Reduce stage seeds coupling between pipeline and YAML

This is a first step to decouple pipeline from YAML processing. It
reduces the coupling by removing some methods that introduce coupling
and by moving logic into separate chain element that is being used to
populate pipelines with stages and builds.

app/models/ci/pipeline.rb

@@ -365,18 +365,10 @@ module Ci
       @stage_seeds ||= config_processor.stage_seeds(self)
     end
-    def seeds_size
-      @seeds_size ||= stage_seeds.sum(&:size)
-    end
-
     def has_kubernetes_active?
       project.deployment_platform&.active?
     end
-    def has_stage_seeds?
-      stage_seeds.any?
-    end
-
     def has_warnings?
       builds.latest.failed_but_allowed.any?
     end

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

@@ -21,15 +21,19 @@ module Gitlab
               pipeline.stages << seed.to_resource
             end
+            if pipeline.stages.none?
+              return error('No stages / jobs for this pipeline.')
+            end
+
             if pipeline.invalid?
-              error('Failed to build the pipeline!')
+              return error('Failed to build the pipeline!')
             end
             raise Populate::PopulateError if pipeline.persisted?
           end
           def break?
-            pipeline.invalid?
+            pipeline.errors.any?
           end
         end
       end

lib/gitlab/ci/pipeline/chain/validate/config.rb

@@ -18,10 +18,6 @@ module Gitlab
                 return error(@pipeline.yaml_errors)
               end
-
-              unless @pipeline.has_stage_seeds?
-                return error('No stages / jobs for this pipeline.')
-              end
             end
             def break?

spec/lib/gitlab/ci/pipeline/chain/populate_spec.rb

@@ -42,6 +42,32 @@ describe Gitlab::Ci::Pipeline::Chain::Populate do
     end
   end
+  context 'when pipeline is empty' do
+    let(:config) do
+      { rspec: {
+          script: 'ls',
+          only: ['something']
+      } }
+    end
+
+    let(:pipeline) do
+      build(:ci_pipeline, project: project, config: config)
+    end
+
+    before do
+      step.perform!
+    end
+
+    it 'breaks the chain' do
+      expect(step.break?).to be true
+    end
+
+    it 'appends an error about missing stages' do
+      expect(pipeline.errors.to_a)
+        .to include 'No stages / jobs for this pipeline.'
+    end
+  end
+
   context 'when pipeline has validation errors' do
     let(:pipeline) do
       build(:ci_pipeline, project: project, ref: nil)

spec/lib/gitlab/ci/pipeline/chain/validate/config_spec.rb

@@ -76,28 +76,6 @@ describe Gitlab::Ci::Pipeline::Chain::Validate::Config do
     end
   end
-  context 'when pipeline has no stages / jobs' do
-    let(:config) do
-      { rspec: {
-          script: 'ls',
-          only: ['something']
-      } }
-    end
-
-    let(:pipeline) do
-      build(:ci_pipeline, project: project, config: config)
-    end
-
-    it 'appends an error about missing stages' do
-      expect(pipeline.errors.to_a)
-        .to include 'No stages / jobs for this pipeline.'
-    end
-
-    it 'breaks the chain' do
-      expect(step.break?).to be true
-    end
-  end
-
   context 'when pipeline contains configuration validation errors' do
     let(:config) { { rspec: {} } }

spec/models/ci/pipeline_spec.rb

@@ -253,14 +253,6 @@ describe Ci::Pipeline, :mailer do
       end
     end
-    describe '#seeds_size' do
-      let(:pipeline) { build(:ci_pipeline_with_one_job) }
-
-      it 'returns number of jobs in stage seeds' do
-        expect(pipeline.seeds_size).to eq 1
-      end
-    end
-
     describe '#legacy_stages' do
       subject { pipeline.legacy_stages }
@@ -590,20 +582,6 @@ describe Ci::Pipeline, :mailer do
     end
   end
-  describe '#has_stage_seeds?' do
-    context 'when pipeline has stage seeds' do
-      subject { build(:ci_pipeline_with_one_job) }
-
-      it { is_expected.to have_stage_seeds }
-    end
-
-    context 'when pipeline does not have stage seeds' do
-      subject { create(:ci_pipeline_without_jobs) }
-
-      it { is_expected.not_to have_stage_seeds }
-    end
-  end
-
   describe '#has_warnings?' do
     subject { pipeline.has_warnings? }