Add onBlocked handler for schema upgrade (#963)

Author: schmidt-sebastian

packages/firestore/src/local/simple_db.ts

@@ -22,6 +22,7 @@ import { PersistencePromise } from './persistence_promise';
 import { SCHEMA_VERSION } from './indexeddb_schema';
 import { Deferred } from '../util/promise';
 import { PersistenceTransaction } from './persistence';
+import { Code, FirestoreError } from '../util/error';
 
 const LOG_TAG = 'SimpleDb';
 
@@ -62,6 +63,16 @@ export class SimpleDb {
         resolve(new SimpleDb(db));
       };
 
+      request.onblocked = () => {
+        reject(
+          new FirestoreError(
+            Code.FAILED_PRECONDITION,
+            'Cannot upgrade IndexedDB schema while another tab is open. ' +
+              'Close all tabs that access Firestore and reload this page to proceed.'
+          )
+        );
+      };
+
       request.onerror = (event: ErrorEvent) => {
         reject((event.target as IDBOpenDBRequest).error);
       };