feat(flux): convert walltime to minutes/FSD for batch_system="flux" #5017

Summary

Flux's -t/--time-limit accepts minutes or Flux Standard Duration (FSD, RFC 23) — not HH:MM:SS. Tuolumne is currently the only E3SM machine running Flux, so this is handled as a Flux-specific code path in env_batch.py, leaving the generic format_time()/walltime_format mechanism untouched (used as-is by other Slurm/PBS-style machines).

Reference: Flux's accepted format

Per flux-submit(1):

-t, --time-limit=MINUTES|FSD
Set a time limit for the job in either minutes or Flux standard duration (RFC 23). FSD is a floating point number with a single character units suffix (s, m, h, or d). The default unit is minutes when no unit is specified.

Root cause recap

CIME/utils.py::format_time() re-arranges H/M/S fields positionally; it does not sum across units. E.g. walltime_format="%M" on 01:10:00 yields 10, not 70. This isn't a bug in format_time() itself — it's simply the wrong tool for Flux's minutes/FSD requirement, and other machines rely on its current field-rearranging behavior, so it should not be changed generically.

Proposed fix

In CIME/XML/env_batch.py::get_job_overrides() (~line 484), add a Flux-specific branch that converts the resolved walltime directly, rather than reusing the generic format_time()/walltime_format path for this scheduler:

if batch_system == "flux":
    seconds = convert_to_seconds(walltime)
    # convert to minutes and/or FSD as appropriate
    walltime = <flux-appropriate conversion>
else:
    walltime_format = self.get_value("walltime_format")
    if walltime_format:
        ...  # existing format_time() path, unchanged

Implementation should support both Flux-accepted formats:

• Minutes (plain integer, e.g. 70)
• FSD (floating point + unit suffix s/m/h/d, e.g. 70m, 1.17h)

Exact choice of which format to emit by default (and any rounding behavior for partial minutes) is an implementation detail to work out in the PR.

Out of scope

• Changing the generic format_time() field-rearranging behavior used by other (non-flux) machines.
• Rounding/ceiling-division correctness improvements to the general walltime_format path.

Validation

• Unit tests covering batch_system="flux" walltime conversion for both minutes and FSD output paths, e.g. 01:10:00 → 70 (minutes) and equivalent FSD form.
• Confirm non-flux batch systems are unaffected (existing format_time()/walltime_format behavior unchanged).
• End-to-end: submit a Tuolumne test case with env_batch.xml walltime 01:10:00; confirm the generated flux batch/flux submit -t value is valid and the enforced job time limit matches.

References

• Project
• GitHub
• Docs