Add caching to branches API

Adds caching around entity generation for branches
on the API list endpoint.

config/feature_flags/development/api_caching_branches.yml

+---
+name: api_caching_branches
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61157
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330371
+milestone: '13.12'
+type: development
+group: group::source code
+default_enabled: false

lib/api/branches.rb

@@ -47,13 +47,25 @@ module API
 
         merged_branch_names = repository.merged_branch_names(branches.map(&:name))
 
-        present(
-          branches,
-          with: Entities::Branch,
-          current_user: current_user,
-          project: user_project,
-          merged_branch_names: merged_branch_names
-        )
+        if Feature.enabled?(:api_caching_branches, user_project, type: :development, default_enabled: :yaml)
+          present_cached(
+            branches,
+            with: Entities::Branch,
+            current_user: current_user,
+            project: user_project,
+            merged_branch_names: merged_branch_names,
+            expires_in: 10.minutes,
+            cache_context: -> (branch) { [current_user&.cache_key, merged_branch_names.include?(branch.name)] }
+          )
+        else
+          present(
+            branches,
+            with: Entities::Branch,
+            current_user: current_user,
+            project: user_project,
+            merged_branch_names: merged_branch_names
+          )
+        end
       end
 
       resource ':id/repository/branches/:branch', requirements: BRANCH_ENDPOINT_REQUIREMENTS do

lib/gitlab/git/branch.rb

@@ -28,6 +28,10 @@ module Gitlab
       def state
         active? ? :active : :stale
       end
+
+      def cache_key
+        "branch:" + Digest::SHA1.hexdigest([name, target, dereferenced_target&.sha].join(':'))
+      end
     end
   end
 end

spec/lib/gitlab/git/branch_spec.rb

@@ -44,6 +44,16 @@ RSpec.describe Gitlab::Git::Branch, :seed_helper do
     end
   end
 
+  describe "#cache_key" do
+    subject { repository.branches.first }
+
+    it "returns a cache key that changes based on changeable values" do
+      digest = Digest::SHA1.hexdigest([subject.name, subject.target, subject.dereferenced_target.sha].join(":"))
+
+      expect(subject.cache_key).to eq("branch:#{digest}")
+    end
+  end
+
   describe '#size' do
     subject { super().size }
 

spec/requests/api/branches_spec.rb

@@ -53,7 +53,7 @@ RSpec.describe API::Branches do
           end
 
           it 'determines only a limited number of merged branch names' do
-            expect(API::Entities::Branch).to receive(:represent).with(anything, has_up_to_merged_branch_names_count(2)).and_call_original
+            expect(API::Entities::Branch).to receive(:represent).with(anything, has_up_to_merged_branch_names_count(2)).at_least(:once).and_call_original
 
             get api(route, current_user), params: base_params.merge(per_page: 2)
 
@@ -111,7 +111,7 @@ RSpec.describe API::Branches do
             end
 
             it 'determines only a limited number of merged branch names' do
-              expect(API::Entities::Branch).to receive(:represent).with(anything, has_up_to_merged_branch_names_count(2)).and_call_original
+              expect(API::Entities::Branch).to receive(:represent).with(anything, has_up_to_merged_branch_names_count(2)).at_least(:once).and_call_original
 
               get api(route, current_user), params: base_params.merge(per_page: 2)