Provide Fake KV Store for Mocking

[jarred wilson]

It seems that we have run into several issues surrounding race conditions and the concurrency of tox when using the kv store provided by charm helpers. Examples of this can be found below:

https://bugs.launchpad.net/charm-neutron-openvswitch/+bug/1911430
https://bugs.launchpad.net/charm-hacluster/+bug/1908282
https://bugs.launchpad.net/charm-nova-cloud-controller/+bug/1908282

Can we provide a non-SQLite version of the kv for mocking as part of charm helpers? It should reduce the number of independent implementations across our charms.

[Cory Johns (johnsca)]

You can make it use an in-memory database by setting this environment var:

UNIT_STATE_DB=:memory:

Or by doing the following before kv() is called:

charmhelpers.core.unitdata._KV = charmhelpers.core.unitdata.Storage(':memory:')

Reference: https://charm-helpers.readthedocs.io/en/latest/api/charmhelpers.core.unitdata.html#charmhelpers.core.unitdata.Storage

[Danny Cocks (dannycocks)]

I've created a PR for this at https://github.com/juju/charm-helpers/pull/823

[Danny Cocks (dannycocks)]

After merging in https://github.com/juju/charm-helpers/pull/823 this should now be addressed for unittests.