Add LocalStorageCryptoStore

To avoid throwing away all the data for anyone running firefox in
one of the modes where indexedDB is broken.

src/crypto/store/indexeddb-crypto-store.js

@@ -16,6 +16,7 @@ limitations under the License.
 
 import Promise from 'bluebird';
 
+import LocalStorageCryptoStore from './localStorage-crypto-store';
 import MemoryCryptoStore from './memory-crypto-store';
 import * as IndexedDBCryptoStoreBackend from './indexeddb-crypto-store-backend';
 
@@ -93,7 +94,12 @@ export default class IndexedDBCryptoStore {
         }).catch((e) => {
             console.warn(
                 `unable to connect to indexeddb ${this._dbName}` +
-                    `: falling back to in-memory store: ${e}`,
+                    `: falling back to localStorage store: ${e}`,
+            );
+            return new LocalStorageCryptoStore();
+        }).catch((e) => {
+            console.warn(
+                `unable to open localStorage: falling back to in-memory store: ${e}`,
             );
             return new MemoryCryptoStore();
         });

src/crypto/store/localStorage-crypto-store.js

@@ -0,0 +1,59 @@
+/*
+Copyright 2017 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';
+import MemoryCryptoStore from './memory-crypto-store.js';
+
+/**
+ * Internal module. Partial localStorage backed storage for e2e.
+ * This is not a full crypto store, just the in-memory store with
+ * some things backed by localStorage. It exists because indexedDB
+ * is broken in Firefox private mode or set to, "will not remember
+ * history".
+ * 
+ *
+ * @module
+ */
+
+const E2E_PREFIX = "crypto.";
+const KEY_END_TO_END_ACCOUNT = E2E_PREFIX + "account";
+
+/**
+ * @implements {module:crypto/store/base~CryptoStore}
+ */
+export default class LocalStorageCryptoStore extends MemoryCryptoStore {
+    constructor() {
+        super();
+        this.store = global.localStorage;
+    }
+
+    /**
+     * Delete all data from this store.
+     *
+     * @returns {Promise} Promise which resolves when the store has been cleared.
+     */
+    deleteAllData() {
+        this.store.removeItem(KEY_END_TO_END_ACCOUNT);
+        return Promise.resolve();
+    }
+
+    endToEndAccountTransaction(func) {
+        const account = this.store.getItem(KEY_END_TO_END_ACCOUNT);
+        return Promise.resolve(func(account, (newData) => {
+            this.store.setItem(KEY_END_TO_END_ACCOUNT, newData);
+        }));
+    }
+}