Add enabled endpoint to Pages Internal API

Add this new simple endpoint to allow GitLab Pages to verify that
the internal API is enabled and ready to be used. Details on the issue
https://gitlab.com/gitlab-org/gitlab/-/issues/210010

changelogs/unreleased/add-internal-api-pages-enabled.yml

+---
+title: Add status endpoint to Pages Internal API
+merge_request: 28743
+author:
+type: added

lib/api/internal/pages.rb

@@ -16,6 +16,13 @@ module API
 
       namespace 'internal' do
         namespace 'pages' do
+          desc 'Indicates that pages API is enabled and auth token is valid' do
+            detail 'This feature was introduced in GitLab 12.10.'
+          end
+          get "status" do
+            no_content!
+          end
+
           desc 'Get GitLab Pages domain configuration by hostname' do
             detail 'This feature was introduced in GitLab 12.3.'
           end

spec/requests/api/internal/pages_spec.rb

@@ -3,13 +3,36 @@
 require 'spec_helper'
 
 describe API::Internal::Pages do
-  describe "GET /internal/pages" do
+  let(:auth_headers) do
+    jwt_token = JWT.encode({ 'iss' => 'gitlab-pages' }, Gitlab::Pages.secret, 'HS256')
+    { Gitlab::Pages::INTERNAL_API_REQUEST_HEADER => jwt_token }
+  end
   let(:pages_secret) { SecureRandom.random_bytes(Gitlab::Pages::SECRET_LENGTH) }
 
   before do
     allow(Gitlab::Pages).to receive(:secret).and_return(pages_secret)
   end
 
+  describe "GET /internal/pages/status" do
+    def query_enabled(headers = {})
+      get api("/internal/pages/status"), headers: headers
+    end
+
+    it 'responds with 401 Unauthorized' do
+      query_enabled
+
+      expect(response).to have_gitlab_http_status(:unauthorized)
+    end
+
+    it 'responds with 204 no content' do
+      query_enabled(auth_headers)
+
+      expect(response).to have_gitlab_http_status(:no_content)
+      expect(response.body).to be_empty
+    end
+  end
+
+  describe "GET /internal/pages" do
     def query_host(host, headers = {})
       get api("/internal/pages"), headers: headers, params: { host: host }
     end