Add an IndexedDB existence check to the main store

This will be useful for future storage diagnostics as part of
https://github.com/vector-im/riot-web/issues/9271.

src/indexeddb-helpers.js

@@ -0,0 +1,50 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import Promise from 'bluebird';
+
+/**
+ * Check if an IndexedDB database exists. The only way to do so is to try opening it, so
+ * we do that and then delete it did not exist before.
+ *
+ * @param {Object} indexedDB The `indexedDB` interface
+ * @param {string} dbName The database name to test for
+ * @returns {boolean} Whether the database exists
+ */
+export function exists(indexedDB, dbName) {
+    return new Promise((resolve, reject) => {
+        let exists = true;
+        const req = indexedDB.open(dbName);
+        req.onupgradeneeded = function(ev) {
+            // Since we did not provide an explicit version when opening, this event
+            // should only fire if the DB did not exist before at any version.
+            exists = false;
+        };
+        req.onblocked = () => reject();
+        req.onsuccess = function() {
+            if (!exists) {
+                // The DB did not exist before, but has been created as part of this
+                // existence check. Delete it now to restore previous state. Delete can
+                // actually take a while to complete in some browsers, so don't wait for
+                // it. This won't block future open calls the might issue next to properly
+                // set up the DB.
+                indexedDB.deleteDatabase(dbName);
+            }
+            resolve(exists);
+        };
+        req.onerror = ev => reject(ev.target.error);
+    });
+}

src/store/indexeddb-local-backend.js

@@ -18,6 +18,7 @@ limitations under the License.
 import Promise from 'bluebird';
 import SyncAccumulator from "../sync-accumulator";
 import utils from "../utils";
+import * as IndexedDBHelpers from "../indexeddb-helpers";
 
 const VERSION = 3;
 
@@ -132,6 +133,10 @@ const LocalIndexedDBStoreBackend = function LocalIndexedDBStoreBackend(
     this._isNewlyCreated = false;
 };
 
+LocalIndexedDBStoreBackend.exists = function(indexedDB, dbName) {
+    dbName = "matrix-js-sdk:" + (dbName || "default");
+    return IndexedDBHelpers.exists(indexedDB, dbName);
+};
 
 LocalIndexedDBStoreBackend.prototype = {
     /**

src/store/indexeddb.js

@@ -111,6 +111,10 @@ const IndexedDBStore = function IndexedDBStore(opts) {
 };
 utils.inherits(IndexedDBStore, MemoryStore);
 
+IndexedDBStore.exists = function(indexedDB, dbName) {
+    return LocalIndexedDBStoreBackend.exists(indexedDB, dbName);
+};
+
 /**
  * @return {Promise} Resolved when loaded from indexed db.
   */