We have recently upgraded to foreman 2.3.2 and somehow we also got fog-libvirt 0.8.0 during that upgrade. Now, listing the VM details fails for some of our VMs with the following exception:

Backtrace for 'Failed to fetch vm information' error (ActionView::Template::Error): undefined method `allocation' for nil:NilClass
 | /usr/share/foreman/app/views/compute_resources_vms/show/_libvirt.html.erb:46:in `block in _app_views_compute_resources_vms_show__libvirt_html_erb___3550598388922153960_47210195739420'
 | /usr/share/foreman/app/views/compute_resources_vms/show/_libvirt.html.erb:43:in `each'
 | /usr/share/foreman/app/views/compute_resources_vms/show/_libvirt.html.erb:43:in `_app_views_compute_resources_vms_show__libvirt_html_erb___3550598388922153960_47210195739420'
 ...

We are running with ruby 2.5 on Ubuntu 18.04.

I've discovered that this bug happens, when a VM has a block disk in the XML as follows:

<disk type="block" ...>
  ...
  <source dev="/foo/...">
</disk>

If I modify the XML and replace the "block" type with a "file" type, it works. The XML would look like this:

<disk type="file" ...>
  ...
  <source file="/foo/...">
</disk>
`

I have also determined that the issue happens due to the following change in fog-libvirt: https://github.com/fog/fog-libvirt/commit/f9df2f8d5e3c7008b33e8e642e1c62677e95f36f#diff-202c61eaa5442b1562357d52fe8d9ccfc84d82ff3b24439e8ec3db10bacf7b38R73

More specifically, it happens because the no-match return value of get_volume() has been changed from "{}" to "nil". If I change this return value back to "{}" the exception disappears and the VM details are shown in the webinterface again. Though, even then, I can only see real data for the "type=file" disks and I get some default/incorrect data for the "type=block" disk. Alternatively, I can change the return value of list_volumes() from the same file to "[{}]" in which case I get default/incorrect data for all disks of the VM, but at least it works. If that return value is hardcoded to "[]" I get the same exception no matter what get_volume() returns. I expect that there is some additional processing between the linked file and the template that raises the exception, but I cannot find it.

Note that there are two workarounds for this issue: 1) switch the VM disk from "block" to "file" or 2) "fix" the return value of get_volume(). I'm not sure what a proper fix would look like.

If you need any further information to reproduce this issue or if you want me to test any fixes, please do not hesitate to ask.