Updating domains definitions via API

[Darragh Bailey]

On Fri, 13 May 2022 at 00:17, Darragh Bailey <daragh.bailey at gmail.com>
wrote:

> Hi,
>
> On Thu 12 May 2022, 21:34 Laine Stump, <laine at redhat.com> wrote:
>
>> The virDomainDefineXMLFlags API (and also the older/deprecated
>> virDomainDefineXML API) doesn't require that the domain first be
>> undefined (with one notable exception - see below[*]). If you define a
>> domain that already exists with the same name and uuid, then the effect
>> is to "redefine" (or "update" if you prefer) the existing domain of that
>> name. If the domain is currently active, then the changes will take
>> effect the next time the domain is shut down ("Destroy"ed in libvirt API
>> parlance) and re-started.
>>
>
Unfortunately trying to call this via ruby-libvirt doesn't appear to behave
as expected. It appears that if I add an nvram element without a loader
element to the os block, the following code block will execute without
issue but also without changing the domain XML:

# Apply XML changes directly
if descr_changed
  begin
    env[:ui].info("Updating domain definition due to configuration change")
    new_descr = String.new
    xml_descr.write new_descr
    # env[:machine].provider.driver.connection.client provides access to
the ruby-libvirt connection object
    libvirt_domain =
env[:machine].provider.driver.connection.client.define_domain_xml(new_descr,
1)
    domain =
env[:machine].provider.driver.connection.servers.get(env[:machine].id.to_s)
  rescue Fog::Errors::Error => e
    raise Errors::UpdateServerError, error_message: e.message
  end
end

I've by-passing the fog-libvirt to call the ruby-libvirt connection object
directly to try and avoid any unexpected interference when testing the API
call out e.g.
https://libvirt.org/ruby/api/Libvirt/Connect.html#method-i-define_domain_xml

So not only is there no exception thrown if the XML change is ignored, I've
noticed there doesn't appear to be an easy way to check using the API
either, in that
https://libvirt.org/ruby/api/Libvirt/Domain.html#method-i-updated-3F
doesn't indicate that the domain has been updated whether I call it on the
libvirt_domain object returned, or an instance from before the
define_domain_xml call is made.

It appears that the only way is to perform a call to get the xml definition
of the libvirt_domain object returned in the above block and see if that
matches the xml that was sent, if not, error.

Is this the expected usage of the API? Or should the call to
define_domain_xml raise an exception if it cannot update the domain XML? as
opposed to a schema validation error which does appear to be detected when
I did something stupid.

--
Darragh
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