Fix for nested cache calls

lib/api/helpers/caching.rb

@@ -81,7 +81,13 @@ module API
       # @return [Gitlab::Json::PrecompiledJson]
       def cache_action(key, **cache_opts)
         json = cache.fetch(key, **apply_default_cache_options(cache_opts)) do
-          Gitlab::Json.dump(yield.as_json)
+          response = yield
+
+          if response.is_a?(Gitlab::Json::PrecompiledJson)
+            response.to_s
+          else
+            Gitlab::Json.dump(response.as_json)
+          end
         end
 
         body Gitlab::Json::PrecompiledJson.new(json)

spec/lib/api/helpers/caching_spec.rb

@@ -177,6 +177,16 @@ RSpec.describe API::Helpers::Caching, :use_clean_rails_redis_caching do
 
       5.times { perform }
     end
+
+    it "handles nested cache calls" do
+      nested_call = instance.cache_action(cache_key, **kwargs) do
+        instance.cache_action([:nested], **kwargs) do
+          expensive_thing.do_very_expensive_action
+        end
+      end
+
+      expect(nested_call.to_s).to eq(subject.to_s)
+    end
   end
 
   describe "#cache_action_if" do

spec/requests/api/branches_spec.rb

@@ -20,7 +20,7 @@ RSpec.describe API::Branches do
     stub_feature_flags(branch_list_keyset_pagination: false)
   end
 
-  describe "GET /projects/:id/repository/branches" do
+  describe "GET /projects/:id/repository/branches", :use_clean_rails_redis_caching do
     let(:route) { "/projects/#{project_id}/repository/branches" }
 
     shared_examples_for 'repository branches' do