Fix inventory cache interface

[s-hertel]

SUMMARY

There are a number of problems with the current cache interface, the primary ones being that the developer interface is terrible and it only handles file-backed cache plugins.

Some of the interface problems are that the cache was difficult to set up, get() was expected to raise a KeyError, and the cache had to be set a particular way to be reflected in the backing plugin.

This allows self._cache to be used as a dictionary - e.g. if you want a KeyError, use __getattr__ instead of get. It also updates the backing cache plugin with the value of self._cache if it has been modified after parse is called so the developer doesn't need to worry about the discrepancy between the cache object being exposed (a dictionary) and the permanent cache store (CacheModule).

Added tests and developer documentation.

ISSUE TYPE

• Bugfix Pull Request
• Feature Pull Request

[ansibot]

cc @acozine
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[s-hertel]

Rebased for porting guide conflict.

[s-hertel]

@FutureTecSystems It looks like you have some valuable pieces in lib/ansible/plugins/inventory/foreman.py in that PR. I think it might be out of the scope of this pull request. I've tried to limit my changes to only those necessary for getting cache plugins to be functional with inventory plugins, so optimizing foreman may be better as a follow up PR.

[acozine]

Thanks @s-hertel for creating and updating the docs for this. Last docs changes suggested.

[AlanCoding]

I'm hitting this message

[DEPRECATION WARNING]: InventoryModule should utilize self._cache as a dict 
instead of self.cache. To set the self._cache dictionary, use self._cache[key] 
= value instead of self.cache.set(key, value). To force update the underlying 
cache plugin with the contents of self._cache before parse() is complete, call 
self.set_cache_plugin and it will use the self._cache dictionary to update the 
cache plugin. This feature will be removed in version 2.12. Deprecation 
warnings can be disabled by setting deprecation_warnings=False in ansible.cfg.

with the openstack inventory plugin.

What does this mean? Is this automatically worthy of creating a new issue in your issue queue, since it looks like the message is directed at the maintainers of the inventory plugin itself, and the openstack inventory plugin is a part of Ansible.