Ensure that xtrigger OR fails validation

[wxtim]

Closes #6771

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 (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• 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.

[hjoliver]

This fails here and on master, but I wonder if it needs to?

@x & (a | b) => c

$ cylc val .
WorkflowConfigError: Xtriggers cannot be used in conditional graph expressions:
@x&(a:succeeded|b:succeeded) => c

It's been a long time since I looked at - and probably wrote - the associated code, so I'm not sure of the reason for the restriction anymore, and whether it should apply to this sort of expression (and I'm out of time today). But possibly it should not fail because it's equivalent to this:

@x => c
a | b => c

[oliver-sanders]

This fails here and on master, but I wonder if it needs to?

If this PR isn't changing behaviour, suggest moving this to another issue. Opened #6949.

[oliver-sanders]

As Hillary pointed out, there is already logic intended to catch xtriggers used in conditional expressions:

cylc-flow/cylc/flow/scripts/validate.py

Lines 196 to 210 in 700eb02

	# force trigger evaluation now
	try:
	itask.state.prerequisites_eval_all()
	except TriggerExpressionError as exc:
	err = str(exc)
	if '@' in err:
	print(
	f"ERROR, {name}: xtriggers can't be in conditional"
	f" expressions: {err}",
	file=sys.stderr,
	)
	else:
	print(
	'ERROR, %s: bad trigger: %s' % (name, err), file=sys.stderr
	)

However, this check isn't catching situations where there are no tasks on the LHS, only xtriggers.

Sidenote: Strangely, this code is in cylc.flow.validate (which means you can bypass this error using cylc install; cylc play). I suspect this is purely historical, this should really be in cylc.flow.config.

If the new test covers all of the logic of the old one, suggest ripping the old check out. There are probably tests for the old check which could do with being combined / erased.

[oliver-sanders]

I ran a bunch of our workflows through cylc validate on this branch, no spurious failures :)

However, beyond @wall_clock xtriggers are not used at our site, might be worth getting Hillary to make sure it doesn't cause any false positives with NIWA workflows?

[hjoliver]

However, beyond @wall_clock xtriggers are not used at our site, might be worth getting Hillary to make sure it doesn't cause any false positives with NIWA workflows?

Unfortunately, due to enforced home dir privacy here, I can't see our workflows.