Add a feature flag to try to redirect to latest template

This commit adds a feature flag for measuring impact of
overriding stable templates by the latest templates.

config/feature_flags/development/redirect_to_latest_template_jobs_browser_performance_testing.yml

+---
+name: redirect_to_latest_template_jobs_browser_performance_testing
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63144
+rollout_issue_url: 
+milestone: '14.0'
+type: development
+group: group::pipeline authoring
+default_enabled: false

config/feature_flags/development/redirect_to_latest_template_jobs_deploy.yml

+---
+name: redirect_to_latest_template_jobs_deploy
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63144
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/332660
+milestone: '14.0'
+type: development
+group: group::pipeline authoring
+default_enabled: false

config/feature_flags/development/redirect_to_latest_template_security_api_fuzzing.yml

+---
+name: redirect_to_latest_template_security_api_fuzzing
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63144
+rollout_issue_url: 
+milestone: '14.0'
+type: development
+group: group::pipeline authoring
+default_enabled: false

config/feature_flags/development/redirect_to_latest_template_security_dast.yml

+---
+name: redirect_to_latest_template_security_dast
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63144
+rollout_issue_url: 
+milestone: '14.0'
+type: development
+group: group::pipeline authoring
+default_enabled: false

config/feature_flags/development/redirect_to_latest_template_terraform.yml

+---
+name: redirect_to_latest_template_terraform
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63144
+rollout_issue_url: 
+milestone: '14.0'
+type: development
+group: group::pipeline authoring
+default_enabled: false

config/feature_flags/development/redirect_to_latest_template_verify_browser_performance.yml

+---
+name: redirect_to_latest_template_verify_browser_performance
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63144
+rollout_issue_url: 
+milestone: '14.0'
+type: development
+group: group::pipeline authoring
+default_enabled: false

doc/development/cicd/templates.md

@@ -307,6 +307,22 @@ include:
   - remote: https://gitlab.com/gitlab-org/gitlab/-/raw/v13.0.1-ee/lib/gitlab/ci/templates/Jobs/Deploy.gitlab-ci.yml
 ```
 
+### Use a feature flag to rollout the latest template
+
+At a GitLab major version update, we should renew [stable templates](#stable-version) by the corresponding [latest templates](#latest-version),
+but in some cases, you'd want to gauge the impact at first, so that we can be
+more confident that the change wouldn't require revert/rollback on production.
+
+To do so, you can use the feature flag, named `redirect_to_latest_template_<name>`.
+For example, you can redirect to the latest template in 25% of projects on `gitlab.com`, by the following chatops command:
+
+```shell
+/chatops run feature set redirect_to_latest_template_jobs_deploy 25 --actors
+```
+
+Once you're confident on the latest template, you should renew the stable template by the latest content,
+remove the latest template and remove the corresponding feature flag.
+
 ### Further reading
 
 There is an [open issue](https://gitlab.com/gitlab-org/gitlab/-/issues/17716) about

ee/lib/ee/gitlab/template/gitlab_ci_yml_template.rb

@@ -6,6 +6,10 @@ module EE
       module GitlabCiYmlTemplate
         extend ActiveSupport::Concern
 
+        EE_TEMPLATES_WITH_LATEST_VERSION = {
+          'Verify/Browser-Performance' => true
+        }.freeze
+
         class_methods do
           extend ::Gitlab::Utils::Override
 
@@ -26,6 +30,12 @@ module EE
               'Security' => 'Security'
             }
           end
+
+          override :templates_with_latest_version
+          def templates_with_latest_version
+            @templates_with_latest_version ||=
+              super.merge(EE_TEMPLATES_WITH_LATEST_VERSION)
+          end
         end
       end
     end

ee/spec/lib/ee/gitlab/ci/templates/templates_spec.rb

@@ -11,6 +11,14 @@ RSpec.describe "CI YML Templates" do
     Gitlab::Template::GitlabCiYmlTemplate.all.map(&:full_name)
   end
 
+  before do
+    stub_feature_flags(
+      redirect_to_latest_template_terraform: false,
+      redirect_to_latest_template_security_dast: false,
+      redirect_to_latest_template_security_api_fuzzing: false,
+      redirect_to_latest_template_jobs_browser_performance_testing: false)
+  end
+
   with_them do
     let(:content) do
       if template_name == 'Security/DAST-API.gitlab-ci.yml'

ee/spec/lib/gitlab/ci/templates/Jobs/browser_performance_testing_gitlab_ci_yaml_spec.rb

@@ -34,6 +34,7 @@ RSpec.describe 'Jobs/Browser-Performance-Testing.gitlab-ci.yml' do
 
     before do
       stub_ci_pipeline_yaml_file(template)
+      stub_feature_flags(redirect_to_latest_template_jobs_browser_performance_testing: false)
 
       allow_next_instance_of(Ci::BuildScheduleWorker) do |worker|
         allow(worker).to receive(:perform).and_return(true)

ee/spec/lib/gitlab/ci/templates/Verify/browser_performance_testing_gitlab_ci_yaml_spec.rb

@@ -30,6 +30,7 @@ RSpec.describe 'Verify/Browser-Performance.gitlab-ci.yml' do
 
     before do
       stub_ci_pipeline_yaml_file(template)
+      stub_feature_flags(redirect_to_latest_template_verify_browser_performance: false)
       allow_next_instance_of(Ci::BuildScheduleWorker) do |worker|
         allow(worker).to receive(:perform).and_return(true)
       end

lib/gitlab/ci/config/external/file/template.rb

@@ -6,7 +6,7 @@ module Gitlab
       module External
         module File
           class Template < Base
-            attr_reader :location, :project
+            attr_reader :location
 
             SUFFIX = '.gitlab-ci.yml'
 
@@ -41,7 +41,7 @@ module Gitlab
             end
 
             def fetch_template_content
-              Gitlab::Template::GitlabCiYmlTemplate.find(template_name, project)&.content
+              Gitlab::Template::GitlabCiYmlTemplate.find(template_name, context.project)&.content
             end
           end
         end

lib/gitlab/template/gitlab_ci_yml_template.rb

@@ -5,6 +5,14 @@ module Gitlab
     class GitlabCiYmlTemplate < BaseTemplate
       BASE_EXCLUDED_PATTERNS = [%r{\.latest\.}].freeze
 
+      TEMPLATES_WITH_LATEST_VERSION = {
+        'Jobs/Deploy' => true,
+        'Jobs/Browser-Performance-Testing' => true,
+        'Security/API-Fuzzing' => true,
+        'Security/DAST' => true,
+        'Terraform' => true
+      }.freeze
+
       def description
         "# This file is a template, and might need editing before it works on your project."
       end
@@ -54,6 +62,30 @@ module Gitlab
             excluded_patterns: self.excluded_patterns
           )
         end
+
+        def find(key, project = nil)
+          if try_redirect_to_latest?(key, project)
+            key += '.latest'
+          end
+
+          super(key, project)
+        end
+
+        private
+
+        # To gauge the impact of the latest template,
+        # you can redirect the stable template to the latest template by enabling the feature flag.
+        # See https://docs.gitlab.com/ee/development/cicd/templates.html#versioning for more information.
+        def try_redirect_to_latest?(key, project)
+          return false unless templates_with_latest_version[key]
+
+          flag_name = "redirect_to_latest_template_#{key.underscore.tr('/', '_')}"
+          ::Feature.enabled?(flag_name, project, default_enabled: :yaml)
+        end
+
+        def templates_with_latest_version
+          TEMPLATES_WITH_LATEST_VERSION
+        end
       end
     end
   end

spec/lib/gitlab/ci/templates/templates_spec.rb

@@ -20,6 +20,13 @@ RSpec.describe 'CI YML Templates' do
       all_templates - excluded_templates
     end
 
+    before do
+      stub_feature_flags(
+        redirect_to_latest_template_terraform: false,
+        redirect_to_latest_template_security_api_fuzzing: false,
+        redirect_to_latest_template_security_dast: false)
+    end
+
     with_them do
       let(:content) do
         if template_name == 'Security/DAST-API.gitlab-ci.yml'

spec/lib/gitlab/template/gitlab_ci_yml_template_spec.rb

@@ -21,6 +21,54 @@ RSpec.describe Gitlab::Template::GitlabCiYmlTemplate do
     end
   end
 
+  describe '.find' do
+    let_it_be(:project) { create(:project) }
+
+    described_class::TEMPLATES_WITH_LATEST_VERSION.keys.each do |key|
+      it "finds the latest template for #{key}" do
+        result = described_class.find(key, project)
+        expect(result.full_name).to eq("#{key}.latest.gitlab-ci.yml")
+        expect(result.content).to be_present
+      end
+
+      context 'when `redirect_to_latest_template` feature flag is disabled' do
+        before do
+          stub_feature_flags("redirect_to_latest_template_#{key.underscore.tr('/', '_')}".to_sym => false)
+        end
+
+        it "finds the stable template for #{key}" do
+          result = described_class.find(key, project)
+          expect(result.full_name).to eq("#{key}.gitlab-ci.yml")
+          expect(result.content).to be_present
+        end
+      end
+
+      context 'when `redirect_to_latest_template` feature flag is enabled on the project' do
+        before do
+          stub_feature_flags("redirect_to_latest_template_#{key.underscore.tr('/', '_')}".to_sym => project)
+        end
+
+        it "finds the latest template for #{key}" do
+          result = described_class.find(key, project)
+          expect(result.full_name).to eq("#{key}.latest.gitlab-ci.yml")
+          expect(result.content).to be_present
+        end
+      end
+
+      context 'when `redirect_to_latest_template` feature flag is enabled on the other project' do
+        before do
+          stub_feature_flags("redirect_to_latest_template_#{key.underscore.tr('/', '_')}".to_sym => create(:project))
+        end
+
+        it "finds the stable template for #{key}" do
+          result = described_class.find(key, project)
+          expect(result.full_name).to eq("#{key}.gitlab-ci.yml")
+          expect(result.content).to be_present
+        end
+      end
+    end
+  end
+
   describe '#content' do
     it 'loads the full file' do
       gitignore = subject.new(Rails.root.join('lib/gitlab/ci/templates/Ruby.gitlab-ci.yml'))