Graceful degradation for refs endpoint

**Problem**

We have an unhandled exception when Gitaly is not available.

**Solution**

Return 503 error with an error message.

Changelog: added

app/controllers/projects_controller.rb

@@ -305,12 +305,7 @@ class ProjectsController < Projects::ApplicationController
     end
 
     if find_tags && @repository.tag_count.nonzero?
-      tags = begin
-        TagsFinder.new(@repository, refs_params).execute
-      rescue Gitlab::Git::CommandError
-        []
-      end
-
+      tags = TagsFinder.new(@repository, refs_params).execute
       options['Tags'] = tags.take(100).map(&:name)
     end
 
@@ -321,6 +316,8 @@ class ProjectsController < Projects::ApplicationController
     end
 
     render json: options.to_json
+  rescue Gitlab::Git::CommandError
+    render json: { error: _('Unable to load refs') }, status: :service_unavailable
   end
   # rubocop: enable CodeReuse/ActiveRecord
 

locale/gitlab.pot

@@ -39636,6 +39636,9 @@ msgstr ""
 msgid "Unable to load file contents. Try again later."
 msgstr ""
 
+msgid "Unable to load refs"
+msgstr ""
+
 msgid "Unable to load the diff"
 msgstr ""
 

spec/controllers/projects_controller_spec.rb

@@ -1159,16 +1159,15 @@ RSpec.describe ProjectsController do
     context 'when gitaly is unavailable' do
       before do
         expect_next_instance_of(TagsFinder) do |finder|
-          allow(finder).to receive(:execute).and_raise(Gitlab::Git::CommandError)
+          allow(finder).to receive(:execute).and_raise(Gitlab::Git::CommandError, 'something went wrong')
         end
       end
 
-      it 'gets an empty list of tags' do
+      it 'responds with 503 error' do
         get :refs, params: { namespace_id: project.namespace, id: project, ref: "123456" }
 
-        expect(json_response["Branches"]).to include("master")
-        expect(json_response["Tags"]).to eq([])
-        expect(json_response["Commits"]).to include("123456")
+        expect(response).to have_gitlab_http_status(:service_unavailable)
+        expect(json_response['error']).to eq 'Unable to load refs'
       end
     end