat_map(&:errors).compact.presence
-              return error(seeds_errors.join("\n"))
+              return error(seeds_errors.join("\n"), config_error: true)
             end
             ##

spec/services/ci/create_pipeline_service_spec.rb

@@ -1140,10 +1140,26 @@ describe Ci::CreatePipelineService do
       context 'when pipeline on feature is created' do
         let(:ref_name) { 'refs/heads/feature' }
-        it 'does not create a pipeline as test_a depends on build_a' do
-          expect(pipeline).not_to be_persisted
-          expect(pipeline.builds).to be_empty
-          expect(pipeline.errors[:base]).to contain_exactly("test_a: needs 'build_a'")
+        context 'when save_on_errors is enabled' do
+          let(:pipeline) { execute_service(save_on_errors: true) }
+
+          it 'does create a pipeline as test_a depends on build_a' do
+            expect(pipeline).to be_persisted
+            expect(pipeline.builds).to be_empty
+            expect(pipeline.yaml_errors).to eq("test_a: needs 'build_a'")
+            expect(pipeline.errors[:base]).to contain_exactly("test_a: needs 'build_a'")
+          end
+        end
+
+        context 'when save_on_errors is disabled' do
+          let(:pipeline) { execute_service(save_on_errors: false) }
+
+          it 'does not create a pipeline as test_a depends on build_a' do
+            expect(pipeline).not_to be_persisted
+            expect(pipeline.builds).to be_empty
+            expect(pipeline.yaml_errors).to be_nil
+            expect(pipeline.errors[:base]).to contain_exactly("test_a: needs 'build_a'")
+          end
         end
       end