Optimize ProjectWiki#empty? check

The `empty?` check was iterating over all Wiki pages to determine
whether it was empty. Using the limit parameter allows us to bail
out early once we found a page.

Note that this currently only improves performance for GitLab 11.0
until https://gitlab.com/gitlab-org/gitaly/issues/1204 is fixed.

Relates to #40101

app/models/project_wiki.rb

@@ -20,7 +20,6 @@ class ProjectWiki
     @user = user
   end
 
-  delegate :empty?, to: :pages
   delegate :repository_storage, :hashed_storage?, to: :project
 
   def path
@@ -74,6 +73,10 @@ class ProjectWiki
     !!find_page('home')
   end
 
+  def empty?
+    pages(limit: 1).empty?
+  end
+
   # Returns an Array of Gitlab WikiPage instances or an
   # empty Array if this Wiki has no pages.
   def pages(limit: nil)

changelogs/unreleased/sh-optimize-wiki-empty-check.yml

@@ -0,0 +1,5 @@
+---
+title: Optimize ProjectWiki#empty? check
+merge_request: 20573
+author:
+type: performance

spec/models/project_wiki_spec.rb

@@ -1,3 +1,4 @@
+# coding: utf-8
 require "spec_helper"
 
 describe ProjectWiki do
@@ -10,7 +11,6 @@ describe ProjectWiki do
 
   subject { project_wiki }
 
-  it { is_expected.to delegate_method(:empty?).to :pages }
   it { is_expected.to delegate_method(:repository_storage).to :project }
   it { is_expected.to delegate_method(:hashed_storage?).to :project }
 
@@ -92,11 +92,19 @@ describe ProjectWiki do
     context "when the wiki has pages" do
       before do
         project_wiki.create_page("index", "This is an awesome new Gollum Wiki")
+        project_wiki.create_page("another-page", "This is another page")
       end
 
       describe '#empty?' do
         subject { super().empty? }
         it { is_expected.to be_falsey }
+
+        # Re-enable this when https://gitlab.com/gitlab-org/gitaly/issues/1204 is fixed
+        xit 'only instantiates a Wiki page once' do
+          expect(WikiPage).to receive(:new).once.and_call_original
+
+          subject
+        end
       end
     end
   end