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On 08/08/14 11:04, Matthew Booth wrote:
> On 07/08/14 18:54, Kevin L. Mitchell wrote:
>> On Thu, 2014-08-07 at 17:46 +0100, Matthew Booth wrote:
>>>> In any case, the operative point is that CONF.<attribute> must
>>> always be
>>>> evaluated inside run-time code, never at module load time.
>>>
>>> ...unless you call register_opts() safely, which is what I'm
>>> proposing.
>>
>> No, calling register_opts() at a different point only fixes the import
>> issue you originally complained about; it does not fix the problem that
>> the configuration option is evaluated at the wrong time.  The example
>> code you included in your original email evaluates the configuration
>> option at module load time, BEFORE the configuration has been loaded,
>> which means that the argument default will be the default of the
>> configuration option, rather than the configured value of the
>> configuration option.  Configuration options must be evaluated at
>> RUN-TIME, after configuration is loaded; they must not be evaluated at
>> LOAD-TIME, which is what your original code does.
> 
> Ah, thanks, Kevin. The pertinent information is that the config has not
> been loaded at module import time, and you'll therefore always get a
> default.

Ironically, the specific instance which prompted this investigation[1]
is in a driver. As drivers are imported dynamically after the config has
been loaded, using a config variable in import context will actually
work as intended. Relying on that behaviour seems pretty nasty to me,
though. We could probably do with a guideline on this.

I did a quick scan of Nova and found 12 instances which aren't in a
driver which appear to be broken due to this at first glance, resulting
in 11 config variables whose specified values appear to be ignored. Bug
here:

https://bugs.launchpad.net/nova/+bug/1354403

Matt

[1]
https://review.openstack.org/#/c/104145/14/nova/virt/vmwareapi/vmware_images.py
-- 
Matthew Booth
Red Hat Engineering, Virtualisation Team

Phone: +442070094448 (UK)
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