Harden the wiki against missing last_versions

Currently, we assume a "last_version" always exists for a wiki page.
In production, this is not always true. So, guard uses of it with a
null check.

app/models/wiki_page.rb

@@ -160,7 +160,9 @@ class WikiPage
   # Returns boolean True or False if this instance
   # is an old version of the page.
   def historical?
-    @page.historical? && last_version.sha != version.sha
+    return false unless last_commit_sha && version
+
+    @page.historical? && last_commit_sha != version.sha
   end
 
   # Returns boolean True or False if this instance

app/views/projects/wikis/_pages_wiki_page.html.haml

@@ -2,4 +2,5 @@
   = link_to wiki_page.title, project_wiki_path(@project, wiki_page)
   %small (#{wiki_page.format})
   .float-right
-    %small= (s_("Last edited %{date}") % { date: time_ago_with_tooltip(wiki_page.last_version.authored_date) }).html_safe
+    - if wiki_page.last_version
+      %small= (s_("Last edited %{date}") % { date: time_ago_with_tooltip(wiki_page.last_version.authored_date) }).html_safe

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

@@ -11,8 +11,9 @@
   .nav-text
     %h2.wiki-page-title= @page.title.capitalize
     %span.wiki-last-edit-by
-      = (_("Last edited by %{name}") % { name: "<strong>#{@page.last_version.author_name}</strong>" }).html_safe
-      #{time_ago_with_tooltip(@page.last_version.authored_date)}
+      - if @page.last_version
+        = (_("Last edited by %{name}") % { name: "<strong>#{@page.last_version.author_name}</strong>" }).html_safe
+        #{time_ago_with_tooltip(@page.last_version.authored_date)}
 
   .nav-controls
     = render 'main_links'

changelogs/unreleased/52527-harden-wiki-against-missing-last-version.yml

+---
+title: Fix a bug displaying certain wiki pages
+merge_request: 22377
+author:
+type: fixed

spec/models/wiki_page_spec.rb

@@ -457,6 +457,12 @@ describe WikiPage do
   end
 
   describe '#historical?' do
+    let(:page) { wiki.find_page('Update') }
+    let(:old_version) { page.versions.last.id }
+    let(:old_page) { wiki.find_page('Update', old_version) }
+    let(:latest_version) { page.versions.first.id }
+    let(:latest_page) { wiki.find_page('Update', latest_version) }
+
     before do
       create_page('Update', 'content')
       @page = wiki.find_page('Update')
@@ -468,23 +474,27 @@ describe WikiPage do
     end
 
     it 'returns true when requesting an old version' do
-      old_version = @page.versions.last.id
-      old_page = wiki.find_page('Update', old_version)
-
-      expect(old_page.historical?).to eq true
+      expect(old_page.historical?).to be_truthy
     end
 
     it 'returns false when requesting latest version' do
-      latest_version = @page.versions.first.id
-      latest_page = wiki.find_page('Update', latest_version)
-
-      expect(latest_page.historical?).to eq false
+      expect(latest_page.historical?).to be_falsy
     end
 
     it 'returns false when version is nil' do
-      latest_page = wiki.find_page('Update', nil)
+      expect(latest_page.historical?).to be_falsy
+    end
+
+    it 'returns false when the last version is nil' do
+      expect(old_page).to receive(:last_version) { nil }
+
+      expect(old_page.historical?).to be_falsy
+    end
+
+    it 'returns false when the version is nil' do
+      expect(old_page).to receive(:version) { nil }
 
-      expect(latest_page.historical?).to eq false
+      expect(old_page.historical?).to be_falsy
     end
   end