feat(service-worker): recover from `EXISTING_CLIENTS_ONLY` mode when there is a valid update (#31865)

Previously, when the ServiceWorker entered a degraded mode
(`EXISTING_CLIENTS_ONLY` or `SAFE_MODE`) it would remain in that mode
for the rest of the lifetime of ServiceWorker instance. Note that
ServiceWorkers are stopped by the browser after a certain period of
inactivity and a new instance is created as soon as the ServiceWorker
needs to handle an event (such as a request from the page). Those new
instances would start from the `NORMAL` mode.

The reason for this behavior is to err on the side of caution: If we
can't be sure why the ServiceWorker entered the degraded mode, it is
risky to try recovering on the same instance and might lead to
unexpected behavior.

However, it turns out that the `EXISTING_CLIENTS_ONLY` mode can only be
a result of some error happening with the latest version (e.g. a hash
mismatch in the manifest). Therefore, it is safe to recover from that
mode once a new, valid update is successfully installed and to start
accepting new clients.

This commit ensures that the mode is set back to `NORMAL`, when (a) an
update is successfully installed and (b) the current mode is
`EXISTING_CLIENTS_ONLY`.

Besides making the behavior more predictable (instead of relying on the
browser to decide when to terminate the current ServiceWorker instance
and create a new one), this change can also improve the developer
experience:
When people notice the error during debugging and fix it by deploying a
new version (either to production or locally), it is confusing that the
ServiceWorker will fetch and install the update (as seen by the requests
in the Network panel in DevTools) but not serve it to clients. With this
change, the update will be served to new clients as soon as it is
installed.

Fixes #31109

PR Close #31865

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

@@ -802,8 +802,15 @@ export class Driver implements Debuggable, UpdateSource {
     // Future new clients will use this hash as the latest version.
     this.latestHash = hash;
 
+    // If we are in `EXISTING_CLIENTS_ONLY` mode (meaning we didn't have a clean copy of the last
+    // latest version), we can now recover to `NORMAL` mode and start accepting new clients.
+    if (this.state === DriverReadyState.EXISTING_CLIENTS_ONLY) {
+      this.state = DriverReadyState.NORMAL;
+      this.stateMessage = '(nominal)';
+    }
+
     await this.sync();
-    await this.notifyClientsAboutUpdate();
+    await this.notifyClientsAboutUpdate(newVersion);
   }
 
   async checkForUpdate(): Promise<boolean> {
@@ -959,19 +966,19 @@ export class Driver implements Debuggable, UpdateSource {
 
   async lookupResourceWithoutHash(url: string): Promise<CacheState|null> {
     await this.initialized;
-    const version = this.versions.get(this.latestHash !) !;
-    return version.lookupResourceWithoutHash(url);
+    const version = this.versions.get(this.latestHash !);
+    return version ? version.lookupResourceWithoutHash(url) : null;
   }
 
   async previouslyCachedResources(): Promise<string[]> {
     await this.initialized;
-    const version = this.versions.get(this.latestHash !) !;
-    return version.previouslyCachedResources();
+    const version = this.versions.get(this.latestHash !);
+    return version ? version.previouslyCachedResources() : [];
   }
 
-  recentCacheStatus(url: string): Promise<UpdateCacheStatus> {
-    const version = this.versions.get(this.latestHash !) !;
-    return version.recentCacheStatus(url);
+  async recentCacheStatus(url: string): Promise<UpdateCacheStatus> {
+    const version = this.versions.get(this.latestHash !);
+    return version ? version.recentCacheStatus(url) : UpdateCacheStatus.NOT_CACHED;
   }
 
   private mergeHashWithAppData(manifest: Manifest, hash: string): {hash: string, appData: Object} {
@@ -981,11 +988,10 @@ export class Driver implements Debuggable, UpdateSource {
     };
   }
 
-  async notifyClientsAboutUpdate(): Promise<void> {
+  async notifyClientsAboutUpdate(next: AppVersion): Promise<void> {
     await this.initialized;
 
     const clients = await this.scope.clients.matchAll();
-    const next = this.versions.get(this.latestHash !) !;
 
     await clients.reduce(async(previous, client) => {
       await previous;

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

@@ -1269,6 +1269,22 @@ import {SwTestHarness, SwTestHarnessBuilder} from '../testing/scope';
         brokenLazyServer.assertNoOtherRequests();
       });
 
+      it('recovers from degraded `EXISTING_CLIENTS_ONLY` mode as soon as there is a valid update',
+         async() => {
+           await driver.initialized;
+           expect(driver.state).toBe(DriverReadyState.NORMAL);
+
+           // Install a broken version.
+           scope.updateServerState(brokenServer);
+           await driver.checkForUpdate();
+           expect(driver.state).toBe(DriverReadyState.EXISTING_CLIENTS_ONLY);
+
+           // Install a good version.
+           scope.updateServerState(serverUpdate);
+           await driver.checkForUpdate();
+           expect(driver.state).toBe(DriverReadyState.NORMAL);
+         });
+
       it('ignores invalid `only-if-cached` requests ', async() => {
         const requestFoo = (cache: RequestCache | 'only-if-cached', mode: RequestMode) =>
             makeRequest(scope, '/foo.txt', undefined, {cache, mode});