fix(service-worker): treat 503 as offline (angular#35595)

Prior to this commit the service worker only treated 504 errors as "effectively offline".
This commit changes the behaviour to treat both 503 (Service Unavailable) and 504 as "offline".

Fixes angular#35571

PR Close angular#35595

Author: alexwiese

packages/service-worker/worker/src/driver.ts

@@ -684,7 +684,7 @@ export class Driver implements Debuggable, UpdateSource {
   /**
    * Retrieve a copy of the latest manifest from the server.
    * Return `null` if `ignoreOfflineError` is true (default: false) and the server or client are
-   * offline (detected as response status 504).
+   * offline (detected as response status 503 (service unavailable) or 504 (gateway timeout)).
    */
   private async fetchLatestManifest(ignoreOfflineError?: false): Promise<Manifest>;
   private async fetchLatestManifest(ignoreOfflineError: true): Promise<Manifest|null>;
@@ -695,7 +695,7 @@ export class Driver implements Debuggable, UpdateSource {
       if (res.status === 404) {
         await this.deleteAllCaches();
         await this.scope.registration.unregister();
-      } else if (res.status === 504 && ignoreOfflineError) {
+      } else if ((res.status === 503 || res.status === 504) && ignoreOfflineError) {
         return null;
       }
       throw new Error(`Manifest fetch failed! (status: ${res.status})`);

packages/service-worker/worker/test/happy_spec.ts

@@ -12,7 +12,7 @@ import {Driver, DriverReadyState} from '../src/driver';
 import {AssetGroupConfig, DataGroupConfig, Manifest} from '../src/manifest';
 import {sha1} from '../src/sha1';
 import {MockCache, clearAllCaches} from '../testing/cache';
-import {MockRequest} from '../testing/fetch';
+import {MockRequest, MockResponse} from '../testing/fetch';
 import {MockFileSystemBuilder, MockServerStateBuilder, tmpHashTableForFs} from '../testing/mock';
 import {SwTestHarness, SwTestHarnessBuilder} from '../testing/scope';
 
@@ -872,6 +872,21 @@ import {SwTestHarness, SwTestHarnessBuilder} from '../testing/scope';
       expect(await scope.caches.keys()).not.toEqual([]);
     });
 
+    it('does not unregister or change state when status code is 503 (service unavailable)',
+       async() => {
+         expect(await makeRequest(scope, '/foo.txt')).toEqual('this is foo');
+         await driver.initialized;
+         spyOn(server, 'fetch').and.callFake((req: Request) => new MockResponse(null, {
+                                               status: 503,
+                                               statusText: 'Service Unavailable'
+                                             }));
+
+         expect(await driver.checkForUpdate()).toEqual(false);
+         expect(driver.state).toEqual(DriverReadyState.NORMAL);
+         expect(scope.unregistered).toBeFalsy();
+         expect(await scope.caches.keys()).not.toEqual([]);
+       });
+
     describe('cache naming', () => {
       // Helpers
       const cacheKeysFor = (baseHref: string) =>