Use Gitaly to fetch overview branches

The problem is that we're fetching all branches and then take 5
the most active and then iterate over the whole array until we get
5 branches with committer-date older than 3 months ago.
If we display the stalest branches on the overview, we'll be able
to fetch 5 the most recent and then fetch 5 the oldest.

We perform 2 requests, but they are smaller and don't depend
on the number of branches.

app/controllers/projects/branches_controller.rb

@@ -175,19 +175,32 @@ class Projects::BranchesController < Projects::ApplicationController
   end
 
   def fetch_branches_by_mode
-    if @mode == 'overview'
-      # overview mode
-      @active_branches, @stale_branches = BranchesFinder.new(@repository, sort: sort_value_recently_updated).execute.partition(&:active?)
-      # Here we get one more branch to indicate if there are more data we're not showing
-      @active_branches = @active_branches.first(@overview_max_branches + 1)
-      @stale_branches = @stale_branches.first(@overview_max_branches + 1)
-      @branches = @active_branches + @stale_branches
+    return fetch_branches_for_overview if @mode == 'overview'
+
+    # active/stale/all view mode
+    @branches = BranchesFinder.new(@repository, params.merge(sort: @sort)).execute
+    @branches = @branches.select { |b| b.state.to_s == @mode } if %w[active stale].include?(@mode)
+    @branches = Kaminari.paginate_array(@branches).page(params[:page])
+  end
+
+  def fetch_branches_for_overview
+    # Here we get one more branch to indicate if there are more data we're not showing
+    limit = @overview_max_branches + 1
+
+    if Feature.enabled?(:branch_list_keyset_pagination, project, default_enabled: true)
+      @active_branches =
+        BranchesFinder.new(@repository, { per_page: limit, sort: sort_value_recently_updated })
+          .execute(gitaly_pagination: true).select(&:active?)
+      @stale_branches =
+        BranchesFinder.new(@repository, { per_page: limit, sort: sort_value_oldest_updated })
+          .execute(gitaly_pagination: true).select(&:stale?)
     else
-      # active/stale/all view mode
-      @branches = BranchesFinder.new(@repository, params.merge(sort: @sort)).execute
-      @branches = @branches.select { |b| b.state.to_s == @mode } if %w[active stale].include?(@mode)
-      @branches = Kaminari.paginate_array(@branches).page(params[:page])
+      @active_branches, @stale_branches = BranchesFinder.new(@repository, sort: sort_value_recently_updated).execute.partition(&:active?)
+      @active_branches = @active_branches.first(limit)
+      @stale_branches = @stale_branches.first(limit)
     end
+
+    @branches = @active_branches + @stale_branches
   end
 
   def confidential_issue_project

changelogs/unreleased/id-optimize-branches-overview.yml

+---
+title: Improve UI and performance of branches overview page
+merge_request: 50096
+author:
+type: performance

spec/controllers/projects/branches_controller_spec.rb

@@ -639,6 +639,34 @@ RSpec.describe Projects::BranchesController do
         expect(response).to redirect_to project_branches_filtered_path(project, state: 'all')
       end
     end
+
+    context 'fetching branches for overview' do
+      before do
+        get :index, format: :html, params: {
+          namespace_id: project.namespace, project_id: project, state: 'overview'
+        }
+      end
+
+      it 'sets active and stale branches' do
+        expect(assigns[:active_branches]).to eq([])
+        expect(assigns[:stale_branches].map(&:name)).to eq(
+          ["feature", "improve/awesome", "merge-test", "markdown", "feature_conflict", "'test'"]
+        )
+      end
+
+      context 'branch_list_keyset_pagination is disabled' do
+        before do
+          stub_feature_flags(branch_list_keyset_pagination: false)
+        end
+
+        it 'sets active and stale branches' do
+          expect(assigns[:active_branches]).to eq([])
+          expect(assigns[:stale_branches].map(&:name)).to eq(
+            ["feature", "improve/awesome", "merge-test", "markdown", "feature_conflict", "'test'"]
+          )
+        end
+      end
+    end
   end
 
   describe 'GET diverging_commit_counts' do