Emit an event for crypto store migration

src/client.js

@@ -391,6 +391,7 @@ MatrixClient.prototype.initCrypto = async function() {
     this.reEmitter.reEmit(crypto, [
         "crypto.roomKeyRequest",
         "crypto.roomKeyRequestCancellation",
+        "crypto.warning",
     ]);
 
     await crypto.init();

src/crypto/OlmDevice.js

@@ -166,7 +166,6 @@ OlmDevice.getOlmVersion = function() {
 
 OlmDevice.prototype._migrateFromSessionStore = async function() {
     // account
-    let migratedAccount = false;
     await this._cryptoStore.doTxn(
         'readwrite', [IndexedDBCryptoStore.STORE_ACCOUNT], (txn) => {
             this._cryptoStore.getAccount(txn, (pickledAccount) => {
@@ -175,18 +174,16 @@ OlmDevice.prototype._migrateFromSessionStore = async function() {
                     pickledAccount = this._sessionStore.getEndToEndAccount();
                     if (pickledAccount !== null) {
                         console.log("Migrating account from session store");
-                        migratedAccount = true;
                         this._cryptoStore.storeAccount(txn, pickledAccount);
                     }
                 }
             },
         );
     });
 
-    // only remove this once it's safely saved to the crypto store
-    if (migratedAccount) {
-        this._sessionStore.removeEndToEndAccount();
-    }
+    // remove the old account now the transaction has completed. Either we've
+    // migrated it or decided not to, either way we want to blow away the old data.
+    this._sessionStore.removeEndToEndAccount();
 
     // sessions
     const sessions = this._sessionStore.getAllEndToEndSessions();

src/crypto/index.js

@@ -33,6 +33,7 @@ const DeviceVerification = DeviceInfo.DeviceVerification;
 const DeviceList = require('./DeviceList').default;
 
 import OutgoingRoomKeyRequestManager from './OutgoingRoomKeyRequestManager';
+import IndexedDBCryptoStore from './store/indexeddb-crypto-store';
 
 /**
  * Cryptography bits
@@ -108,6 +109,24 @@ utils.inherits(Crypto, EventEmitter);
  * Returns a promise which resolves once the crypto module is ready for use.
  */
 Crypto.prototype.init = async function() {
+    const sessionStoreHasAccount = Boolean(this._sessionStore.getEndToEndAccount());
+    let cryptoStoreHasAccount;
+    await this._cryptoStore.doTxn(
+        'readonly', [IndexedDBCryptoStore.STORE_ACCOUNT], (txn) => {
+            this._cryptoStore.getAccount(txn, (pickledAccount) => {
+                cryptoStoreHasAccount = Boolean(pickledAccount);
+            });
+        },
+    );
+    if (sessionStoreHasAccount && !cryptoStoreHasAccount) {
+        // we're about to migrate to the crypto store
+        this.emit("crypto.warning", 'CRYPTO_WARNING_ACCOUNT_MIGRATED');
+    } else if (sessionStoreHasAccount && cryptoStoreHasAccount) {
+        // There's an account in both stores: an old version of
+        // the code has been run against this store.
+        this.emit("crypto.warning", 'CRYPTO_WARNING_OLD_VERSION_DETECTED');
+    }
+
     await this._olmDevice.init();
 
     // build our device keys: these will later be uploaded
@@ -1332,5 +1351,24 @@ class IncomingRoomKeyRequestCancellation {
  * @param {module:crypto~IncomingRoomKeyRequestCancellation} req
  */
 
+/**
+ * Fires when the app may wish to warn the user about something related
+ * the end-to-end crypto.
+ *
+ * Comes with a type which is one of:
+ * * CRYPTO_WARNING_ACCOUNT_MIGRATED: Account data has been migrated from an older
+ *       version of the store in such a way that older clients will no longer be
+ *       able to read it. The app may wish to warn the user against going back to
+ *       an older version of the app.
+ * * CRYPTO_WARNING_OLD_VERSION_DETECTED: js-sdk has detected that an older version
+ *       of js-sdk has been run against the same store after a migration has been
+ *       performed. This is likely have caused unexpected behaviour in the old
+ *       version. For example, the old version and the new version may have two
+ *       different identity keys.
+ *
+ * @event module:client~MatrixClient#"crypto.warning"
+ * @param {string} type One of the strings listed above
+ */
+
 /** */
 module.exports = Crypto;