Allow diffing changes in wiki history

Introduces a new `diff` action on the wiki controllers which shows
the diff for the selected file and commit.

- Add `Gitlab::Diff::FileCollection::WikiPage`.
- Override diff helpers in wiki controller.
- Unify wiki views:
  - Always use limited container width.
  - Always add breadcrumbs for wiki page path.
  - Show sidebar in diff view.
  - Align title in diff view with history view.
  - Add helpers for wiki page headers.

app/controllers/concerns/wiki_actions.rb

 # frozen_string_literal: true
 
 module WikiActions
+  include DiffHelper
   include SendsBlob
   include Gitlab::Utils::StrongMemoize
   extend ActiveSupport::Concern
@@ -11,8 +12,9 @@ module WikiActions
     before_action :authorize_admin_wiki!, only: :destroy
 
     before_action :wiki
-    before_action :page, only: [:show, :edit, :update, :history, :destroy]
+    before_action :page, only: [:show, :edit, :update, :history, :destroy, :diff]
     before_action :load_sidebar, except: [:pages]
+    before_action :set_content_class
 
     before_action only: [:show, :edit, :update] do
       @valid_encoding = valid_encoding?
@@ -25,6 +27,8 @@ module WikiActions
         redirect_to wiki_path(wiki)
       end
     end
+
+    helper_method :view_file_button, :diff_file_html_data
   end
 
   def new
@@ -136,6 +140,19 @@ module WikiActions
   end
   # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
+  def diff
+    return render_404 unless page
+
+    apply_diff_view_cookie!
+
+    @diffs = page.diffs(diff_options)
+    @diff_notes_disabled = true
+
+    render 'shared/wikis/diff'
+  end
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
+
   # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def destroy
     WikiPages::DestroyService.new(container: container, current_user: current_user).execute(page)
@@ -207,7 +224,7 @@ module WikiActions
 
   def page_params
     keys = [:id]
-    keys << :version_id if params[:action] == 'show'
+    keys << :version_id if %w[show diff].include?(params[:action])
 
     params.values_at(*keys)
   end
@@ -233,4 +250,25 @@ module WikiActions
       wiki.repository.blob_at(commit.id, params[:id])
     end
   end
+
+  def set_content_class
+    @content_class = 'limit-container-width' unless fluid_layout # rubocop:disable Gitlab/ModuleWithInstanceVariables
+  end
+
+  # Override CommitsHelper#view_file_button
+  def view_file_button(commit_sha, *args)
+    path = wiki_page_path(wiki, page, version_id: page.version.id)
+
+    helpers.link_to(path, class: 'btn') do
+      helpers.raw(_('View page @ ')) + helpers.content_tag(:span, Commit.truncate_sha(commit_sha), class: 'commit-sha')
+    end
+  end
+
+  # Override DiffHelper#diff_file_html_data
+  def diff_file_html_data(_project, _diff_file_path, diff_commit_id)
+    {
+      blob_diff_path: wiki_page_path(wiki, page, action: :diff, version_id: diff_commit_id),
+      view: diff_view
+    }
+  end
 end

app/helpers/commits_helper.rb

@@ -181,15 +181,11 @@ module CommitsHelper
   end
 
   def view_file_button(commit_sha, diff_new_path, project, replaced: false)
+    path = project_blob_path(project, tree_join(commit_sha, diff_new_path))
     title = replaced ? _('View replaced file @ ') : _('View file @ ')
 
-    link_to(
-      project_blob_path(project,
-                                  tree_join(commit_sha, diff_new_path)),
-      class: 'btn view-file js-view-file'
-    ) do
-      raw(title) + content_tag(:span, Commit.truncate_sha(commit_sha),
-                                       class: 'commit-sha')
+    link_to(path, class: 'btn') do
+      raw(title) + content_tag(:span, truncate_sha(commit_sha), class: 'commit-sha')
     end
   end
 

app/helpers/diff_helper.rb

@@ -135,8 +135,7 @@ module DiffHelper
 
   def diff_file_html_data(project, diff_file_path, diff_commit_id)
     {
-      blob_diff_path: project_blob_diff_path(project,
-                                                       tree_join(diff_commit_id, diff_file_path)),
+      blob_diff_path: project_blob_diff_path(project, tree_join(diff_commit_id, diff_file_path)),
       view: diff_view
     }
   end

app/helpers/nav_helper.rb

@@ -27,7 +27,7 @@ module NavHelper
       end
     elsif current_path?('jobs#show')
       %w[page-gutter build-sidebar right-sidebar-expanded]
-    elsif current_controller?('wikis') && current_action?('show', 'create', 'edit', 'update', 'history', 'git_access', 'destroy')
+    elsif current_controller?('wikis') && current_action?('show', 'create', 'edit', 'update', 'history', 'git_access', 'destroy', 'diff')
       %w[page-gutter wiki-sidebar right-sidebar-expanded]
     else
       []

app/helpers/wiki_helper.rb

@@ -3,6 +3,30 @@
 module WikiHelper
   include API::Helpers::RelatedResourcesHelpers
 
+  def wiki_page_title(page, action = nil)
+    titles = [_('Wiki')]
+
+    if page.persisted?
+      titles << page.human_title
+      breadcrumb_title(page.human_title)
+      wiki_breadcrumb_dropdown_links(page.slug)
+    end
+
+    titles << action if action
+    page_title(*titles.reverse)
+    add_to_breadcrumbs(_('Wiki'), wiki_path(page.wiki))
+  end
+
+  def link_to_wiki_page(page, **options)
+    link_to page.human_title, wiki_page_path(page.wiki, page), **options
+  end
+
+  def wiki_sidebar_toggle_button
+    content_tag :button, class: 'btn btn-default sidebar-toggle js-sidebar-wiki-toggle', role: 'button', type: 'button' do
+      sprite_icon('chevron-double-lg-left')
+    end
+  end
+
   # Produces a pure text breadcrumb for a given page.
   #
   # page_slug - The slug of a WikiPage object.

app/models/wiki_page.rb

@@ -301,6 +301,10 @@ class WikiPage
     version&.commit&.committed_date
   end
 
+  def diffs(diff_options = {})
+    Gitlab::Diff::FileCollection::WikiPage.new(self, diff_options: diff_options)
+  end
+
   private
 
   def serialize_front_matter(hash)

app/views/discussions/_discussion.html.haml

@@ -37,7 +37,7 @@
                     an outdated change in
                   commit
 
-                  %span.commit-sha= Commit.truncate_sha(discussion.commit_id)
+                  %span.commit-sha= truncate_sha(discussion.commit_id)
                 - else
                   - unless discussion.active?
                     an old version of

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

@@ -16,6 +16,8 @@
           = diff_merge_request_whitespace_link(diffs.project, @merge_request, class: 'd-none d-sm-inline-block')
         - elsif current_controller?(:compare)
           = diff_compare_whitespace_link(diffs.project, params[:from], params[:to], class: 'd-none d-sm-inline-block')
+        - elsif current_controller?(:wikis)
+          = toggle_whitespace_link(url_for(params_with_whitespace), class: 'd-none d-sm-inline-block')
       .btn-group
         = inline_diff_btn
         = parallel_diff_btn

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

@@ -2,8 +2,7 @@
 - page_title s_("WikiClone|Git Access"), _("Wiki")
 
 .wiki-page-header.top-area.has-sidebar-toggle.py-3.flex-column.flex-lg-row
-  %button.btn.btn-default.d-block.d-sm-block.d-md-none.float-right.sidebar-toggle.js-sidebar-wiki-toggle{ role: "button", type: "button" }
-    = sprite_icon('chevron-double-lg-left')
+  = wiki_sidebar_toggle_button
 
   .git-access-header.w-100.d-flex.flex-column.justify-content-center
     %span

app/views/shared/wikis/diff.html.haml

+- wiki_page_title @page, _('Changes')
+- commit = @diffs.diffable
+
+.wiki-page-header.top-area.has-sidebar-toggle.flex-column.flex-lg-row
+  = wiki_sidebar_toggle_button
+
+  .nav-text
+    %h2.wiki-page-title
+      = link_to_wiki_page @page
+      %span.light
+        ·
+        = _('Changes')
+
+  .nav-controls.pb-md-3.pb-lg-0
+    = link_to wiki_page_path(@wiki, @page, action: :history), class: 'btn', role: 'button', data: { qa_selector: 'page_history_button' } do
+      = s_('Wiki|Page history')
+
+.page-content-header
+  .header-main-content
+    %strong= markdown_field(commit, :title)
+    %span.d-none.d-sm-inline= _('authored')
+    #{time_ago_with_tooltip(commit.authored_date)}
+    %span= s_('ByAuthor|by')
+    = author_avatar(commit, size: 24, has_tooltip: false)
+    %strong
+      = commit_author_link(commit, avatar: true, size: 24)
+    - if commit.description.present?
+      %pre.commit-description<
+        = preserve(markdown_field(commit, :description))
+
+= render 'projects/diffs/diffs', diffs: @diffs
+= render 'shared/wikis/sidebar'

app/views/shared/wikis/edit.html.haml

-- @content_class = "limit-container-width" unless fluid_layout
-- add_to_breadcrumbs _("Wiki"), wiki_page_path(@wiki, @page)
-- breadcrumb_title @page.persisted? ? _("Edit") : _("New")
-- page_title @page.persisted? ? _("Edit") : _("New"), @page.human_title, _("Wiki")
+- wiki_page_title @page, @page.persisted? ? _('Edit') : _('New')
 
 = wiki_page_errors(@error)
 
 .wiki-page-header.top-area.has-sidebar-toggle.flex-column.flex-lg-row
-  %button.btn.btn-default.sidebar-toggle.js-sidebar-wiki-toggle{ role: "button", type: "button" }
-    = sprite_icon('chevron-double-lg-left')
+  = wiki_sidebar_toggle_button
 
   .nav-text
     %h2.wiki-page-title
       - if @page.persisted?
-        = link_to @page.human_title, wiki_page_path(@wiki, @page)
+        = link_to_wiki_page @page
         %span.light
           ·
           = s_("Wiki|Edit Page")

app/views/shared/wikis/history.html.haml

-- page_title _("History"), @page.human_title, _("Wiki")
+- wiki_page_title @page, _('History')
 
 .wiki-page-header.top-area.has-sidebar-toggle.flex-column.flex-lg-row
-  %button.btn.btn-default.sidebar-toggle.js-sidebar-wiki-toggle{ role: "button", type: "button" }
-    = sprite_icon('chevron-double-lg-left')
+  = wiki_sidebar_toggle_button
 
   .nav-text
     %h2.wiki-page-title
-      = link_to @page.human_title, wiki_page_path(@wiki, @page)
+      = link_to_wiki_page @page
       %span.light
         ·
-        = _("History")
+        = _('History')
 
-.table-holder
-  %table.table
+.prepend-top-default.gl-mb-3
+  .table-holder
+    %table.table.wiki-history
       %thead
         %tr
-        %th= s_("Wiki|Page version")
-        %th= _("Author")
-        %th= _("Commit Message")
-        %th= _("Last updated")
-        %th= _("Format")
+          %th= s_('Wiki|Page version')
+          %th= _('Author')
+          %th= _('Changes')
+          %th= _('Last updated')
       %tbody
-      - @page_versions.each_with_index do |version, index|
-        - commit = version
+        - @page_versions.each do |commit|
           %tr
             %td
-            = link_to wiki_page_path(@wiki, @page, version_id: index == 0 ? nil : commit.id) do
+              = link_to wiki_page_path(@wiki, @page, version_id: commit.id) do
                 = truncate_sha(commit.id)
             %td
               = commit.author_name
             %td
+              %span.str-truncated-60
+                = link_to wiki_page_path(@wiki, @page, action: :diff, version_id: commit.id), { title: commit.message } do
                   = commit.message
             %td
-            #{time_ago_with_tooltip(version.authored_date)}
-          %td
-            %strong
-              = version.format
-= paginate @page_versions, theme: 'gitlab'
+              = time_ago_with_tooltip(commit.authored_date)
+  = paginate @page_versions, theme: 'gitlab'
 
 = render 'shared/wikis/sidebar'

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

-- @content_class = "limit-container-width" unless fluid_layout
-- breadcrumb_title @page.human_title
-- wiki_breadcrumb_dropdown_links(@page.slug)
-- page_title @page.human_title, _("Wiki")
-- add_to_breadcrumbs _("Wiki"), wiki_path(@wiki)
+- wiki_page_title @page
 
 .wiki-page-header.top-area.has-sidebar-toggle.flex-column.flex-lg-row
-  %button.btn.btn-default.sidebar-toggle.js-sidebar-wiki-toggle{ role: "button", type: "button" }
-    = sprite_icon('chevron-double-lg-left')
+  = wiki_sidebar_toggle_button
 
   .nav-text.flex-fill
     %h2.wiki-page-title{ data: { qa_selector: 'wiki_page_title' } }= @page.human_title
     %span.wiki-last-edit-by
       - 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)}
+        = time_ago_with_tooltip(@page.last_version.authored_date)
 
   .nav-controls.pb-md-3.pb-lg-0
     = render 'shared/wikis/main_links'

changelogs/unreleased/15242-wiki-diff.yml

+---
+title: Allow diffing changes in wiki history
+merge_request: 35330
+author: gwhyte, Steve Mokris
+type: added

config/routes/wiki.rb

@@ -10,6 +10,7 @@ scope(controller: :wikis) do
   scope(path: 'wikis/*id', as: :wiki, format: false) do
     get :edit
     get :history
+    get :diff
     post :preview_markdown
     get '/', action: :show
     put '/', action: :update

lib/gitlab/diff/file_collection/wiki_page.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Diff
+    module FileCollection
+      class WikiPage < Base
+        def initialize(page, diff_options:)
+          commit = page.wiki.commit(page.version.commit)
+          diff_options = diff_options.merge(
+            expanded: true,
+            paths: [page.path]
+          )
+
+          super(commit,
+            # TODO: Uncouple diffing from projects
+            # https://gitlab.com/gitlab-org/gitlab/-/issues/217752
+            project: page.wiki,
+            diff_options: diff_options,
+            diff_refs: commit.diff_refs)
+        end
+      end
+    end
+  end
+end

lib/gitlab/git/wiki.rb

@@ -101,6 +101,10 @@ module Gitlab
         wrapped_gitaly_errors do
           gitaly_find_page(title: title, version: version, dir: dir)
         end
+      rescue Gitlab::Git::CommandError
+        # Return nil for invalid versions.
+        # This can be removed with https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2323 in place.
+        nil
       end
 
       def file(name, version)

locale/gitlab.pot

@@ -10178,9 +10178,6 @@ msgstr ""
 msgid "Forks"
 msgstr ""
 
-msgid "Format"
-msgstr ""
-
 msgid "Format: %{dateFormat}"
 msgstr ""
 
@@ -25387,6 +25384,9 @@ msgstr ""
 msgid "View open merge request"
 msgstr ""
 
+msgid "View page @ "
+msgstr ""
+
 msgid "View performance dashboard."
 msgstr ""
 

spec/features/projects/wiki/user_views_wiki_page_spec.rb

@@ -8,9 +8,10 @@ RSpec.describe 'User views a wiki page' do
   let(:user) { create(:user) }
   let(:project) { create(:project, :wiki_repo, namespace: user.namespace) }
   let(:path) { 'image.png' }
+  let(:wiki) { project.wiki }
   let(:wiki_page) do
     create(:wiki_page,
-           wiki: project.wiki,
+           wiki: wiki,
            title: 'home', content: "Look at this [image](#{path})\n\n ![alt text](#{path})")
   end
 
@@ -70,11 +71,13 @@ RSpec.describe 'User views a wiki page' do
 
       click_on('Page history')
 
-      page.within(:css, '.nav-text') do
+      within('.nav-text') do
         expect(page).to have_content('History')
       end
 
-      find('a[href*="?version_id"]')
+      within('.wiki-history') do
+        expect(page).to have_css('a[href*="?version_id"]', count: 4)
+      end
     end
   end
 
@@ -92,8 +95,8 @@ RSpec.describe 'User views a wiki page' do
       let(:path) { upload_file_to_wiki(project, user, 'dk.png') }
 
       it do
-        expect(page).to have_xpath("//img[@data-src='#{project.wiki.wiki_base_path}/#{path}']")
-        expect(page).to have_link('image', href: "#{project.wiki.wiki_base_path}/#{path}")
+        expect(page).to have_xpath("//img[@data-src='#{wiki.wiki_base_path}/#{path}']")
+        expect(page).to have_link('image', href: "#{wiki.wiki_base_path}/#{path}")
 
         click_on('image')
 
@@ -103,7 +106,7 @@ RSpec.describe 'User views a wiki page' do
     end
 
     it 'shows the creation page if file does not exist' do
-      expect(page).to have_link('image', href: "#{project.wiki.wiki_base_path}/#{path}")
+      expect(page).to have_link('image', href: "#{wiki.wiki_base_path}/#{path}")
 
       click_on('image')
 
@@ -114,7 +117,7 @@ RSpec.describe 'User views a wiki page' do
 
   context 'when a page has history' do
     before do
-      wiki_page.update(message: 'updated home', content: 'updated [some link](other-page)')
+      wiki_page.update(message: 'updated home', content: 'updated [some link](other-page)') # rubocop:disable Rails/SaveBang
     end
 
     it 'shows the page history' do
@@ -134,13 +137,74 @@ RSpec.describe 'User views a wiki page' do
 
       expect(page).not_to have_selector('a.btn', text: 'Edit')
     end
+
+    context 'show the diff' do
+      def expect_diff_links(commit)
+        diff_path = wiki_page_path(wiki, wiki_page, version_id: commit, action: :diff)
+
+        expect(page).to have_link('Hide whitespace changes', href: "#{diff_path}&w=1")
+        expect(page).to have_link('Inline', href: "#{diff_path}&view=inline")
+        expect(page).to have_link('Side-by-side', href: "#{diff_path}&view=parallel")
+        expect(page).to have_link("View page @ #{commit.short_id}", href: wiki_page_path(wiki, wiki_page, version_id: commit))
+        expect(page).to have_css('.diff-file[data-blob-diff-path="%s"]' % diff_path)
+      end
+
+      it 'links to the correct diffs' do
+        visit project_wiki_history_path(project, wiki_page)
+
+        commit1 = wiki.commit('HEAD^')
+        commit2 = wiki.commit
+
+        expect(page).to have_link('created page: home', href: wiki_page_path(wiki, wiki_page, version_id: commit1, action: :diff))
+        expect(page).to have_link('updated home', href: wiki_page_path(wiki, wiki_page, version_id: commit2, action: :diff))
+      end
+
+      it 'between the current and the previous version of a page' do
+        commit = wiki.commit
+        visit wiki_page_path(wiki, wiki_page, version_id: commit, action: :diff)
+
+        expect(page).to have_content('by John Doe')
+        expect(page).to have_content('updated home')
+        expect(page).to have_content('Showing 1 changed file with 1 addition and 3 deletions')
+        expect(page).to have_content('some link')
+
+        expect_diff_links(commit)
+      end
+
+      it 'between two old versions of a page' do
+        wiki_page.update(message: 'latest home change', content: 'updated [another link](other-page)') # rubocop:disable Rails/SaveBang:
+        commit = wiki.commit('HEAD^')
+        visit wiki_page_path(wiki, wiki_page, version_id: commit, action: :diff)
+
+        expect(page).to have_content('by John Doe')
+        expect(page).to have_content('updated home')
+        expect(page).to have_content('Showing 1 changed file with 1 addition and 3 deletions')
+        expect(page).to have_content('some link')
+        expect(page).not_to have_content('latest home change')
+        expect(page).not_to have_content('another link')
+
+        expect_diff_links(commit)
+      end
+
+      it 'for the oldest version of a page' do
+        commit = wiki.commit('HEAD^')
+        visit wiki_page_path(wiki, wiki_page, version_id: commit, action: :diff)
+
+        expect(page).to have_content('by John Doe')
+        expect(page).to have_content('created page: home')
+        expect(page).to have_content('Showing 1 changed file with 4 additions and 0 deletions')
+        expect(page).to have_content('Look at this')
+
+        expect_diff_links(commit)
+      end
+    end
   end
 
   context 'when a page has special characters in its title' do
     let(:title) { '<foo> !@#$%^&*()[]{}=_+\'"\\|<>? <bar>' }
 
     before do
-      wiki_page.update(title: title )
+      wiki_page.update(title: title ) # rubocop:disable Rails/SaveBang
     end
 
     it 'preserves the special characters' do
@@ -155,7 +219,7 @@ RSpec.describe 'User views a wiki page' do
     let(:title) { '<script>alert("title")<script>' }
 
     before do
-      wiki_page.update(title: title, content: 'foo <script>alert("content")</script> bar')
+      wiki_page.update(title: title, content: 'foo <script>alert("content")</script> bar') # rubocop:disable Rails/SaveBang
     end
 
     it 'safely displays the page' do
@@ -168,7 +232,7 @@ RSpec.describe 'User views a wiki page' do
 
   context 'when a page has XSS in its message' do
     before do
-      wiki_page.update(message: '<script>alert(true)<script>', content: 'XSS update')
+      wiki_page.update(message: '<script>alert(true)<script>', content: 'XSS update') # rubocop:disable Rails/SaveBang
     end
 
     it 'safely displays the message' do

spec/helpers/commits_helper_spec.rb

@@ -51,6 +51,20 @@ RSpec.describe CommitsHelper do
     end
   end
 
+  describe '#view_file_button' do
+    let(:project) { build(:project) }
+    let(:path) { 'path/to/file' }
+    let(:sha) { '1234567890' }
+
+    subject do
+      helper.view_file_button(sha, path, project)
+    end
+
+    it 'links to project files' do
+      expect(subject).to have_link('1234567', href: helper.project_blob_path(project, "#{sha}/#{path}"))
+    end
+  end
+
   describe '#view_on_environment_button' do
     let(:project) { create(:project) }
     let(:environment) { create(:environment, external_url: 'http://example.com') }

spec/helpers/diff_helper_spec.rb

@@ -303,6 +303,20 @@ RSpec.describe DiffHelper do
     end
   end
 
+  describe '#diff_file_html_data' do
+    let(:project) { build(:project) }
+    let(:path) { 'path/to/file' }
+    let(:sha) { '1234567890' }
+
+    subject do
+      helper.diff_file_html_data(project, path, sha)
+    end
+
+    it 'returns data for project files' do
+      expect(subject).to include(blob_diff_path: helper.project_blob_diff_path(project, "#{sha}/#{path}"))
+    end
+  end
+
   describe '#diff_file_path_text' do
     it 'returns full path by default' do
       expect(diff_file_path_text(diff_file)).to eq(diff_file.new_path)

spec/helpers/wiki_helper_spec.rb

@@ -3,6 +3,38 @@
 require 'spec_helper'
 
 RSpec.describe WikiHelper do
+  describe '#wiki_page_title' do
+    let_it_be(:page) { create(:wiki_page) }
+
+    it 'sets the title for the show action' do
+      expect(helper).to receive(:breadcrumb_title).with(page.human_title)
+      expect(helper).to receive(:wiki_breadcrumb_dropdown_links).with(page.slug)
+      expect(helper).to receive(:page_title).with(page.human_title, 'Wiki')
+      expect(helper).to receive(:add_to_breadcrumbs).with('Wiki', helper.wiki_path(page.wiki))
+
+      helper.wiki_page_title(page)
+    end
+
+    it 'sets the title for a custom action' do
+      expect(helper).to receive(:breadcrumb_title).with(page.human_title)
+      expect(helper).to receive(:wiki_breadcrumb_dropdown_links).with(page.slug)
+      expect(helper).to receive(:page_title).with('Edit', page.human_title, 'Wiki')
+      expect(helper).to receive(:add_to_breadcrumbs).with('Wiki', helper.wiki_path(page.wiki))
+
+      helper.wiki_page_title(page, 'Edit')
+    end
+
+    it 'sets the title for an unsaved page' do
+      expect(page).to receive(:persisted?).and_return(false)
+      expect(helper).not_to receive(:breadcrumb_title)
+      expect(helper).not_to receive(:wiki_breadcrumb_dropdown_links)
+      expect(helper).to receive(:page_title).with('Wiki')
+      expect(helper).to receive(:add_to_breadcrumbs).with('Wiki', helper.wiki_path(page.wiki))
+
+      helper.wiki_page_title(page)
+    end
+  end
+
   describe '#breadcrumb' do
     context 'when the page is at the root level' do
       it 'returns the capitalized page name' do

spec/models/wiki_page_spec.rb

@@ -864,6 +864,24 @@ RSpec.describe WikiPage do
     end
   end
 
+  describe '#diffs' do
+    subject { existing_page }
+
+    it 'returns a diff instance' do
+      diffs = subject.diffs(foo: 'bar')
+
+      expect(diffs).to be_a(Gitlab::Diff::FileCollection::WikiPage)
+      expect(diffs.diffable).to be_a(Commit)
+      expect(diffs.diffable.id).to eq(subject.version.id)
+      expect(diffs.project).to be(subject.wiki)
+      expect(diffs.diff_options).to include(
+        expanded: true,
+        paths: [subject.path],
+        foo: 'bar'
+      )
+    end
+  end
+
   private
 
   def get_slugs(page_or_dir)

spec/support/shared_examples/controllers/wiki_actions_shared_examples.rb

@@ -104,6 +104,35 @@ RSpec.shared_examples 'wiki controller actions' do
     end
   end
 
+  describe 'GET #diff' do
+    context 'when commit exists' do
+      it 'renders the diff' do
+        get :diff, params: routing_params.merge(id: wiki_title, version_id: wiki.repository.commit.id)
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(response).to render_template('shared/wikis/diff')
+        expect(assigns(:diffs)).to be_a(Gitlab::Diff::FileCollection::Base)
+        expect(assigns(:diff_notes_disabled)).to be(true)
+      end
+    end
+
+    context 'when commit does not exist' do
+      it 'returns a 404 error' do
+        get :diff, params: routing_params.merge(id: wiki_title, version_id: 'invalid')
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
+    context 'when page does not exist' do
+      it 'returns a 404 error' do
+        get :diff, params: routing_params.merge(id: 'invalid')
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+  end
+
   describe 'GET #show' do
     render_views