Added unit tests for ClusterRequirer interface class

[lorenzo-medici]

Issue

Missing test coverage for src/coordinated_workers/interface.py in class ClusterRequirer

Solution

Create additional tests with invalid databags, specifically when the cluster relation changes

Context

The ClusterProvider object is a relation endpoint wrapper for the 'cluster' relation interface.
That piece of code is responsible for walking through all units related over 'cluster' and collect a piece of data from their databags.

Testing Instructions

Run tox -e unit

Upgrade Notes

No code changes, only effect is increased test coverage.

[lorenzo-medici]

I forgot to run the linter before committing, I have already fixed everything locally and will push the changes together with the fixes from the review.

[PietroPasotti]

In addition to the issues I commented about, if I understand these tests correctly, you might have just found a bug.

I'm not sure why there should be a behavioural difference between 'invalid local app data' and 'invalid local unit data' when it comes to emitting 'cluster removed' events.

Could you mash those two tests into one, parametrize it with an app boolean field, depending on which value you choose what to patch and adapt your assertion?
And make a note that the assertion special-casing should not be necessary.

@lucabello do you agree that this is an issue?

Note for @lorenzo-medici: it will be out of scope for you and this PR to fix the bug, I'm just wondering if we should be opening a new one to track or not.

[lorenzo-medici]

I merged the two unit tests into one with the app parameter making sure to leave a note regarding the difference in behavior.
I also applied the review suggestions.

[PietroPasotti]

one final nit

[PietroPasotti]

much better! last pass :)

[lorenzo-medici]

Is this failure my fault? All tests pass locally, and the failure point in the GH workflow is

/home/runner/work/cos-lib/cos-lib/src/charm.py
  /home/runner/work/cos-lib/cos-lib/src/charm.py:27:48 - error: "DatabagModel" is not exported from module "cosl.coordinated_workers.interface"
    Import from "cosl.interfaces.utils" instead (reportPrivateImportUsage)

which I don't know how to fix as DatabagModel is always imported from cosl.interfaces.utils.

[PietroPasotti]

Is this failure my fault? All tests pass locally, and the failure point in the GH workflow is

/home/runner/work/cos-lib/cos-lib/src/charm.py
  /home/runner/work/cos-lib/cos-lib/src/charm.py:27:48 - error: "DatabagModel" is not exported from module "cosl.coordinated_workers.interface"
    Import from "cosl.interfaces.utils" instead (reportPrivateImportUsage)

which I don't know how to fix as DatabagModel is always imported from cosl.interfaces.utils.

No that's a recent change in cos-lib, it could be that the tempo coordinator's integration tests are broken because of it.