Introduce service for merge request pipeline creation

We don't have a way to create MR pipeline today, this is one
of the headaches for users.
This change is a preliminary work for the capability.

app/services/merge_requests/base_service.rb

@@ -60,31 +60,7 @@ module MergeRequests
     end
 
     def create_pipeline_for(merge_request, user)
-      return unless can_create_pipeline_for?(merge_request)
-
-      create_detached_merge_request_pipeline(merge_request, user)
-    end
-
-    def create_detached_merge_request_pipeline(merge_request, user)
-      if can_use_merge_request_ref?(merge_request)
-        Ci::CreatePipelineService.new(merge_request.source_project, user,
-                                      ref: merge_request.ref_path)
-          .execute(:merge_request_event, merge_request: merge_request)
-      else
-        Ci::CreatePipelineService.new(merge_request.source_project, user,
-                                      ref: merge_request.source_branch)
-          .execute(:merge_request_event, merge_request: merge_request)
-      end
-    end
-
-    def can_create_pipeline_for?(merge_request)
-      ##
-      # UpdateMergeRequestsWorker could be retried by an exception.
-      # pipelines for merge request should not be recreated in such case.
-      return false if merge_request.find_actual_head_pipeline&.triggered_by_merge_request?
-      return false if merge_request.has_no_commits?
-
-      true
+      MergeRequests::CreatePipelineService.new(project, user).execute(merge_request)
     end
 
     def can_use_merge_request_ref?(merge_request)

app/services/merge_requests/create_pipeline_service.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+module MergeRequests
+  class CreatePipelineService < MergeRequests::BaseService
+    def execute(merge_request)
+      return unless can_create_pipeline_for?(merge_request)
+
+      create_detached_merge_request_pipeline(merge_request)
+    end
+
+    def create_detached_merge_request_pipeline(merge_request)
+      if can_use_merge_request_ref?(merge_request)
+        Ci::CreatePipelineService.new(merge_request.source_project, current_user,
+                                      ref: merge_request.ref_path)
+          .execute(:merge_request_event, merge_request: merge_request)
+      else
+        Ci::CreatePipelineService.new(merge_request.source_project, current_user,
+                                      ref: merge_request.source_branch)
+          .execute(:merge_request_event, merge_request: merge_request)
+      end
+    end
+
+    def can_create_pipeline_for?(merge_request)
+      ##
+      # UpdateMergeRequestsWorker could be retried by an exception.
+      # pipelines for merge request should not be recreated in such case.
+      return false if !allow_duplicate && merge_request.find_actual_head_pipeline&.triggered_by_merge_request?
+      return false if merge_request.has_no_commits?
+
+      true
+    end
+
+    def allow_duplicate
+      params[:allow_duplicate]
+    end
+  end
+end

spec/services/merge_requests/create_pipeline_service_spec.rb

@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe MergeRequests::CreatePipelineService do
+  set(:project) { create(:project, :repository) }
+  set(:user) { create(:user) }
+  let(:service) { described_class.new(project, user, params) }
+  let(:params) { {} }
+
+  before do
+    project.add_developer(user)
+  end
+
+  describe '#execute' do
+    subject { service.execute(merge_request) }
+
+    before do
+      stub_ci_pipeline_yaml_file(YAML.dump(config))
+    end
+
+    let(:config) do
+      { rspec: { script: 'echo', only: ['merge_requests'] } }
+    end
+
+    let(:merge_request) do
+      create(:merge_request,
+        source_branch: 'feature',
+        source_project: project,
+        target_branch: 'master',
+        target_project: project)
+    end
+
+    it 'creates a detached merge request pipeline' do
+      expect { subject }.to change { Ci::Pipeline.count }.by(1)
+
+      expect(subject).to be_persisted
+      expect(subject).to be_detached_merge_request_pipeline
+    end
+
+    context 'when service is called multiple times' do
+      it 'creates a pipeline once' do
+        expect do
+          service.execute(merge_request)
+          service.execute(merge_request)
+        end.to change { Ci::Pipeline.count }.by(1)
+      end
+
+      context 'when allow_duplicate option is true' do
+        let(:params) { { allow_duplicate: true } }
+
+        it 'creates pipelines multiple times' do
+          expect do
+            service.execute(merge_request)
+            service.execute(merge_request)
+          end.to change { Ci::Pipeline.count }.by(2)
+        end
+      end
+    end
+
+    context 'when .gitlab-ci.yml does not have only: [merge_requests] keyword' do
+      let(:config) do
+        { rspec: { script: 'echo' } }
+      end
+
+      it 'does not create a pipeline' do
+        expect { subject }.not_to change { Ci::Pipeline.count }
+      end
+    end
+  end
+end