Add feature flag disabling release notes in tags API

We deprecated it a long time ago, but I'm afraid of just removing it
without an option to quickly roll it back on .com

So adding a feature flag, and will remove all these very soon

config/feature_flags/development/remove_release_notes_from_tags_api.yml

+---
+name: remove_release_notes_from_tags_api
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63392
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/290311
+milestone: '14.0'
+type: development
+group: group::release
+default_enabled: false

lib/api/tags.rb

@@ -61,6 +61,8 @@ module API
         optional :release_description, type: String, desc: 'Specifying release notes stored in the GitLab database (deprecated in GitLab 11.7)'
       end
       post ':id/repository/tags', :release_orchestration do
+        deprecate_release_notes unless params[:release_description].blank?
+
         authorize_admin_tag
 
         result = ::Tags::CreateService.new(user_project, current_user)
@@ -119,6 +121,7 @@ module API
         requires :description, type: String, desc: 'Release notes with markdown support'
       end
       post ':id/repository/tags/:tag_name/release', requirements: TAG_ENDPOINT_REQUIREMENTS, feature_category: :release_orchestration do
+        deprecate_release_notes
         authorize_create_release!
 
         ##
@@ -151,6 +154,7 @@ module API
         requires :description, type: String, desc: 'Release notes with markdown support'
       end
       put ':id/repository/tags/:tag_name/release', requirements: TAG_ENDPOINT_REQUIREMENTS, feature_category: :release_orchestration do
+        deprecate_release_notes
         authorize_update_release!
 
         result = ::Releases::UpdateService
@@ -177,6 +181,12 @@ module API
       def release
         @release ||= user_project.releases.find_by_tag(params[:tag])
       end
+
+      def deprecate_release_notes
+        return unless Feature.enabled?(:remove_release_notes_from_tags_api, user_project, default_enabled: :yaml)
+
+        render_api_error!("Release notes modification via tags API is deprecated, see https://gitlab.com/gitlab-org/gitlab/-/issues/290311", 400)
+      end
     end
   end
 end

spec/requests/api/tags_spec.rb

@@ -358,8 +358,17 @@ RSpec.describe API::Tags do
         expect(json_response['message']).to eq('Target foo is invalid')
       end
 
-      context 'lightweight tags with release notes' do
-        it 'creates a new tag' do
+      context 'when release_description is passed' do
+        it 'returns error' do
+          post api(route, current_user), params: { tag_name: tag_name, ref: 'master', release_description: 'Wow' }
+
+          expect(response).to have_gitlab_http_status(:bad_request)
+          expect(json_response["message"]).to eq("Release notes modification via tags API is deprecated, see https://gitlab.com/gitlab-org/gitlab/-/issues/290311")
+        end
+
+        it 'creates a new tag with release if feature is enabled' do
+          stub_feature_flags(remove_release_notes_from_tags_api: false)
+
           post api(route, current_user), params: { tag_name: tag_name, ref: 'master', release_description: 'Wow' }
 
           expect(response).to have_gitlab_http_status(:created)
@@ -445,6 +454,10 @@ RSpec.describe API::Tags do
     let(:route) { "/projects/#{project_id}/repository/tags/#{tag_name}/release" }
     let(:description) { 'Awesome release!' }
 
+    before do
+      stub_feature_flags(remove_release_notes_from_tags_api: false)
+    end
+
     shared_examples_for 'repository new release' do
       it 'creates description for existing git tag' do
         post api(route, user), params: { description: description }
@@ -455,6 +468,15 @@ RSpec.describe API::Tags do
         expect(json_response['description']).to eq(description)
       end
 
+      it 'returns error if feature is removed' do
+        stub_feature_flags(remove_release_notes_from_tags_api: true)
+
+        post api(route, user), params: { description: description }
+
+        expect(response).to have_gitlab_http_status(:bad_request)
+        expect(json_response["message"]).to eq("Release notes modification via tags API is deprecated, see https://gitlab.com/gitlab-org/gitlab/-/issues/290311")
+      end
+
       context 'when tag does not exist' do
         let(:tag_name) { 'unknown' }
 
@@ -502,6 +524,10 @@ RSpec.describe API::Tags do
     let(:description) { 'Awesome release!' }
     let(:new_description) { 'The best release!' }
 
+    before do
+      stub_feature_flags(remove_release_notes_from_tags_api: false)
+    end
+
     shared_examples_for 'repository update release' do
       context 'on tag with existing release' do
         let!(:release) do
@@ -519,6 +545,15 @@ RSpec.describe API::Tags do
           expect(json_response['tag_name']).to eq(tag_name)
           expect(json_response['description']).to eq(new_description)
         end
+
+        it 'returns error if feature is removed' do
+          stub_feature_flags(remove_release_notes_from_tags_api: true)
+
+          put api(route, current_user), params: { description: new_description }
+
+          expect(response).to have_gitlab_http_status(:bad_request)
+          expect(json_response["message"]).to eq("Release notes modification via tags API is deprecated, see https://gitlab.com/gitlab-org/gitlab/-/issues/290311")
+        end
       end
 
       context 'when tag does not exist' do