[Libvir] A 'mock' driver enabling application unit testing

[Daniel Veillard]

On Fri, May 26, 2006 at 07:35:37PM +0100, Daniel P. Berrange wrote:
> The attached patch adds a new driver which provides a 'mock' hypervisor
> connection. The idea behind this is to provide a completely predictable
> and isolated hypervisor implementation, which will facilitate creation 
> of unit tests in applications using libvirt.

  Sounds good on two counts:
    - this forces to clean up the code internally and test the driver based
      approach
    - testing testing testing :-)
 thanks !

> The code attached is not a complete implementation, providing support only 
> for connecting, listing domains, getting a domain by id/uuid/name, getting
> domain info, getting node info. Further work will hook up methods for 
> creating domains, exporting XML & the other various libvirt APIs. The driver
> is intended to operate as a black box, closed system - the only interaction
> an application will have is via the libVirt entry points. It starts off with
> a single domain present, and fixed node info, and updates domain CPU time
> based on elapsed wall clock time. When the domain create API is hooked up
> this will let apps create new dummy domains within the context of the process. 

  Okay, simplistic but a good first start :-)

> As I said, I delibrately implemented this driver to only have process local
> state. When the process shuts down all state is discarded. This keeps application
> usage very simple. Longer term I think it would be useful to have a 2nd, dynamic,
> which would allow fault injection, cross-process interaction & more to allow
> advanced scripting of a test scenario. I don't have time to create such a driver
> myself though, so I stuck with the simplest possible impl which will allow 95%
> of application unit testing needs to be satisfied.

  Understandable, but I would really like to be able to go one step behind
and be able to provide a runtime scenario in a config file.

> To use this driver, simply pass 'TestSimple' as the name parameter to 
> virConnectOpen / virConnectOpenReadonly instead of NULL. If you are using
> 'virsh' then my previous patch will let you call 'virsh --connect TestSimple'
> I've also tested this with the 'gnome-vm-applet' panel applet.

  That's probably the only part where I have a slight concern, shouldn't
we just use something like "test:///default" for this simple fallback case
and probably something like "test:///path/to/config" in the more general 
case where there will be some state. But it really should not be a big deal.

> The only problem I have found is that the 'xend_internal.c' driver will
> always return success from its 'xenDaemonOpen' method, regardless of 
> whether there is actually a Xen Daemon present. So when using the test
> driver, every method will first try to ue the XenD driver impl, fail (printing
> an error message) and then go onto use the test driver. The xenDaemonOpen
> method really needs to be fixed to only succeed when Xen is actually present.

  Right this need some cleanup I think. I though I had left a TODO there.

> Or perhaps it should only try to run when the 'name' passed to virConnectOpen
> is NULL or 'Xen' - ie be a no op if the name is 'TestSimple' / QEMU / any
> other driver implementation.
> 
> I'm not sure whether we want to commit this to the libvirt codebae just,
> since there is a fair bit more work to be done to hook up additional
> methods.

  Do you think you will need to change radically that code when completing
it ? Looks to me it's just additional code, so what's there should be commited
because it's code you expect to be present in the end. So please commit
(I think your CVS account works, right ?)

Daniel

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