Fix spawning of absolute-triggered tasks.

[hjoliver]

These changes close #5002

I've run the example out to 50 cycle points several times, with nothing strange happening

I haven't thought of a reliable way to reproduce the problem (which was more of an inefficiency + confusing warning than a balls-to-the-wall bug) in a short test. But codecov says the change is covered in a strictly technical sense at least.

Requirements 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.
• Already covered by existing tests.
• Appropriate change log entry included.
• No documentation update required.

[hjoliver]

Explanation:

Consider a task foo that depends only on an absolute trigger:

P1 = "init[3] => foo"

In principle, init[3] should spawn all foo instances, but that's not practically possible (there could be a large of infinite number of foo instances). So we have to autospawn foo instances as they were parentless, but make sure that none of them run before init[3] succeeds, or not spawn any foo until init[3] succeeds.

On current master, we spawn 3/foo after init:succeed, and then autospawn other foo instances (out to the runahead limit) as if they are parentless. However, that's tricky and has an edge case problem.

On this branch, we autospawn all foo immediately (to the runahead limit), but they stay in the hidden pool (which holds partially satisfied prerequisites) with an unsatisfied prerequisite on init:succeed, until that gets satisfied.

This is simpler, and it works in all cases.

[dwsutherland]

Was able to reproduce:

(flow) sutherlander@graphic-vbox:~$ cylc cat-log parentless | grep WARNING
2022-07-25T15:00:07+12:00 WARNING - Backward compatibility mode ON
2022-07-25T15:00:14+12:00 WARNING - Task 3/foo already spawned in {1}
2022-07-25T15:00:14+12:00 WARNING - Task 5/foo already spawned in {1}
2022-07-25T15:00:14+12:00 WARNING - Task 5/foo already spawned in {1}
2022-07-25T15:00:57+12:00 WARNING - Task 23/foo already spawned in {1}

Fixed after this change:

(flow) sutherlander@graphic-vbox:~$ cylc cat-log parentless | grep WARNING
2022-07-25T15:02:59+12:00 WARNING - Backward compatibility mode ON

Code change looks fine, & tests passing 👍

[wxtim]

Booted @MetRonnie as reviewer because I was able to reproduce issue locally.

[wxtim]

• Read the code
• Checked out and run tests locally
• Run Local Proof of pudding tests up to cycle 50

[MetRonnie]

Can we add a task to this test that doesn't get spawned to the runahead limit? (That was the point of pub originally, I think)

[hjoliver]

Yes, I'd like to spend a bit of time making these integration tests more comprehensive. As a quick fix, I just adapted the existing test to what's actually going on in the scheduler now.

[hjoliver]

New issue: #5012