Memoize pipelines for project download buttons

This adds Project#latest_successful_pipeline_for and
Project#latest_successful_pipeline_for_default_branch. The 2nd method
memoizes the result (taking nil values into account) to ensure the
underlying query isn't executed multiple times when viewing a project's
homepage.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/36878#note_40073607
for more information.

app/models/project.rb

@@ -1163,6 +1163,23 @@ class Project < ActiveRecord::Base
     pipelines.order(id: :desc).find_by(sha: sha, ref: ref)
   end
 
+  def latest_successful_pipeline_for_default_branch
+    if defined?(@latest_successful_pipeline_for_default_branch)
+      return @latest_successful_pipeline_for_default_branch
+    end
+
+    @latest_successful_pipeline_for_default_branch =
+      pipelines.latest_successful_for(default_branch)
+  end
+
+  def latest_successful_pipeline_for(ref = nil)
+    if ref && ref != default_branch
+      pipelines.latest_successful_for(ref)
+    else
+      latest_successful_pipeline_for_default_branch
+    end
+  end
+
   def enable_ci
     project_feature.update_attribute(:builds_access_level, ProjectFeature::ENABLED)
   end

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

-- pipeline = local_assigns.fetch(:pipeline) { project.pipelines.latest_successful_for(ref) }
+- pipeline = local_assigns.fetch(:pipeline) { project.latest_successful_pipeline_for(ref) }
 
 - if !project.empty_repo? && can?(current_user, :download_code, project)
   .project-action-button.dropdown.inline>

changelogs/unreleased/projects-controller-show.yml

+---
+title: Memoize pipelines for project download buttons
+merge_request:
+author:
+type: other

spec/models/project_spec.rb

@@ -2682,4 +2682,60 @@ describe Project do
       end
     end
   end
+
+  describe '#latest_successful_builds_for' do
+    let(:project) { build(:project) }
+
+    before do
+      allow(project).to receive(:default_branch).and_return('master')
+    end
+
+    context 'without a ref' do
+      it 'returns a pipeline for the default branch' do
+        expect(project)
+          .to receive(:latest_successful_pipeline_for_default_branch)
+
+        project.latest_successful_pipeline_for
+      end
+    end
+
+    context 'with the ref set to the default branch' do
+      it 'returns a pipeline for the default branch' do
+        expect(project)
+          .to receive(:latest_successful_pipeline_for_default_branch)
+
+        project.latest_successful_pipeline_for(project.default_branch)
+      end
+    end
+
+    context 'with a ref that is not the default branch' do
+      it 'returns the latest successful pipeline for the given ref' do
+        expect(project.pipelines).to receive(:latest_successful_for).with('foo')
+
+        project.latest_successful_pipeline_for('foo')
+      end
+    end
+  end
+
+  describe '#latest_successful_pipeline_for_default_branch' do
+    let(:project) { build(:project) }
+
+    before do
+      allow(project).to receive(:default_branch).and_return('master')
+    end
+
+    it 'memoizes and returns the latest successful pipeline for the default branch' do
+      pipeline = double(:pipeline)
+
+      expect(project.pipelines).to receive(:latest_successful_for)
+        .with(project.default_branch)
+        .and_return(pipeline)
+        .once
+
+      2.times do
+        expect(project.latest_successful_pipeline_for_default_branch)
+          .to eq(pipeline)
+      end
+    end
+  end
 end