Graceful degradation for Branches controller

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/340817

**Problem**

We return a 500 error page when we Gitaly is not available. It is not
a great user experience.

**Solution**

Handle Gitaly exception and successfully render the page with an error
message block.

Changelog: added

app/controllers/projects/branches_controller.rb

@@ -33,6 +33,11 @@ class Projects::BranchesController < Projects::ApplicationController
         Gitlab::GitalyClient.allow_n_plus_1_calls do
           render
         end
+      rescue Gitlab::Git::CommandError => e
+        Gitlab::ErrorTracking.track_exception(e)
+
+        @gitaly_unavailable = true
+        render
       end
       format.json do
         branches = BranchesFinder.new(@repository, params).execute

app/views/projects/branches/index.html.haml

@@ -31,7 +31,10 @@
     %h5
       = s_('Branches|Protected branches can be managed in %{project_settings_link}.').html_safe % { project_settings_link: project_settings_link }
 
-- if @mode == 'overview' && (@active_branches.any? || @stale_branches.any?)
+- if @gitaly_unavailable
+  = render 'shared/errors/gitaly_unavailable', reason: s_('Branches|Unable to load branches')
+
+- elsif @mode == 'overview' && (@active_branches.any? || @stale_branches.any?)
   = render "projects/branches/panel", branches: @active_branches, state: 'active', panel_title: s_('Branches|Active branches'), show_more_text: s_('Branches|Show more active branches'), project: @project, overview_max_branches: @overview_max_branches
   = render "projects/branches/panel", branches: @stale_branches, state: 'stale', panel_title: s_('Branches|Stale branches'), show_more_text: s_('Branches|Show more stale branches'), project: @project, overview_max_branches: @overview_max_branches
 

locale/gitlab.pot

@@ -5758,6 +5758,9 @@ msgstr ""
 msgid "Branches|To discard the local changes and overwrite the branch with the upstream version, delete it here and choose 'Update Now' above."
 msgstr ""
 
+msgid "Branches|Unable to load branches"
+msgstr ""
+
 msgid "Branches|Yes, delete branch"
 msgstr ""
 

spec/controllers/projects/branches_controller_spec.rb

@@ -656,6 +656,26 @@ RSpec.describe Projects::BranchesController do
         )
       end
     end
+
+    context 'when gitaly is not available' do
+      before do
+        allow_next_instance_of(Gitlab::GitalyClient::RefService) do |ref_service|
+          allow(ref_service).to receive(:local_branches).and_raise(GRPC::DeadlineExceeded)
+        end
+
+        get :index, format: :html, params: {
+           namespace_id: project.namespace, project_id: project
+        }
+      end
+
+      it 'returns with a status 200' do
+        expect(response).to have_gitlab_http_status(:ok)
+      end
+
+      it 'sets gitaly_unavailable variable' do
+        expect(assigns[:gitaly_unavailable]).to be_truthy
+      end
+    end
   end
 
   describe 'GET diverging_commit_counts' do

spec/views/projects/branches/index.html.haml_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'projects/branches/index.html.haml' do
+  let_it_be(:project) { create(:project, :repository) }
+  let_it_be(:repository) { project.repository }
+
+  let(:branches) { repository.branches }
+  let(:active_branch) { branches.find { |b| b.name == 'master' } }
+  let(:stale_branch) { branches.find { |b| b.name == 'feature' } }
+
+  before do
+    assign(:project, project)
+    assign(:repository, repository)
+    assign(:mode, 'overview')
+    assign(:active_branches, [active_branch])
+    assign(:stale_branches, [stale_branch])
+    assign(:overview_max_branches, 5)
+    assign(:branch_pipeline_statuses, {})
+    assign(:refs_pipelines, {})
+  end
+
+  it 'renders list of active and stale branches' do
+    content = render
+
+    expect(content).to include(active_branch.name)
+    expect(content).to include(stale_branch.name)
+  end
+
+  context 'when Gitaly is unavailable' do
+    it 'renders an error' do
+      assign(:gitaly_unavailable, true)
+
+      content = render
+
+      expect(content).to include('Unable to load branches')
+      expect(content).to include(
+        'The git server, Gitaly, is not available at this time. Please contact your administrator.'
+      )
+    end
+  end
+end