Restore lazy loading of pipeline commits in preloader

lib/gitlab/ci/pipeline/preloader.rb

@@ -7,9 +7,16 @@ module Gitlab
       # authors.
       class Preloader
         def self.preload!(pipelines)
+          ##
+          # This preloads all commits at once, because `Ci::Pipeline#commit` is
+          # using a lazy batch loading, what results in only one batched Gitaly
+          # call.
+          #
+          pipelines.each(&:commit)
+
           pipelines.each do |pipeline|
             self.new(pipeline).tap do |preloader|
-              preloader.preload_commits
+              preloader.preload_commit_authors
               preloader.preload_pipeline_warnings
               preloader.preload_stages_warnings
             end
@@ -20,10 +27,7 @@ module Gitlab
           @pipeline = pipeline
         end
 
-        def preload_commits
-          # This ensures that all the pipeline commits are eager loaded before we
-          # start using them.
-          #
+        def preload_commit_authors
           # This also preloads the author of every commit. We're using "lazy_author"
           # here since "author" immediately loads the data on the first call.
           @pipeline.commit.try(:lazy_author)

spec/lib/gitlab/ci/pipeline/preloader_spec.rb

 # frozen_string_literal: true
 
-require 'fast_spec_helper'
+require 'spec_helper'
 
 describe Gitlab::Ci::Pipeline::Preloader do
   let(:stage) { double(:stage) }
@@ -11,6 +11,23 @@ describe Gitlab::Ci::Pipeline::Preloader do
   end
 
   describe '.preload!' do
+    context 'when preloading multiple commits' do
+      let(:project) { create(:project, :repository) }
+
+      it 'preloads all commits once' do
+        expect(Commit).to receive(:decorate).once.and_call_original
+
+        pipelines = [build_pipeline(ref: 'HEAD'),
+                     build_pipeline(ref: 'HEAD~1')]
+
+        described_class.preload!(pipelines)
+      end
+
+      def build_pipeline(ref:)
+        build_stubbed(:ci_pipeline, project: project, sha: project.commit(ref).id)
+      end
+    end
+
     it 'preloads commit authors and number of warnings' do
       expect(commit).to receive(:lazy_author)
       expect(pipeline).to receive(:number_of_warnings)