Added "maintenance" app serving 503 page

refs #12568

- When the server is in maintenance mode (e.g. during migration) it needs to serve a 503 page. Previously this role was delegated to the "frontend" which tightly coupled server bootup and frontend.
- With a dedicated HTTP application serving up 503 server is no longer coupled to the frontend during maintenance

core/server/ghost-server.js

@@ -196,6 +196,15 @@ class GhostServer {
         }
     }
 
+    /**
+    * @param  {Object} externalApp - express app instance
+    * @return {Promise} Resolves once Ghost has switched HTTP Servers
+    */
+    async swapHttpApp(externalApp) {
+        await this._stopServer();
+        await this.start(externalApp);
+    }
+
     /**
      * ### Hammertime
      * To be called after `stop`

core/server/index.js

@@ -93,6 +93,10 @@ const initExpressApps = async () => {
     return parentApp;
 };
 
+const initMaintenanceApp = () => {
+    return require('./web/maintenance')();
+};
+
 /**
  * - initialise models
  * - initialise i18n
@@ -138,25 +142,26 @@ const minimalRequiredSetupToStartGhost = async (dbState) => {
         config.set('maintenance:enabled', true);
         logging.info('Blog is in maintenance mode.');
 
+        ghostServer.rootApp = initMaintenanceApp();
+
         try {
-            // const delay= ms => new Promise(resolve => setTimeout(resolve ,ms));
-            await migrator.migrate();
+            migrator.migrate()
+                .then(async () => {
+                    await settings.init();
+                    debug('Settings done');
 
-            // await delay(10000);
-            await settings.init();
-            debug('Settings done');
+                    const parentApp = await initExpressApps();
+                    ghostServer.swapHttpApp(parentApp);
 
-            const parentApp = await initExpressApps();
-            ghostServer.rootApp = parentApp;
+                    events.emit('db.ready');
 
-            events.emit('db.ready');
+                    await initialiseServices();
 
-            await initialiseServices();
+                    config.set('maintenance:enabled', false);
+                    logging.info('Blog is out of maintenance mode.');
 
-            config.set('maintenance:enabled', false);
-            logging.info('Blog is out of maintenance mode.');
-
-            await GhostServer.announceServerReadiness();
+                    await GhostServer.announceServerReadiness();
+                });
         } catch (err) {
             try {
                 await GhostServer.announceServerReadiness(err);

core/server/web/maintenance/index.js

@@ -0,0 +1,51 @@
+const hbs = require('express-hbs');
+const compress = require('compression');
+const debug = require('ghost-ignition').debug('web:maintenance');
+const constants = require('@tryghost/constants');
+const express = require('../../../shared/express');
+const config = require('../../../shared/config');
+const {servePublicFile} = require('../site/middleware');
+
+const createHbsEngine = () => {
+    const engine = hbs.create();
+    engine.registerHelper('asset', require('../../../frontend/helpers/asset'));
+
+    return engine.express4();
+};
+
+module.exports = function setupMaintenanceApp() {
+    debug('MaintenanceApp setup start');
+    const app = express('maintenance');
+
+    // enabled gzip compression by defaulti
+    if (config.get('compress') !== false) {
+        app.use(compress());
+    }
+
+    app.engine('hbs', createHbsEngine());
+    app.set('view engine', 'hbs');
+    app.set('views', config.get('paths').defaultViews);
+
+    // Serve stylesheets for default templates
+    app.use(servePublicFile('public/ghost.css', 'text/css', constants.ONE_HOUR_S));
+    app.use(servePublicFile('public/ghost.min.css', 'text/css', constants.ONE_YEAR_S));
+
+    // Serve images for default templates
+    app.use(servePublicFile('public/404-ghost@2x.png', 'image/png', constants.ONE_HOUR_S));
+    app.use(servePublicFile('public/404-ghost.png', 'image/png', constants.ONE_HOUR_S));
+
+    app.use('/', (req, res) => {
+        const data = {
+            message: 'Maintenance',
+            statusCode: 503
+        };
+
+        res.render('error', data, (err, html) => {
+            return res.send(html);
+        });
+    });
+
+    debug('MaintenanceApp setup end');
+
+    return app;
+};