Null provider and failing early

[Andrew Wilkins]

On Tue, Aug 27, 2013 at 8:44 AM, Tim Penhey <tim.penhey at canonical.com>wrote:

> Perhaps we should have a sanity-check type callback into the provider
> with the constraints at the time we want to add a machine.  This would
> give the null provider the early fail mechanism, and could also allow
> other providers to error if people as asking for constraints that really
> don't make sense.
>

I'm sort of thinking out aloud here: maybe this could used for checking
environment-specific constraints too? Some kind of
"VerifyMachineConstraints" method that will ensure your add-machine/--to
constraints are valid for the current provider/environment. In this case
constraints may be nil, but the null provider would just always return
false.
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