@Override
        public void onProgress(ProgressToken token) {
            super.onProgress(token);
            builder.append("onProgress : ");
            builder.append(token.getActiveBucketCount()).append(" active, ");
            builder.append(token.getPendingBucketCount()).append(" pending, ");
            builder.append(token.getFinishedBucketCount()).append(" finished, ");
            builder.append(token.getTotalBucketCount()).append(" total\n");
            if (exceptionOnProgress != null) {
                throw exceptionOnProgress;
            }
        }
Test that using an id.user=foo selection only tries to visit a single
superbucket for that user.
documentapi/src/test/java/com/yahoo/documentapi/messagebus/MessageBusVisitorSessionTestCase.java