Null provider and failing early

[David Cheney]

I think they should be ignored, but preserved.

On Tue, Aug 27, 2013 at 11:40 AM, Tim Penhey <tim.penhey at canonical.com> wrote:
> On 27/08/13 11:50, Andrew Wilkins wrote:
>> On Tue, Aug 27, 2013 at 8:44 AM, Tim Penhey <tim.penhey at canonical.com
>> <mailto: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.
>
> This is exactly what I'm thinking as well, but I had forgotten about the
> provider specific constraints.
>
> It does raise the question of what should happen if a provider specific
> constraint is passed to a provider that can't handle it.  My suggestion
> is we fail.
>
> Tim
>
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