Add skeleton Pages internal API

Basic `/internal/pages` endpoint that will be used for Pages virtual
domains internal API. The endpoint is currently behind feature flag and
provides authetication similar to how Workhorse is authenticating with
the GitLab.

.gitignore

@@ -65,6 +65,7 @@ eslint-report.html
 /vendor/gitaly-ruby
 /builds*
 /.gitlab_workhorse_secret
+/.gitlab_pages_shared_secret
 /webpack-report/
 /knapsack/
 /rspec_flaky/

config/gitlab.yml.example

@@ -321,6 +321,9 @@ production: &base
     # external_https: ["1.1.1.1:443", "[2001::1]:443"] # If defined, enables custom domain and certificate support in GitLab Pages
     admin:
       address: unix:/home/git/gitlab/tmp/sockets/private/pages-admin.socket # TCP connections are supported too (e.g. tcp://host:port)
+    # File that contains the shared secret key for verifying access for gitlab-pages.
+    # Default is '.gitlab_pages_shared_secret' relative to Rails.root (i.e. root of the GitLab app).
+    # secret_file: /home/git/gitlab/.gitlab_pages_shared_secret
 
   ## Mattermost
   ## For enabling Add to Mattermost button

config/initializers/1_settings.rb

@@ -292,6 +292,7 @@ Settings.pages['artifacts_server']  ||= Settings.pages['enabled'] if Settings.pa
 
 Settings.pages['admin'] ||= Settingslogic.new({})
 Settings.pages.admin['certificate'] ||= ''
+Settings.pages['secret_file'] ||= Rails.root.join('.gitlab_pages_shared_secret')
 
 #
 # Geo

lib/api/api.rb

@@ -119,6 +119,7 @@ module API
     mount ::API::GroupVariables
     mount ::API::ImportGithub
     mount ::API::Internal::Base
+    mount ::API::Internal::Pages
     mount ::API::Issues
     mount ::API::JobArtifacts
     mount ::API::Jobs

lib/api/internal/pages.rb

+# frozen_string_literal: true
+
+module API
+  # Pages Internal API
+  module Internal
+    class Pages < Grape::API
+      before do
+        not_found! unless Feature.enabled?(:pages_internal_api)
+        authenticate_gitlab_pages_request!
+      end
+
+      helpers do
+        def authenticate_gitlab_pages_request!
+          unauthorized! unless Gitlab::Pages.verify_api_request(headers)
+        end
+      end
+
+      namespace 'internal' do
+        namespace 'pages' do
+          get "/" do
+            status :ok
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/pages.rb

 # frozen_string_literal: true
 
 module Gitlab
-  module Pages
+  class Pages
     VERSION = File.read(Rails.root.join("GITLAB_PAGES_VERSION")).strip.freeze
+    INTERNAL_API_REQUEST_HEADER = 'Gitlab-Pages-Api-Request'.freeze
+
+    include JwtAuthenticatable
+
+    class << self
+      def verify_api_request(request_headers)
+        decode_jwt_for_issuer('gitlab-pages', request_headers[INTERNAL_API_REQUEST_HEADER])
+      rescue JWT::DecodeError
+        false
+      end
+
+      def secret_path
+        Gitlab.config.pages.secret_file
+      end
+    end
   end
 end

spec/lib/gitlab/pages_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Pages do
+  let(:pages_shared_secret) { SecureRandom.random_bytes(Gitlab::Pages::SECRET_LENGTH) }
+
+  before do
+    allow(described_class).to receive(:secret).and_return(pages_shared_secret)
+  end
+
+  describe '.verify_api_request' do
+    let(:payload) { { 'iss' => 'gitlab-pages' } }
+
+    it 'returns false if fails to validate the JWT' do
+      encoded_token = JWT.encode(payload, 'wrongsecret', 'HS256')
+      headers = { described_class::INTERNAL_API_REQUEST_HEADER => encoded_token }
+
+      expect(described_class.verify_api_request(headers)).to eq(false)
+    end
+
+    it 'returns the decoded JWT' do
+      encoded_token = JWT.encode(payload, described_class.secret, 'HS256')
+      headers = { described_class::INTERNAL_API_REQUEST_HEADER => encoded_token }
+
+      expect(described_class.verify_api_request(headers)).to eq([{ "iss" => "gitlab-pages" }, { "alg" => "HS256" }])
+    end
+  end
+end

spec/requests/api/internal/pages_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe API::Internal::Pages do
+  describe "GET /internal/pages" do
+    let(:pages_shared_secret) { SecureRandom.random_bytes(Gitlab::Pages::SECRET_LENGTH) }
+
+    before do
+      allow(Gitlab::Pages).to receive(:secret).and_return(pages_shared_secret)
+    end
+
+    def query_host(host, headers = {})
+      get api("/internal/pages"), headers: headers, params: { host: host }
+    end
+
+    context 'feature flag disabled' do
+      before do
+        stub_feature_flags(pages_internal_api: false)
+      end
+
+      it 'responds with 404 Not Found' do
+        query_host('pages.gitlab.io')
+
+        expect(response).to have_gitlab_http_status(404)
+      end
+    end
+
+    context 'feature flag enabled' do
+      context 'not authenticated' do
+        it 'responds with 401 Unauthorized' do
+          query_host('pages.gitlab.io')
+
+          expect(response).to have_gitlab_http_status(401)
+        end
+      end
+
+      context 'authenticated' do
+        def query_host(host)
+          jwt_token = JWT.encode({ 'iss' => 'gitlab-pages' }, Gitlab::Pages.secret, 'HS256')
+          headers = { Gitlab::Pages::INTERNAL_API_REQUEST_HEADER => jwt_token }
+
+          super(host, headers)
+        end
+
+        it 'responds with 200 OK' do
+          query_host('pages.gitlab.io')
+
+          expect(response).to have_gitlab_http_status(200)
+        end
+      end
+    end
+  end
+end