Cylc-flow: platforms upgrade of root settings

[hjoliver]

The first deprecation example in the platforms proposal works as advertised:

[scheduling]
    [[graph]]
          R1 = alpha
[runtime]
    [[alpha]]
        [[[remote]]]
            host = localhost
        [[[job]]]
            batch system = background

But if I change [[alpha]] to [[root]] under [runtime] the scheduler aborts with:

...
2020-09-14T15:09:14+12:00 INFO - Initial point: 1
2020-09-14T15:09:14+12:00 INFO - Final point: 1
2020-09-14T15:09:14+12:00 ERROR - PlatformLookupError: No platform found matching your task
2020-09-14T15:09:15+12:00 INFO - DONE

[hjoliver]

Huh, actually the upgrade works fine for any explicit inheritance from a family (root or otherwise). It only fails if alpha is a "naked dummy task" (i.e. no explicit section under runtime).

So this fails:

R1 = alpha
[runtime]
    [[root]]
        [[[remote]]]
            host = localhost
        [[[job]]]
            batch system = background

But this passes:

R1 = alpha
[runtime]
    [[root]]
        [[[remote]]]
            host = localhost
        [[[job]]]
            batch system = background
    [[alpha]]  # <---