def _list_backing_images(self):
        inuse_images = []
        for ent in os.listdir(CONF.instances_path):
            if ent in self.instance_names:
                LOG.debug('%s is a valid instance name', ent)
                disk_path = os.path.join(CONF.instances_path, ent, 'disk')
                if os.path.exists(disk_path):
                    LOG.debug('%s has a disk file', ent)
                    try:
                        backing_file = libvirt_utils.get_disk_backing_file(
                            disk_path)
                    except processutils.ProcessExecutionError:
                        # (for bug 1261442)
                        if not os.path.exists(disk_path):
                            LOG.debug('Failed to get disk backing file: %s',
                                      disk_path)
                            continue
                        else:
                            raise
                    LOG.debug('Instance %(instance)s is backed by '
                              '%(backing)s',
                              {'instance': ent,
                               'backing': backing_file})

                    if backing_file:
                        backing_path = os.path.join(
                            CONF.instances_path,
                            CONF.image_cache_subdirectory_name,
                            backing_file)
                        if backing_path not in inuse_images:
                            inuse_images.append(backing_path)

                        if backing_path in self.unexplained_images:
                            LOG.warning(_LW('Instance %(instance)s is using a '
                                         'backing file %(backing)s which '
                                         'does not appear in the image '
                                         'service'),
                                        {'instance': ent,
                                         'backing': backing_file})
                            self.unexplained_images.remove(backing_path)
        return inuse_images
List the backing images currently in use.
dataset/ETHPy150Open BU-NU-CLOUD-SP16/Trusted-Platform-Module-nova/nova/virt/libvirt/imagecache.py/ImageCacheManager._list_backing_images