[V2] Loading page: avoid seeing infinite reloads

packages/pwa-kit-cli/src/ssr/server/build-dev-server.js

@@ -153,7 +153,7 @@ export const DevServerMixin = {
             if (app.__webpackReady()) {
                 middleware(req, res, next)
             } else {
-                res.redirect(301, '/__mrt/loading-screen/index.html?loading=1')
+                this._onAllRequestsBeforeWebpackReady(req, res, next)
             }
         })
 
@@ -173,6 +173,11 @@ export const DevServerMixin = {
         })
     },
 
+    // eslint-disable-next-line no-unused-vars
+    _onAllRequestsBeforeWebpackReady(req, res, next) {
+        res.redirect('/__mrt/loading-screen/index.html?loading=1')
+    },
+
     _getDevServerHostAndPort(options) {
         const split = options.devServerHostName.split(':')
         const hostname = split.length === 2 ? split[0] : options.devServerHostName

packages/pwa-kit-cli/src/ssr/server/build-dev-server.test.js

@@ -130,6 +130,21 @@ describe('DevServer startup', () => {
     })
 })
 
+describe('DevServer loading page', () => {
+    test('requesting homepage would temporarily redirect to the loading page, when build is not ready', async () => {
+        const options = opts()
+        const app = NoWebpackDevServerFactory.createApp(options)
+        app.use('/', DevServerFactory._onAllRequestsBeforeWebpackReady)
+
+        return request(app)
+            .get('/')
+            .expect(302) // Expecting the 302 temporary redirect (not 301)
+            .then((response) => {
+                expect(response.headers.location).toBe('/__mrt/loading-screen/index.html?loading=1')
+            })
+    })
+})
+
 describe('DevServer request processor support', () => {
     const helloWorld = '<div>hello world</div>'
 
@@ -230,7 +245,7 @@ describe('DevServer request processor support', () => {
     })
 })
 
-describe('DevServer startup', () => {
+describe('DevServer listening on http/https protocol', () => {
     let server
     let originalEnv