mixed parentless/non-parentless task cause premature shutdown

[dwsutherland]

Description

I ran into this problem while thinking of ways to break my inbound parentless sequential wall clock task spawning (which I may work the solution into). A workflow with a mixed parentless/not-parentless task on different cycles causes/may-cause premature workflow shutdown.

Reproducible Example

i.e. this workflow shutdowns after 2015 has run

[scheduler]
    cycle point format = CCYY
[scheduling]
    initial cycle point = 2010
    [[xtriggers]]
        clock_1 = wall_clock()
    [[graph]]
        P2Y = """
@clock_1 => a
a => b
"""
        +P1Y/P2Y = """
a => b
b[-P1Y] => a
"""

[runtime]
    [[root]]
        script = sleep 5
    [[a,b]]

(clock trigger not really needed, stalls without)

Expected Behaviour

Workflow should never stop.

Discussion

I think the trouble occurs when a non-parentless task ends up the final or next at the runahead limit.

A possible solution is to check for non-spawned parentless successor (the next occurrence) of a task that enters the active pool (from initial spawn or RH pool).. Perhaps we can narrow down the checking somehow (i.e. those who could possibly be parentless, via config or w/e)..