data_store: fix traceback

[oliver-sanders]

Traceback spotted in the wild.

Found this traceback in a scheduler log. It happened immediately after workflow restart.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included - no unreproducible
• Changelog entry included - no unreproducible and unreported
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

Default to a tree_depth of 0.

@dwsutherland does this make sense?

[dwsutherland]

Not sure how
config.get_first_parent_ancestors(pruned=True).values()
could be no value..
Maybe some sort of inheritance error:

    def get_first_parent_ancestors(self, pruned=False):
        if pruned:
            # prune non-task namespaces from ancestors dict
            pruned_ancestors = {}
            for key, val in self.runtime['first-parent ancestors'].items():
                if key not in self.taskdefs:
                    continue
                pruned_ancestors[key] = val
            return pruned_ancestors
        else:
            return self.runtime['first-parent ancestors']

        for name in self.cfg['runtime']:
            try:
                self.runtime['linearized ancestors'][name] = c3.mro(name)
                self.runtime['first-parent ancestors'][name] = (
                    c3_single.mro(name))
            except RecursionError:
                raise WorkflowConfigError(
                    "circular [runtime] inheritance?"
                ) from None
            except Exception as exc:
                # catch inheritance errors
                # TODO - specialise MRO exceptions
                raise WorkflowConfigError(str(exc)) from None

I assume it's always 1 minimum otherwise (root)...

This fix makes sense as a workaround...

[oliver-sanders]

Ok, we don't know why this is happening, but it's easy enough to defend against.