[openstack-dev] [nova][libvirt] Non-readonly connection to libvirt in unit tests

Daniel P. Berrange berrange at redhat.com
Thu Aug 21 15:23:17 UTC 2014

On Thu, Aug 21, 2014 at 11:14:33AM -0400, Solly Ross wrote:
> (reply inline)
> ----- Original Message -----
> > From: "Daniel P. Berrange" <berrange at redhat.com>
> > To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org>
> > Sent: Thursday, August 21, 2014 11:05:18 AM
> > Subject: Re: [openstack-dev] [nova][libvirt] Non-readonly connection to libvirt in unit tests
> > 
> > On Thu, Aug 21, 2014 at 10:52:42AM -0400, Solly Ross wrote:
> > > FYI, the context of this is that I would like to be able to test some
> > > of the libvirt storage pool code against a live file system, as we
> > > currently test the storage pool code.  To do this, we need at least to
> > > be able to get a proper connection to a session daemon.  IMHO, since
> > > these calls aren't "expensive", so to speak, it should be fine to have
> > > them run against a real libvirt.
> > 
> > No it really isn't OK to run against the real libvirt host system when
> > in the unit tests. Unit tests must *not* rely on external system state
> > in this way because it will lead to greater instability and unreliability
> > of our unit tests. If you want to test stuff against the real libvirt
> > storage pools then that becomes a functional / integration test suite
> > which is pretty much what tempest is targetting.
> That's all well and good, but we *currently* manipulates the actual file
> system manually in tests.  Should we then say that we should never manipulate
> the actual file system either?  In that case, there are some tests which
> need to be refactored.

Places where the tests manipulate the filesystem though should be doing
so in an isolated playpen directory, not in the "live" location where
a deployed nova runs, so that's not the same thing.

> > > > So If we require libvirt-python for tests and that requires
> > > > libvirt-bin, what's stopping us from just removing fakelibvirt since
> > > > it's kind of useless now anyway, right?
> > > 
> > > The thing about fakelibvirt is that it allows us to operate against
> > > against a libvirt API without actually doing libvirt-y things like
> > > launching VMs.  Now, libvirt does have a "test:///default" URI that
> > > IIRC has similar functionality, so we could start to phase out fake
> > > libvirt in favor of that.  However, there are probably still some
> > > spots where we'll want to use fakelibvirt.
> > 
> > I'm actually increasingly of the opinion that we should not in fact
> > be trying to use the real libvirt library in the unit tests at all
> > as it is not really adding any value. We typically nmock out all the
> > actual API calls we exercise so despite "using" libvirt-python we
> > are not in fact exercising its code or even validating that we're
> > passing the correct numbers of parameters to API calls. Pretty much
> > all we really relying on is the existance of the various global
> > constants that are defined, and that has been nothing but trouble
> > because the constants may or may not be defined depending on the
> > version.
> Isn't that what 'test:///default' is supposed to be?  A version of libvirt
> with libvirt not actually touching the rest of the system?

Yes, that is what it allows for, however, even if we used that URI we
still wouldn't be actually exercising any of the libvirt code in any
meaningful way because our unit tests mock out all the API calls that
get touched. So using libvirt-python + test:///default URI doesn't
really seem to buy us anything, but it does still mean that developers
need to have libvirt installed in order to run  the unit tests. I'm
not convinced that is a beneficial tradeoff.

> > The downside of fakelibvirt is that it is a half-assed implementation
> > of libvirt that we evolve in an adhoc fashion. I'm exploring the idea
> > of using pythons introspection abilities to query the libvirt-python
> > API and automatically generate a better 'fakelibvirt' that we can
> > guarantee to match the signatures of the real libvirt library. If we
> > had something like that which we had more confidence in, then we could
> > make the unit tests use that unconditionally. This would make our unit
> > tests more reliable since we would not be suspectible to different API
> > coverage in different libvirt module versions which have tripped us up
> > so many times

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