Merge branch 'fix/improve-ci-node-validatable-to-prevent-memory-leak' into 'master'

Memoize CI config node validator to prevent leaks

## What does this MR do?

This MR memoizes CI configuration node validator, to prevent possible memory leak described in #20698.

## Why was this MR needed?

See #20698

## What are the relevant issue numbers?

Closes #20698

## Does this MR meet the acceptance criteria?

- [ ] ~~[CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5733

lib/gitlab/ci/config/node/validatable.rb

@@ -7,13 +7,11 @@ module Gitlab
 
           class_methods do
             def validator
-              validator = Class.new(Node::Validator)
-
+              @validator ||= Class.new(Node::Validator).tap do |validator|
                 if defined?(@validations)
                   @validations.each { |rules| validator.class_eval(&rules) }
                 end
-
-              validator
+              end
             end
 
             private

spec/lib/gitlab/ci/config/node/validatable_spec.rb

@@ -23,6 +23,10 @@ describe Gitlab::Ci::Config::Node::Validatable do
         .to be Gitlab::Ci::Config::Node::Validator
     end
 
+    it 'returns only one validator to mitigate leaks' do
+      expect { node.validator }.not_to change { node.validator }
+    end
+
     context 'when validating node instance' do
       let(:node_instance) { node.new }