Always check backup validity, even during enabling

This ensure we run the full backup validity check even when enabling (rather
than assuming we've signed things correctly) to ensure any problem are reported
right away.

src/client.js

@@ -1506,11 +1506,15 @@ MatrixClient.prototype.createKeyBackupVersion = async function(info) {
         undefined, "POST", "/room_keys/version", undefined, data,
         {prefix: httpApi.PREFIX_UNSTABLE},
     );
-    this.enableKeyBackup({
-        algorithm: info.algorithm,
-        auth_data: info.auth_data,
-        version: res.version,
-    });
+
+    // We could assume everything's okay and enable directly, but this ensures
+    // we run the same signature verification that will be used for future
+    // sessions.
+    await this.checkKeyBackup();
+    if (!this.getKeyBackupEnabled()) {
+        throw new Error("Key backup not usable even though we just created it");
+    }
+
     return res;
 };
 

src/crypto/index.js

@@ -952,8 +952,7 @@ Crypto.prototype.setTrustedBackupPubKey = async function(trustedPubKey) {
  */
 Crypto.prototype.checkKeyBackup = async function() {
     this._checkedForBackup = false;
-    const returnInfo = await this._checkAndStartKeyBackup();
-    return returnInfo;
+    return this._checkAndStartKeyBackup();
 };
 
 /**