Mixing cycle and interval scheduling results in stall

Cylc Cylc Support
2

Hi @srennie

I think you have uncovered some kind of a bug. I’ve reproduced it, but not had time to figure out what’s going on yet. Here’s a slightly simplified case that gets to the stall faster:

[scheduling]
    initial cycle point = 20230101T18
    runahead limit = P3
    [[graph]]
        PT1H = """
             b => c
             c[-PT1H] => c
        """
        T23 = a => b
[runtime]
    [[a, b, c]]

Result (at 8.6.3)

...
INFO - [20230101T2300Z/c/01:running] => succeeded
INFO - [20230102T0000Z/c:waiting(runahead)] => waiting
WARNING - Partially satisfied prerequisites:
      * 20230102T0000Z/c is waiting on ['20230102T0000Z/b:succeeded']
CRITICAL - Workflow stalled
WARNING - PT1H stall timer starts NOW

The active tasks at this point:

$ cylc dump -t sre
20230102T2300Z/a:waiting (runahead)
20230102T0000Z/c:waiting

The log gives the reason for the stall. But 20230102T0000Z/b should just run automatically, because it doesn’t depend on anything in that cycle.

$ cylc log sre | grep '/b' | grep '=> submitted'
2026-03-17T16:57:00+13:00 INFO - [20230101T1800Z/b/01:preparing] => submitted
2026-03-17T16:57:00+13:00 INFO - [20230101T1900Z/b/01:preparing] => submitted
2026-03-17T16:57:00+13:00 INFO - [20230101T2000Z/b/01:preparing] => submitted
2026-03-17T16:57:00+13:00 INFO - [20230101T2100Z/b/01:preparing] => submitted
2026-03-17T16:57:07+13:00 INFO - [20230101T2200Z/b/01:preparing] => submitted
2026-03-17T16:57:15+13:00 INFO - [20230101T2300Z/b/01:preparing] => submitted

I suspect something is wrong with the code that handles tasks that are parented in some cycles and parentless in others (which is a tricky situation, but I thought we had tests covering it…)