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The nova/hooks.py infrastructure has been with us since early Nova. It's
currently only annotated on a few locations - 'build_instance',
'create_instance', 'delete_instance', and 'instance_network_info'. It's
got a couple of unit tests on it, but nothing that actually tests real
behavior of the hooks we have specified.

It does get used in the wild, and we do break it with changes we didn't
ever anticipate would impact it -
https://bugs.launchpad.net/nova/+bug/1518321

However, when you look into how that is used, it's really really odd and
fragile -
https://github.com/richm/rdo-vm-factory/blob/master/rdo-ipa-nova/novahooks.py#L248


    def pre(self, *args, **kwargs):
        # args[7] is the injected_files parameter array
        # the value is ('filename', 'base64 encoded contents')
        ipaotp = str(uuid.uuid4())
        ipainject = ('/tmp/ipaotp', base64.b64encode(ipaotp))
        args[7].extend(self.inject_files)
        args[7].append(ipainject)

In our continued quest on being more explicit about plug points it feels
like we should other document the interface (which means creating
stability on the hook parameters) or we should deprecate this construct
as part of a bygone era.

I lean on deprecation because it feels like a thing we don't really want
to support going forward, but I can go either way.

	-Sean

P.S. I'm starting to look at in tree functional testing for all of this,
in the event that we decide not to deprecate it. It's definitely made a
little hard by the way all exceptions are caught when hooks go wrong.

-- 
Sean Dague
http://dague.net