Rate-limited action caching for branches API

config/feature_flags/development/api_caching_rate_limit_branches.yml

+---
+name: api_caching_rate_limit_branches
+introduced_by_url: 
+rollout_issue_url: 
+milestone: '13.12'
+type: development
+group: group::source code
+default_enabled: false

lib/api/branches.rb

@@ -38,6 +38,9 @@ module API
         optional :page_token, type: String, desc: 'Name of branch to start the paginaition from'
       end
       get ':id/repository/branches' do
+        ff_enabled = Feature.enabled?(:api_caching_rate_limit_branches, user_project, default_enabled: :yaml)
+
+        cache_action_if(ff_enabled, [user_project, :branches, current_user&.cache_key, params], expires_in: 30.seconds) do
           user_project.preload_protected_branches
 
           repository = user_project.repository
@@ -67,6 +70,7 @@ module API
             )
           end
         end
+      end
 
       resource ':id/repository/branches/:branch', requirements: BRANCH_ENDPOINT_REQUIREMENTS do
         desc 'Get a single branch' do

lib/api/helpers/caching.rb

@@ -63,6 +63,32 @@ module API
         body Gitlab::Json::PrecompiledJson.new(json)
       end
 
+      # Action caching implementation
+      #
+      # This allows you to wrap an entire API endpoint call in a cache, useful
+      # for short TTL caches to effectively rate-limit an endpoint.
+      #
+      # @param key [Object] any object that can be converted into a cache key
+      # @param expires_in [ActiveSupport::Duration, Integer] an expiry time for the cache entry
+      # @return [Gitlab::Json::PrecompiledJson]
+      def cache_action(key, **cache_opts)
+        json = cache.fetch(key, **cache_opts) do
+          Gitlab::Json.dump(yield.as_json)
+        end
+
+        body Gitlab::Json::PrecompiledJson.new(json)
+      end
+
+      def cache_action_if(conditional, *opts)
+        if conditional
+          cache_action(*opts) do
+            yield
+          end
+        else
+          yield
+        end
+      end
+
       private
 
       # Optionally uses a `Proc` to add context to a cache key