Throw more helpful errors from indexedb-local-backend (#5282)

* Throw more helpful errors from indexedb-local-backend

* Add a test

* Do not tempt fate

spec/unit/stores/indexeddb.spec.ts

@@ -79,6 +79,41 @@ describe("IndexedDBStore", () => {
         expect(await store.getOutOfBandMembers(roomId)).toHaveLength(2);
     });
 
+    it("should handle failed queries", async () => {
+        const store = new IndexedDBStore({
+            indexedDB: indexedDB,
+            dbName: "database",
+            localStorage,
+        });
+        await store.startup();
+
+        // Simulate a failed query
+        let txn: IDBRequest;
+        (store.backend as LocalIndexedDBStoreBackend)["db"]!.transaction = (): IDBTransaction => {
+            return {
+                objectStore: (name: string) =>
+                    ({
+                        name,
+                        openCursor: (query: unknown) => {
+                            return (txn = {
+                                error: new DOMException("Expected error"),
+                            } as IDBRequest);
+                        },
+                    }) as IDBObjectStore,
+            } as IDBTransaction;
+        };
+
+        // Call backend directly as otherwise the error is masked.
+        const promise = store.backend.getClientOptions();
+        // The function uses a Promise.then(() => trick to delay execution
+        // so we need to wait before we can call the txn onerror handler.
+        process.nextTick(() => {
+            txn!.onerror!(new Event("we-ignore-this"));
+        });
+
+        await expect(() => promise).rejects.toThrow("selectQuery failed for client_options");
+    });
+
     it("Should load presence events on startup", async () => {
         // 1. Create idb database
         const indexedDB = new IDBFactory();

src/store/indexeddb-local-backend.ts

@@ -61,6 +61,7 @@ const VERSION = DB_MIGRATIONS.length;
  * Return the data you want to keep.
  * @returns Promise which resolves to an array of whatever you returned from
  * resultMapper.
+ * @throws If there was an error completing the query.
  */
 function selectQuery<T>(
     store: IDBObjectStore,
@@ -71,7 +72,7 @@ function selectQuery<T>(
     return new Promise((resolve, reject) => {
         const results: T[] = [];
         query.onerror = (): void => {
-            reject(new Error("Query failed: " + query.error?.name));
+            reject(new Error(`selectQuery failed for ${store.name}`, { cause: query.error }));
         };
         // collect results
         query.onsuccess = (): void => {