Require other stages than .pre and .post

Do not create pipeline that only
contains .pre, .post stages.

These pipelines alone does not make
sense.

changelogs/unreleased/do-not-create-pre-post-pipeline.yml

+---
+title: Require other stages than .pre and .post
+merge_request: 23629
+author:
+type: fixed

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

@@ -17,7 +17,7 @@ module Gitlab
             #
             pipeline.stages = @command.stage_seeds.map(&:to_resource)
 
-            if pipeline.stages.none?
+            if stage_names.empty?
               return error('No stages / jobs for this pipeline.')
             end
 
@@ -31,6 +31,15 @@ module Gitlab
           def break?
             pipeline.errors.any?
           end
+
+          private
+
+          def stage_names
+            # We filter out `.pre/.post` stages, as they alone are not considered
+            # a complete pipeline:
+            # https://gitlab.com/gitlab-org/gitlab/issues/198518
+            pipeline.stages.map(&:name) - ::Gitlab::Ci::Config::EdgeStagesInjector::EDGES
+          end
         end
       end
     end

spec/services/ci/create_pipeline_service/pre_post_stages_spec.rb

+# frozen_string_literal: true
+require 'spec_helper'
+
+describe Ci::CreatePipelineService do
+  context '.pre/.post stages' do
+    let_it_be(:user)    { create(:admin) }
+    let_it_be(:project) { create(:project, :repository, creator: user) }
+
+    let(:source)   { :push }
+    let(:service)  { described_class.new(project, user, { ref: ref }) }
+    let(:pipeline) { service.execute(source) }
+
+    let(:config) do
+      <<~YAML
+        validate:
+          stage: .pre
+          script: echo Hello World
+
+        build:
+          stage: build
+          rules:
+            - if: $CI_COMMIT_BRANCH == "master"
+          script: echo Hello World
+
+        notify:
+          stage: .post
+          script: echo Hello World
+      YAML
+    end
+
+    before do
+      stub_ci_pipeline_yaml_file(config)
+    end
+
+    context 'when pipeline contains a build except .pre/.post' do
+      let(:ref) { 'refs/heads/master' }
+
+      it 'creates a pipeline' do
+        expect(pipeline).to be_persisted
+        expect(pipeline.stages.map(&:name)).to contain_exactly(
+          *%w(.pre build .post))
+        expect(pipeline.builds.map(&:name)).to contain_exactly(
+          *%w(validate build notify))
+      end
+    end
+
+    context 'when pipeline does not contain any other build except .pre/.post' do
+      let(:ref) { 'refs/heads/feature' }
+
+      it 'does not create a pipeline' do
+        expect(pipeline).not_to be_persisted
+
+        # we can validate a list of stages, as they are assigned
+        # but not persisted
+        expect(pipeline.stages.map(&:name)).to contain_exactly(
+          *%w(.pre .post))
+      end
+    end
+  end
+end