Enable pagination in the CompareController

Previously, the pagination in the Projects::CompareController was
disabled by default. It lead to the memory usage bloat. We enable it and
set it to a rather aggresive (low) value, unlike it was in the
CommitsController previously.

app/controllers/projects/compare_controller.rb

@@ -26,6 +26,8 @@ class Projects::CompareController < Projects::ApplicationController
 
   feature_category :source_code_management
 
+  COMMIT_DIFFS_PER_PAGE = 10
+
   def index
   end
 

app/helpers/commits_helper.rb

@@ -128,7 +128,7 @@ module CommitsHelper
     %w(btn gpg-status-box) + Array(additional_classes)
   end
 
-  def conditionally_paginate_diff_files(diffs, paginate:, per: Projects::CommitController::COMMIT_DIFFS_PER_PAGE)
+  def conditionally_paginate_diff_files(diffs, paginate:, per:)
     if paginate
       Kaminari.paginate_array(diffs.diff_files.to_a).page(params[:page]).per(per)
     else

app/views/projects/commit/show.html.haml

@@ -12,7 +12,12 @@
 .container-fluid{ class: [limited_container_width, container_class] }
   = render "commit_box"
   = render "ci_menu"
-  = render "projects/diffs/diffs", diffs: @diffs, environment: @environment, diff_page_context: "is-commit", paginate_diffs: true
+  = render "projects/diffs/diffs",
+    diffs: @diffs,
+    environment: @environment,
+    diff_page_context: "is-commit",
+    paginate_diffs: true,
+    paginate_diffs_per_page: Projects::CommitController::COMMIT_DIFFS_PER_PAGE
 
   .limited-width-notes
     = render "shared/notes/notes_with_form", :autocomplete => true

app/views/projects/compare/show.html.haml

@@ -7,7 +7,12 @@
 
 - if @commits.present?
   = render "projects/commits/commit_list"
-  = render "projects/diffs/diffs", diffs: @diffs, environment: @environment, diff_page_context: "is-compare"
+  = render "projects/diffs/diffs",
+    diffs: @diffs,
+    environment: @environment,
+    diff_page_context: "is-compare",
+    paginate_diffs: true,
+    paginate_diffs_per_page: Projects::CompareController::COMMIT_DIFFS_PER_PAGE
 - else
   .card.bg-light
     .center

app/views/projects/diffs/_diffs.html.haml

@@ -4,7 +4,8 @@
 - diff_page_context = local_assigns.fetch(:diff_page_context, nil)
 - load_diff_files_async = Feature.enabled?(:async_commit_diff_files, @project) && diff_page_context == "is-commit"
 - paginate_diffs = local_assigns.fetch(:paginate_diffs, false) && !load_diff_files_async
-- diff_files = conditionally_paginate_diff_files(diffs, paginate: paginate_diffs)
+- paginate_diffs_per_page = local_assigns.fetch(:paginate_diffs_per_page, nil)
+- diff_files = conditionally_paginate_diff_files(diffs, paginate: paginate_diffs, per: paginate_diffs_per_page)
 
 .content-block.oneline-block.files-changed.diff-files-changed.js-diff-files-changed
   .files-changed-inner

changelogs/unreleased/326312-project-compare-controller-use-pagination.yml

+---
+title: Enable pagination in the CompareController
+merge_request: 59162
+author:
+type: performance

spec/helpers/commits_helper_spec.rb

@@ -144,7 +144,7 @@ RSpec.describe CommitsHelper do
       }
     end
 
-    subject { helper.conditionally_paginate_diff_files(diffs_collection, paginate: paginate) }
+    subject { helper.conditionally_paginate_diff_files(diffs_collection, paginate: paginate, per: Projects::CommitController::COMMIT_DIFFS_PER_PAGE) }
 
     before do
       allow(helper).to receive(:params).and_return(params)