Cache rendered compare entity

Adds caching for the rendered compare JSON, and adds a new
cache key method to ensure it expires correctly.

Changelog: performance

app/models/compare.rb

@@ -25,6 +25,10 @@ class Compare
     @straight = straight
   end
 
+  def cache_key
+    [@project, :compare, start_commit_sha, base_commit_sha, head_commit_sha, @straight]
+  end
+
   def commits
     @commits ||= Commit.decorate(@compare.commits, project)
   end

config/feature_flags/development/api_caching_repository_compare.yml

+---
+name: api_caching_repository_compare
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64418
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/334264
+milestone: '14.1'
+type: development
+group: group::source code
+default_enabled: false

lib/api/repositories.rb

@@ -138,7 +138,11 @@ module API
           compare = CompareService.new(user_project, params[:to]).execute(target_project, params[:from], straight: params[:straight])
 
           if compare
-            present compare, with: Entities::Compare
+            if Feature.enabled?(:api_caching_repository_compare, user_project, default_enabled: :yaml)
+              present_cached compare, with: Entities::Compare, expires_in: 1.day, cache_context: nil
+            else
+              present compare, with: Entities::Compare
+            end
           else
             not_found!("Ref")
           end

spec/models/compare_spec.rb

@@ -13,7 +13,16 @@ RSpec.describe Compare do
 
   let(:raw_compare) { Gitlab::Git::Compare.new(project.repository.raw_repository, start_commit.id, head_commit.id) }
 
-  subject { described_class.new(raw_compare, project) }
+  subject(:compare) { described_class.new(raw_compare, project) }
+
+  describe '#cache_key' do
+    subject { compare.cache_key }
+
+    it { is_expected.to include(project) }
+    it { is_expected.to include(start_commit.id) }
+    it { is_expected.to include(head_commit.id) }
+    it { is_expected.to include(compare.base_commit_sha) }
+  end
 
   describe '#start_commit' do
     it 'returns raw compare base commit' do