From 393b623aef2921d70383381207528884902c5a0a Mon Sep 17 00:00:00 2001
From: Dominik Henneke <dominik.henneke@sda-se.com>
Date: Fri, 5 Mar 2021 13:41:52 +0100
Subject: [PATCH] Add a `Cache-Control: no-store` header to the `index.html`
 response to instruct the browser to not cache the pages

Signed-off-by: Dominik Henneke <dominik.henneke@sda-se.com>
---
 .changeset/good-humans-draw.md                |  6 ++++
 .../app-backend/src/service/router.test.ts    | 18 +++++++++-
 plugins/app-backend/src/service/router.ts     | 33 +++++++++++++++----
 3 files changed, 49 insertions(+), 8 deletions(-)
 create mode 100644 .changeset/good-humans-draw.md

diff --git a/.changeset/good-humans-draw.md b/.changeset/good-humans-draw.md
new file mode 100644
index 0000000000..ff9bacaa49
--- /dev/null
+++ b/.changeset/good-humans-draw.md
@@ -0,0 +1,6 @@
+---
+'@backstage/plugin-app-backend': patch
+---
+
+Add a `Cache-Control: no-store` header to the `index.html` response to instruct the browser to not cache the pages.
+This is a workaround for a missing `staticFallbackHandler` since an old `index.html` might link to static assets from a previous deployment.
diff --git a/plugins/app-backend/src/service/router.test.ts b/plugins/app-backend/src/service/router.test.ts
index 35dd9287a6..1a6fee01a0 100644
--- a/plugins/app-backend/src/service/router.test.ts
+++ b/plugins/app-backend/src/service/router.test.ts
@@ -14,11 +14,11 @@
  * limitations under the License.
  */
 
-import { resolve as resolvePath } from 'path';
 import { getVoidLogger } from '@backstage/backend-common';
 import { ConfigReader } from '@backstage/config';
 import express from 'express';
 import Router from 'express-promise-router';
+import { resolve as resolvePath } from 'path';
 import request from 'supertest';
 import { createRouter } from './router';
 
@@ -68,6 +68,22 @@ describe('createRouter', () => {
     expect(response.status).toBe(200);
     expect(response.text.trim()).toBe('this is index.html');
   });
+
+  it.each(['/index.html', '/other.html', '/missing.html'])(
+    'returns %s with no-store Cache-Control header',
+    async file => {
+      const response = await request(app).get(file);
+      expect(response.header['cache-control']).toBe('no-store');
+    },
+  );
+
+  it.each(['/static/main.txt'])(
+    'returns %s with default Cache-Control header',
+    async file => {
+      const response = await request(app).get(file);
+      expect(response.header['cache-control']).toBe('public, max-age=0');
+    },
+  );
 });
 
 describe('createRouter with static fallback handler', () => {
diff --git a/plugins/app-backend/src/service/router.ts b/plugins/app-backend/src/service/router.ts
index 4379fcccb9..ba407d9ef8 100644
--- a/plugins/app-backend/src/service/router.ts
+++ b/plugins/app-backend/src/service/router.ts
@@ -14,14 +14,14 @@
  * limitations under the License.
  */
 
-import { resolve as resolvePath } from 'path';
-import express from 'express';
-import Router from 'express-promise-router';
-import { Logger } from 'winston';
 import { notFoundHandler, resolvePackagePath } from '@backstage/backend-common';
 import { Config } from '@backstage/config';
-import { injectConfig, readConfigs } from '../lib/config';
+import express from 'express';
+import Router from 'express-promise-router';
 import fs from 'fs-extra';
+import { resolve as resolvePath } from 'path';
+import { Logger } from 'winston';
+import { injectConfig, readConfigs } from '../lib/config';
 
 export interface RouterOptions {
   config: Config;
@@ -95,9 +95,28 @@ export async function createRouter(
   staticRouter.use(notFoundHandler());
 
   router.use('/static', staticRouter);
-  router.use(express.static(appDistDir));
+  router.use(
+    express.static(appDistDir, {
+      setHeaders: (res, path) => {
+        // The Cache-Control header instructs the browser to not cache html files since it might
+        // link to static assets from recently deployed versions. This is a workaround when no
+        // staticFallbackHandler is configured.
+        // use `as any` since express uses mime v1 while we only have types for mime v2
+        if ((express.static.mime as any).lookup(path) === 'text/html') {
+          res.setHeader('Cache-Control', 'no-store');
+        }
+      },
+    }),
+  );
   router.get('/*', (_req, res) => {
-    res.sendFile(resolvePath(appDistDir, 'index.html'));
+    res.sendFile(resolvePath(appDistDir, 'index.html'), {
+      headers: {
+        // The Cache-Control header instructs the browser to not cache the index.html since it might
+        // link to static assets from recently deployed versions. This is a workaround when no
+        // staticFallbackHandler is configured.
+        'cache-control': 'no-store',
+      },
+    });
   });
 
   return router;