Handle empty containers in zarr chunk checks

[chrisroat]

• Closes write/read to zarr subtly changes array with non-dim coord #4084
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

Continuation of #5019, which closed during the master/main switch.

[chrisroat]

@jhamman Ready for review, though not urgent.

Mostly, I'm curious if this the right way to go about solving the linked issue, or if there is something deeper in xarray to update.

[jhamman]

Thanks @chrisroat for the PR. This looks great. I do think we should add an entry in whats-new.rst though. Once that is done, I'll merge this.

[chrisroat]

In my original PR, I wrote:

In putting this together, I noted that open_zarr to re-read the data triggers this bug, while open_dataset(..., engine='zarr') does not. I'm not sure if my proposed fix is a band-aid, or if something in open_zarr is the real culprit.

I just want to be sure we think this is the correct fix, and there won't be any unintended consequences. I'm really not familiar with the interaction of chunks between zarr, dask, and xarray. I don't feel 100% sure this is the correct solution, though it fixes the immediate solution.

[jhamman]

Thanks @chrisroat - let's see if @rabernat or @shoyer has an opinion here then.

[chrisroat]

Is this change still of potential value?

[rabernat]

@chrisroat - thanks so much for your contribution! Sorry that I never reviewed this PR until now.

LGTM.

[rabernat]

Just for my understanding, could you explain the difference in behavior between var_chunks is None and not var_chunks?

[chrisroat]

I believe I hit this when var_chunks was an empty list instead of None:

>>> var_chunks=[]
>>> not var_chunks
True
>>> var_chunks is None
False
>>> 

I do not know whey the use case I mention above uses an empty list (while the code was designed to expect None). I am not sure if there would be undesired knock-on effects of this change. Thoughts?

I can still add a whats-new if that is desired.

[dcherian]

Thanks @chrisroat Sorry this took so long to get merged