Rename CreateCrossProjectPipelineService to be more generic

With the support for parent-child pipelines this service is
not specific to multi-project pipelines but became more generic.
Both multi-project and child pipelines are considered
`downstream` pipelines.

app/services/ci/create_cross_project_pipeline_service.rb → app/services/ci/create_downstream_pipeline_service.rb

 # frozen_string_literal: true
 
 module Ci
-  # TODO: rename this (and worker) to CreateDownstreamPipelineService
-  class CreateCrossProjectPipelineService < ::BaseService
+  # Takes in input a Ci::Bridge job and creates a downstream pipeline
+  # (either multi-project or child pipeline) according to the Ci::Bridge
+  # specifications.
+  class CreateDownstreamPipelineService < ::BaseService
     include Gitlab::Utils::StrongMemoize
 
     DuplicateDownstreamPipelineError = Class.new(StandardError)

app/workers/ci/create_cross_project_pipeline_worker.rb

@@ -9,7 +9,7 @@ module Ci
 
     def perform(bridge_id)
       ::Ci::Bridge.find_by_id(bridge_id).try do |bridge|
-        ::Ci::CreateCrossProjectPipelineService
+        ::Ci::CreateDownstreamPipelineService
           .new(bridge.project, bridge.user)
           .execute(bridge)
       end

spec/services/ci/create_cross_project_pipeline_service_spec.rb → spec/services/ci/create_downstream_pipeline_service_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Ci::CreateCrossProjectPipelineService, '#execute' do
+RSpec.describe Ci::CreateDownstreamPipelineService, '#execute' do
   let_it_be(:user) { create(:user) }
   let(:upstream_project) { create(:project, :repository) }
   let_it_be(:downstream_project) { create(:project, :repository) }
@@ -130,7 +130,7 @@ RSpec.describe Ci::CreateCrossProjectPipelineService, '#execute' do
         expect(Gitlab::ErrorTracking)
           .to receive(:track_exception)
           .with(
-            instance_of(Ci::CreateCrossProjectPipelineService::DuplicateDownstreamPipelineError),
+            instance_of(described_class::DuplicateDownstreamPipelineError),
             bridge_id: bridge.id, project_id: bridge.project.id)
           .and_call_original
         expect(Ci::CreatePipelineService).not_to receive(:new)
@@ -397,7 +397,7 @@ RSpec.describe Ci::CreateCrossProjectPipelineService, '#execute' do
 
     context 'when pipeline variables are defined' do
       before do
-        upstream_pipeline.variables.create(key: 'PIPELINE_VARIABLE', value: 'my-value')
+        upstream_pipeline.variables.create!(key: 'PIPELINE_VARIABLE', value: 'my-value')
       end
 
       it 'does not pass pipeline variables directly downstream' do

spec/workers/ci/create_cross_project_pipeline_worker_spec.rb

@@ -13,7 +13,7 @@ RSpec.describe Ci::CreateCrossProjectPipelineWorker do
   describe '#perform' do
     context 'when bridge exists' do
       it 'calls cross project pipeline creation service' do
-        expect(Ci::CreateCrossProjectPipelineService)
+        expect(Ci::CreateDownstreamPipelineService)
           .to receive(:new)
           .with(project, user)
           .and_return(service)
@@ -26,7 +26,7 @@ RSpec.describe Ci::CreateCrossProjectPipelineWorker do
 
     context 'when bridge does not exist' do
       it 'does nothing' do
-        expect(Ci::CreateCrossProjectPipelineService)
+        expect(Ci::CreateDownstreamPipelineService)
           .not_to receive(:new)
 
         described_class.new.perform(non_existing_record_id)