Don't show auxiliary blob viewer for README when there is no wiki

app/models/blob_viewer/readme.rb

@@ -10,5 +10,11 @@ module BlobViewer
     def visible_to?(current_user)
       can?(current_user, :read_wiki, project)
     end
+
+    def render_error
+      return if project.has_external_wiki? || (project.wiki_enabled? && project.wiki.has_home_page?)
+
+      :no_wiki
+    end
   end
 end

app/models/project_wiki.rb

@@ -63,6 +63,10 @@ class ProjectWiki
     !!repository.exists?
   end
 
+  def has_home_page?
+    !!find_page('home')
+  end
+
   # Returns an Array of Gitlab WikiPage instances or an
   # empty Array if this Wiki has no pages.
   def pages

app/views/projects/blob/viewers/_readme.html.haml

 = icon('info-circle fw')
 = succeed '.' do
   To learn more about this project, read
-  = link_to "the wiki", project_wikis_path(viewer.project)
+  = link_to "the wiki", get_project_wiki_path(viewer.project)

changelogs/unreleased/dm-readme-auxiliary-blob-viewer-without-wiki.yml

+---
+title: Don't show auxiliary blob viewer for README when there is no wiki
+merge_request:
+author:

spec/models/blob_viewer/readme_spec.rb

+require 'spec_helper'
+
+describe BlobViewer::Readme, model: true do
+  include FakeBlobHelpers
+
+  let(:project) { create(:project, :repository) }
+  let(:blob) { fake_blob(path: 'README.md') }
+  subject { described_class.new(blob) }
+
+  describe '#render_error' do
+    context 'when there is no wiki' do
+      it 'returns :no_wiki' do
+        expect(subject.render_error).to eq(:no_wiki)
+      end
+    end
+
+    context 'when there is an external wiki' do
+      before do
+        project.has_external_wiki = true
+      end
+
+      it 'returns nil' do
+        expect(subject.render_error).to be_nil
+      end
+    end
+
+    context 'when there is a local wiki' do
+      before do
+        project.wiki_enabled = true
+      end
+
+      context 'when the wiki is empty' do
+        it 'returns :no_wiki' do
+          expect(subject.render_error).to eq(:no_wiki)
+        end
+      end
+
+      context 'when the wiki is not empty' do
+        before do
+          WikiPages::CreateService.new(project, project.owner, title: 'home', content: 'Home page').execute
+        end
+
+        it 'returns nil' do
+          expect(subject.render_error).to be_nil
+        end
+      end
+    end
+  end
+end