Memoize the latest builds of a pipeline

This ensures that if a pipeline is present for the last commit on a
project's homepage we only run 1 query to get the builds, instead of
running 2 queries.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/36878#note_40073339
for more information.

app/models/ci/pipeline.rb

@@ -453,6 +453,10 @@ module Ci
         .fabricate!
     end
 
+    def latest_builds_with_artifacts
+      @latest_builds_with_artifacts ||= builds.latest.with_artifacts
+    end
+
     private
 
     def ci_yaml_from_repo

app/views/projects/buttons/_download.html.haml

@@ -26,18 +26,16 @@
           %i.fa.fa-download
           %span= _('Download tar')
 
-      - if pipeline
-        - artifacts = pipeline.builds.latest.with_artifacts
-        - if artifacts.any?
-          %li.dropdown-header Artifacts
-          - unless pipeline.latest?
-            - latest_pipeline = project.pipeline_for(ref)
-            %li
-              .unclickable= ci_status_for_statuseable(latest_pipeline)
-            %li.dropdown-header Previous Artifacts
-          - artifacts.each do |job|
-            %li
-              = link_to latest_succeeded_project_artifacts_path(project, "#{ref}/download", job: job.name), rel: 'nofollow', download: '' do
-                %i.fa.fa-download
-                %span
-                  #{ s_('DownloadArtifacts|Download') } '#{job.name}'
+      - if pipeline && pipeline.latest_builds_with_artifacts.any?
+        %li.dropdown-header Artifacts
+        - unless pipeline.latest?
+          - latest_pipeline = project.pipeline_for(ref)
+          %li
+            .unclickable= ci_status_for_statuseable(latest_pipeline)
+          %li.dropdown-header Previous Artifacts
+        - pipeline.latest_builds_with_artifacts.each do |job|
+          %li
+            = link_to latest_succeeded_project_artifacts_path(project, "#{ref}/download", job: job.name), rel: 'nofollow', download: '' do
+              %i.fa.fa-download
+              %span
+                #{s_('DownloadArtifacts|Download')} '#{job.name}'

changelogs/unreleased/memoize-the-latest-builds-of-a-pipeline.yml

+---
+title: "Memoize the latest builds of a pipeline on a project's homepage"
+merge_request:
+author:
+type: other

spec/models/ci/pipeline_spec.rb

@@ -1439,4 +1439,24 @@ describe Ci::Pipeline, :mailer do
       it_behaves_like 'not sending any notification'
     end
   end
+
+  describe '#latest_builds_with_artifacts' do
+    let!(:pipeline) { create(:ci_pipeline, :success) }
+
+    let!(:build) do
+      create(:ci_build, :success, :artifacts, pipeline: pipeline)
+    end
+
+    it 'returns the latest builds' do
+      expect(pipeline.latest_builds_with_artifacts).to eq([build])
+    end
+
+    it 'memoizes the returned relation' do
+      query_count = ActiveRecord::QueryRecorder
+        .new { 2.times { pipeline.latest_builds_with_artifacts.to_a } }
+        .count
+
+      expect(query_count).to eq(1)
+    end
+  end
 end