Use rate limiting Redis for ApplicationRateLimiter

Here we can and do use a feature flag. The maximum interval for any rate
limit in this section is 3 minutes, so we don't mind too much about
requests that sneak in the gaps when the feature flag is being changed -
especially as feature flags are cached for a minute in process memory
anyway, so it would be hard to be much more precise.

config/feature_flags/development/use_rate_limiting_store_for_application_rate_limiter.yml

+---
+name: use_rate_limiting_store_for_application_rate_limiter
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71196
+rollout_issue_url: https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1249
+milestone: '14.4'
+type: development
+group: group::scalability
+default_enabled: false

ee/spec/services/deployments/auto_rollback_service_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Deployments::AutoRollbackService, :clean_gitlab_redis_cache do
+RSpec.describe Deployments::AutoRollbackService, :clean_gitlab_redis_rate_limiting do
   let_it_be(:maintainer) { create(:user) }
   let_it_be(:project, refind: true) { create(:project, :repository) }
   let_it_be(:environment, refind: true) { create(:environment, project: project) }

lib/gitlab/application_rate_limiter.rb

@@ -73,7 +73,7 @@ module Gitlab
         value = 0
         interval_value = interval || interval(key)
 
-        Gitlab::Redis::Cache.with do |redis|
+        cache_store.with do |redis|
           cache_key = action_key(key, scope)
           value     = redis.incr(cache_key)
           redis.expire(cache_key, interval_value) if value == 1
@@ -109,6 +109,14 @@ module Gitlab
 
       private
 
+      def cache_store
+        if ::Feature.enabled?(:use_rate_limiting_store_for_application_rate_limiter, default_enabled: :yaml)
+          ::Gitlab::Redis::RateLimiting
+        else
+          ::Gitlab::Redis::Cache
+        end
+      end
+
       def threshold(key)
         value = rate_limit_value_by_key(key, :threshold)
 

spec/controllers/projects/pipeline_schedules_controller_spec.rb

@@ -397,7 +397,7 @@ RSpec.describe Projects::PipelineSchedulesController do
     end
   end
 
-  describe 'POST #play', :clean_gitlab_redis_cache do
+  describe 'POST #play', :clean_gitlab_redis_rate_limiting do
     let(:ref) { 'master' }
 
     before do

spec/controllers/projects/raw_controller_spec.rb

@@ -84,7 +84,7 @@ RSpec.describe Projects::RawController do
       include_examples 'single Gitaly request'
     end
 
-    context 'when the endpoint receives requests above the limit', :clean_gitlab_redis_cache do
+    context 'when the endpoint receives requests above the limit', :clean_gitlab_redis_rate_limiting do
       let(:file_path) { 'master/README.md' }
 
       before do

spec/controllers/projects_controller_spec.rb

@@ -1384,12 +1384,12 @@ RSpec.describe ProjectsController do
         end
       end
 
-      context 'when the endpoint receives requests above the limit', :clean_gitlab_redis_cache do
+      context 'when the endpoint receives requests above the limit', :clean_gitlab_redis_rate_limiting do
         include_examples 'rate limits project export endpoint'
       end
     end
 
-    describe '#download_export', :clean_gitlab_redis_cache do
+    describe '#download_export', :clean_gitlab_redis_rate_limiting do
       let(:action) { :download_export }
 
       context 'object storage enabled' do
@@ -1424,7 +1424,7 @@ RSpec.describe ProjectsController do
           end
         end
 
-        context 'when the endpoint receives requests above the limit', :clean_gitlab_redis_cache do
+        context 'when the endpoint receives requests above the limit', :clean_gitlab_redis_rate_limiting do
           before do
             allow(Gitlab::ApplicationRateLimiter)
               .to receive(:increment)
@@ -1496,7 +1496,7 @@ RSpec.describe ProjectsController do
         end
       end
 
-      context 'when the endpoint receives requests above the limit', :clean_gitlab_redis_cache do
+      context 'when the endpoint receives requests above the limit', :clean_gitlab_redis_rate_limiting do
         include_examples 'rate limits project export endpoint'
       end
     end

spec/lib/gitlab/application_rate_limiter_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Gitlab::ApplicationRateLimiter, :clean_gitlab_redis_cache do
+RSpec.describe Gitlab::ApplicationRateLimiter do
   let(:redis) { double('redis') }
   let(:user) { create(:user) }
   let(:project) { create(:project) }
@@ -20,7 +20,6 @@ RSpec.describe Gitlab::ApplicationRateLimiter, :clean_gitlab_redis_cache do
   subject { described_class }
 
   before do
-    allow(Gitlab::Redis::Cache).to receive(:with).and_yield(redis)
     allow(described_class).to receive(:rate_limits).and_return(rate_limits)
   end
 
@@ -49,6 +48,8 @@ RSpec.describe Gitlab::ApplicationRateLimiter, :clean_gitlab_redis_cache do
     end
   end
 
+  # Clean up in https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1249
+  shared_examples 'rate limiting' do
     context 'when the key is an array of only ActiveRecord models' do
       let(:scope) { [user, project] }
 
@@ -118,4 +119,23 @@ RSpec.describe Gitlab::ApplicationRateLimiter, :clean_gitlab_redis_cache do
         end
       end
     end
+  end
+
+  context 'when use_rate_limiting_store_for_application_rate_limiter is enabled' do
+    before do
+      stub_feature_flags(use_rate_limiting_store_for_application_rate_limiter: true)
+      allow(Gitlab::Redis::RateLimiting).to receive(:with).and_yield(redis)
+    end
+
+    it_behaves_like 'rate limiting'
+  end
+
+  context 'when use_rate_limiting_store_for_application_rate_limiter is disabled' do
+    before do
+      stub_feature_flags(use_rate_limiting_store_for_application_rate_limiter: false)
+      allow(Gitlab::Redis::Cache).to receive(:with).and_yield(redis)
+    end
+
+    it_behaves_like 'rate limiting'
+  end
 end

spec/lib/gitlab/rate_limit_helpers_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Gitlab::RateLimitHelpers, :clean_gitlab_redis_cache do
+RSpec.describe Gitlab::RateLimitHelpers, :clean_gitlab_redis_rate_limiting do
   let(:limiter_class) do
     Class.new do
       include ::Gitlab::RateLimitHelpers

spec/services/web_hook_service_spec.rb

@@ -392,7 +392,7 @@ RSpec.describe WebHookService do
         end
       end
 
-      context 'when the hook is throttled (via Redis)', :clean_gitlab_redis_cache do
+      context 'when the hook is throttled (via Redis)', :clean_gitlab_redis_rate_limiting do
         before do
           # Set a high interval to avoid intermittent failures in CI
           allow(Gitlab::ApplicationRateLimiter).to receive(:rate_limits).and_return(