Preload commits markdown cache

This is for improving the performance of
Projects::MergeRequestsController#commits action.

Prevents n+1 requests to Redis when getting the value from cache
on render if the cache is warm.

Previously, it'll make an HMGET request to Redis per commit which
can result to 100 requests when viewing the commits in a MR.

Now, a single Redis pipeline is used via `with_markdown_cache`.

app/controllers/projects/merge_requests_controller.rb

@@ -109,8 +109,8 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     # or from cache if already merged
     @commits =
       set_commits_for_rendering(
-        @merge_request.recent_commits.with_latest_pipeline(@merge_request.source_branch),
-          commits_count: @merge_request.commits_count
+        @merge_request.recent_commits.with_latest_pipeline(@merge_request.source_branch).with_markdown_cache,
+        commits_count: @merge_request.commits_count
       )
 
     render json: { html: view_to_html_string('projects/merge_requests/_commits') }

app/models/commit_collection.rb

@@ -53,6 +53,17 @@ class CommitCollection
     self
   end
 
+  # Returns the collection with markdown fields preloaded.
+  #
+  # Get the markdown cache from redis using pipeline to prevent n+1 requests
+  # when rendering the markdown of an attribute (e.g. title, full_title,
+  # description).
+  def with_markdown_cache
+    Commit.preload_markdown_cache!(commits)
+
+    self
+  end
+
   def unenriched
     commits.reject(&:gitaly_commit?)
   end

changelogs/unreleased/209912-commits-markdown-cache-preload.yml

+---
+title: Preload commits markdown cache
+merge_request: 35314
+author:
+type: performance

spec/models/commit_collection_spec.rb

@@ -75,6 +75,18 @@ RSpec.describe CommitCollection do
     end
   end
 
+  describe '#with_markdown_cache' do
+    let(:commits) { [commit] }
+    let(:collection) { described_class.new(project, commits) }
+
+    it 'preloads commits cache markdown' do
+      aggregate_failures do
+        expect(Commit).to receive(:preload_markdown_cache!).with(commits)
+        expect(collection.with_markdown_cache).to eq(collection)
+      end
+    end
+  end
+
   describe 'enrichment methods' do
     let(:gitaly_commit) { commit }
     let(:hash_commit) { Commit.from_hash(gitaly_commit.to_hash, project) }