Updating domains definitions via API

[Laine Stump]

On 5/14/22 3:23 PM, Darragh Bailey wrote:
> 
> On Fri, 13 May 2022 at 00:17, Darragh Bailey <daragh.bailey at gmail.com 
> <mailto:daragh.bailey at gmail.com>> wrote:
> 
>     Hi,
> 
>     On Thu 12 May 2022, 21:34 Laine Stump, <laine at redhat.com
>     <mailto: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 
> <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 
> <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.

Caveat - I'm completely unfamiliar with ruby and the libvirt-ruby API 
bindings.

If there is a problem that causes the domain config to not be updated, 
libvirt will return an error. So I would suspect one of the two things 
is happening:

1) there may be a problem in the libvirt-ruby bindings that causes the 
error reported by the call (in whatever C code is behind the ruby 
bindings) to libvirt to be properly propagated to ruby. I would hope 
this isn't the case, but "bugs happen", so it should be considered as a 
possibility.

2) As I said in my earlier mail, any changes that are made will take 
effect the next time the domain is destroyed and restarted. This also 
means that the changes won't be reflected in the "live/status" XML of 
the domain until that time. If you want to see the new configuration 
after you've made changes, you should add the VIR_DOMAIN_XML_INACTIVE 
flag when requesting the domain XML. Possibly you haven't included this 
flag, and that's why you think that your change hasn't taken effect?