task_events_mgr: task events should not be able to re-wind task state.

[oliver-sanders]

• Closes data store: task state regression from delayed poll response #7269
• This fixes a bug where a delayed poll notification could cause a final-state task to re-wind to submitted/running.

This fixes the bug, however, it does so by reverting a long-standing behaviour introduced in #2638 (we've been doing it this way ever since we dropped the poll-to-confirm approach).

I'm not sure why we would want to allow task state to re-wind in the event of FLAG_POLLED but not FLAG_RECEIVED, FLAG_INTERNAL might make sense, however, due to implied task state changes, the message will already have been processed and the children spawned, so it's not clear that this logic was required in any case?

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.

[MetRonnie]

From my understanding, if process_message() returns True (indicating an attempt at a backwards state change) then the scheduler will poll the task to confirm. By removing the check for FLAG_RECEIVED is there now the potential for an endless polling loop where the poll always tries to take the task to an earlier state? (It seems unlikely to happen though)

In any case I think this poll to confirm is now useless and ought to be removed?

cylc-flow/cylc/flow/scheduler.py

Lines 990 to 998 in 8b5257a

	if self.task_events_mgr.process_message(
	itask, tm.severity, tm.message, tm.event_time,
	self.task_events_mgr.FLAG_RECEIVED, tm.job_id.submit_num
	):
	should_poll = True
	if should_poll:
	to_poll_tasks.append(itask)
	if to_poll_tasks:
	self.task_job_mgr.poll_task_jobs(to_poll_tasks)