Implement nested expanding environment name and url for deployments

We support variables for YAML attributes of environment:name and
environment:url. However, we expand those variables only 1-level.
In this commit, we start supporting infinite level expansion.

These changes are behind a FF ci_expand_environment_name_and_url

app/models/ci/build.rb

@@ -504,7 +504,11 @@ module Ci
         if metadata&.expanded_environment_name.present?
           metadata.expanded_environment_name
         else
-          ExpandVariables.expand(environment, -> { simple_variables })
+          if ::Feature.enabled?(:ci_expand_environment_name_and_url, project, default_enabled: :yaml)
+            ExpandVariables.expand(environment, -> { simple_variables.sort_and_expand_all })
+          else
+            ExpandVariables.expand(environment, -> { simple_variables })
+          end
         end
       end
     end

app/services/deployments/update_environment_service.rb

@@ -56,7 +56,13 @@ module Deployments
     end
 
     def expanded_environment_url
-      ExpandVariables.expand(environment_url, -> { variables }) if environment_url
+      return unless environment_url
+
+      if ::Feature.enabled?(:ci_expand_environment_name_and_url, deployment.project, default_enabled: :yaml)
+        ExpandVariables.expand(environment_url, -> { variables.sort_and_expand_all })
+      else
+        ExpandVariables.expand(environment_url, -> { variables })
+      end
     end
 
     def environment_url

config/feature_flags/development/ci_expand_environment_name_and_url.yml

+---
+name: ci_expand_environment_name_and_url
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/84969
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/358831
+milestone: '14.10'
+type: development
+group: group::pipeline authoring
+default_enabled: false

spec/models/ci/build_spec.rb

@@ -1585,6 +1585,31 @@ RSpec.describe Ci::Build do
 
         it { is_expected.to eq('review/x') }
       end
+
+      context 'when environment name uses a nested variable' do
+        let(:yaml_variables) do
+          [
+            { key: 'ENVIRONMENT_NAME', value: '${CI_COMMIT_REF_NAME}' }
+          ]
+        end
+
+        let(:build) do
+          create(:ci_build,
+                 ref: 'master',
+                 yaml_variables: yaml_variables,
+                 environment: 'review/$ENVIRONMENT_NAME')
+        end
+
+        it { is_expected.to eq('review/master') }
+
+        context 'when the FF ci_expand_environment_name_and_url is disabled' do
+          before do
+            stub_feature_flags(ci_expand_environment_name_and_url: false)
+          end
+
+          it { is_expected.to eq('review/${CI_COMMIT_REF_NAME}') }
+        end
+      end
     end
 
     describe '#expanded_kubernetes_namespace' do

spec/services/deployments/update_environment_service_spec.rb

@@ -286,6 +286,37 @@ RSpec.describe Deployments::UpdateEnvironmentService do
       end
     end
 
+    context 'when environment url uses a nested variable' do
+      let(:yaml_variables) do
+        [
+          { key: 'MAIN_DOMAIN', value: '${STACK_NAME}.example.com' },
+          { key: 'STACK_NAME', value: 'appname-${ENVIRONMENT_NAME}' },
+          { key: 'ENVIRONMENT_NAME', value: '${CI_COMMIT_REF_SLUG}' }
+        ]
+      end
+
+      let(:job) do
+        create(:ci_build,
+               :with_deployment,
+               pipeline: pipeline,
+               ref: 'master',
+               environment: 'production',
+               project: project,
+               yaml_variables: yaml_variables,
+               options: { environment: { name: 'production', url: 'http://$MAIN_DOMAIN' } })
+      end
+
+      it { is_expected.to eq('http://appname-master.example.com') }
+
+      context 'when the FF ci_expand_environment_name_and_url is disabled' do
+        before do
+          stub_feature_flags(ci_expand_environment_name_and_url: false)
+        end
+
+        it { is_expected.to eq('http://${STACK_NAME}.example.com') }
+      end
+    end
+
     context 'when yaml environment does not have url' do
       let(:job) { create(:ci_build, :with_deployment, pipeline: pipeline, environment: 'staging', project: project) }