@Override
    public Committer closeForCommit() throws IOException {
        lock();
        try {
            closeAndUploadPart();
            return upload.snapshotAndGetCommitter();
        } finally {
            unlock();
        }
    }
@Test
    public void commitEmptyStreamShouldBeSuccessful() throws IOException {
        streamUnderTest.closeForCommit().commit();
    }