human
Complete the below
def _wait_for_vhd_coalesce(session, instance, sr_ref, vdi_ref, original_parent_uuid):

    ''''Spin until the parent VHD is coalesced into its parent VHD
    Before coalesce:
    * original_parent_vhd
    * parent_vhd
    snapshot
    After coalesce:
    * parent_vhd
    snapshot'
    '''
 Write me a the Python program you requested?
assistant
#Here is the Python program you requested.
    def _another_child_vhd():
        if (not original_parent_uuid):
            return False
        vdi_uuid = session.call_xenapi('VDI.get_record', vdi_ref)['uuid']
        parent_vdi_uuid = _get_vhd_parent_uuid(session, vdi_ref)
        for (_ref, rec) in _get_all_vdis_in_sr(session, sr_ref):
            if ((rec['uuid'] != vdi_uuid) and (rec['uuid'] != parent_vdi_uuid) and (rec['sm_config'].get('vhd-parent') == original_parent_uuid)):
                return True
        return False
    if _another_child_vhd():
        parent_uuid = _get_vhd_parent_uuid(session, vdi_ref)
        parent_ref = session.call_xenapi('VDI.get_by_uuid', parent_uuid)
        base_uuid = _get_vhd_parent_uuid(session, parent_ref)
        return (parent_uuid, base_uuid)
    _scan_sr(session, sr_ref)
    max_attempts = CONF.xenapi_vhd_coalesce_max_attempts
    for i in xrange(max_attempts):
        _scan_sr(session, sr_ref)
        parent_uuid = _get_vhd_parent_uuid(session, vdi_ref)
        if (original_parent_uuid and (parent_uuid != original_parent_uuid)):
            LOG.debug(_("Parent    %(parent_uuid)s    doesn't    match    original    parent    %(original_parent_uuid)s,    waiting    for    coalesce..."), locals(), instance=instance)
        else:
            parent_ref = session.call_xenapi('VDI.get_by_uuid', parent_uuid)
            base_uuid = _get_vhd_parent_uuid(session, parent_ref)
            return (parent_uuid, base_uuid)
        greenthread.sleep(CONF.xenapi_vhd_coalesce_poll_interval)
    msg = (_('VHD    coalesce    attempts    exceeded    (%(max_attempts)d),    giving    up...') % locals())
    raise exception.NovaException(msg)