workflow reg argument parsing

[oliver-sanders]

At the moment the parsing of the workflow argument is not fully centralised.

For the "offline" commands (e.g. cylc diff, cylc config, cylc list) we have the centralised parse_workflow_arg function.

For the "online" commands (e.g. pause, stop, trigger) we don't have any centralised function, we usually (but don't always) do os.path.normpath(workflow).

I think we should pull this functionality into a single function something along the lines of:

# cylc.flow.workflow_files
...
def parse_reg(reg, src=False, run=False) -> str:
    """
        Args:
            src: Reg can be a source workflow located by an absolute path.
            run: Reg can be a run directory located by relative path from ~/cylc-run/
    """

# cylc.flow.scripts.pause
...
def main(reg, ...):
     reg = parse_reg(reg, run=True)

This is better for maintenance and would provide us with a good place to handle future changes to registration parsing:

• handling the runN symlink: Cylc cli runN fix #4228
• inferred run number: cli: infer run number where possible #3895
• universal identifier: Universal suite/task/cycle delimiter (universal identifier) #3592
• multi-workflow functionality (same issue as above)

[oliver-sanders]

As of #4228 the runN symlink handling is done in cylc.flow.terminal, this should be moved out into the new centralised thinggy with this issue.

See #4228 (comment)