Scheduler command arg validation

[hjoliver]

Follow-up #5658

The new cylc set command has relatively complicated validation requirements. During discussion there, @oliver-sanders suggested validating async scheduler commands server-side before queueing the command, instead of inside the queued command method itself. This enables validation feedback to the CLI and GUI.

I have a branch in progress, but it impacts other commands too, so it's better done as a quick-fire follow-up.

[oliver-sanders]

It is possible to validate a request and cause it to fail if needed, but only before the command is queued.

Cylc used to have a layer to do this, it looked for a "proxy" command:

cylc-flow/cylc/flow/network/resolvers.py

Lines 733 to 735 in 5398941

	method = getattr(self, command, None)
	if method is not None:
	return method(**kwargs)

If one was provided, it would call it rather than looking for a scheduler method:

cylc-flow/cylc/flow/network/resolvers.py

Lines 776 to 797 in 5398941

	def put_ext_trigger(
	self,
	message,
	id # noqa: A002 (graphql interface)
	):
	"""Server-side external event trigger interface.
	Args:
	message (str): The external trigger message.
	id (str): The unique trigger ID.
	Returns:
	tuple: (outcome, message)
	outcome (bool)
	True if command successfully queued.
	message (str)
	Information about outcome.
	"""
	self.schd.ext_trigger_queue.put((message, id))
	return (True, 'Event queued')

This was the vestigial remains of the old HTTP server where each command had a "proxy" in the server code:

cylc-flow/lib/cylc/network/httpserver.py

Lines 311 to 317 in 4e43fa5

	@cherrypy.expose
	@cherrypy.tools.json_out()
	def get_latest_state(self, full_mode=False):
	"""Return latest suite state (suitable for a GUI update)."""
	client_info = self._check_access_priv_and_report(PRIV_FULL_READ)
	full_mode = self._literal_eval('full_mode', full_mode)
	return self.schd.info_get_latest_state(client_info, full_mode)

The need for this layer dropped away thanks to the better type support and validation features of GraphQL so the "proxy" commands were removed on by one.

The last remains of this interface were removed with #5658 as they were no longer used. To add validation back in, we'll need something like this layer. We could potentially re-implement this with a little syntactic sugar e.g:

def validate(validate_fcn, command_fcn):
    command_fcn.validate = validate_fcn
    return command_fcn

def validate_set(args):
    if ...:
        raise InputError('some argument must be ....')

class Scheduler:
    # ...

    @validate(validate_set)
    @def command set(...):
        ...


class Resolvers:
    # ...

        meth = getattr(schd, command)
        if meth.validate:
            try:
                meth.validate(args)
            except InputError as exc:
                 # TODO: return error
        schd.command_queue.put((meth, args))

[hjoliver]

Yes that's the approach I'm taking.

[hjoliver]

A detail:

I also spotted that the --pre option doesn't appear to work with the --wait option, which makes sense.
Another one for validation?

[hjoliver]

Working on this today, PR tomorrow...

[dwsutherland]

Ha! I haven't seen that httpserver.py in a while..