Ignore errors from transaction rollbacks

app/lib/backend.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub_dartlang_org.backend;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:gcloud/db.dart';
@@ skipping 296 matching lines @@
 
         // Try to load all package versions, sort them by `sort_order` and
         // store them again.
         await _updatePackageSortIndex(package.key);
 
         return new PackageVersion(
             newVersion.package, newVersion.version,
             newVersion.pubspec.jsonString);
       } catch (error, stack) {
         _logger.warning('Error while committing: $error, $stack');
-        await T.rollback();
+
+        // This call might fail if the transaction has already been
+        // committed/rolled back or the transaction failed.
+        //
+        // In which case we simply ignore the rollback error and rethrow the
+        // original error.
+        try {
+          await T.rollback();
+        } catch (_) {}
         rethrow;
       }
     });
   }
 
   Future _updatePackageSortIndex(Key packageKey) async {
     try {
       _logger.info('Trying to update the `sort_order` field.');
       await db.withTransaction((Transaction T) async {
         List<models.PackageVersion> versions = await
@@ skipping 408 matching lines @@
 
   /// The GCS object name of an temporary object [guid] - excluding leading '/'.
   String tmpObjectName(String guid) => 'tmp/$guid';
 
   /// The http URL of a publicly accessable GCS object.
   String tarballObjectUrl(String package, String version) {
     var object = tarballObjectName(package, version);
     return 'https://storage.googleapis.com/${bucket}/${object}';
   }
 }