Comment 34 for bug 1696125

Something to know about detaching from a guest transient domain is that it's an asynchronous request to the guest OS [1], which the guest OS can choose to ignore (e.g. if it has a file open or is otherwise busy).

"Beware that depending on the hypervisor and device type, detaching a device from a running domain may be asynchronous. That is, calling virDomainDetachDeviceFlags may just request device removal while the device is actually removed later (in cooperation with a guest OS)."

So it seems like what you're seeing is the guest OS ignoring the request to detach for some reason and Nova keeps retrying and eventually gives up. The retry simply requests the detach again and then checks for existence of the device. If the device remains, it tries again, until the max retry count is reached.

[1] https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainDetachDeviceFlags