This protocol involves far too much accidental complexity. The original
motivating use-case was to provide a convenient way to send arbitrary
data to layout clients at runtime in order to avoid layout clients
needing to implement their own IPC and do this over a side-channel.
Instead of implementing a quite complex but still rigid options protocol
and storing this state in the compositor, instead we will simply add
events to the layout protocol to support this use case.
Consider the status quo event sequence:
1. send get_option_handle request (riverctl)
2. roundtrip waiting for first event (riverctl)
3. send set_foo_value request (riverctl)
4. receive set_foo_value request (river)
5. send foo_value event to all current handles (river)
6. receive foo_value event (rivertile)
7. send parameters_changed request (rivertile)
8. receive parameters_changed request (river)
9. send layout_demand (river)
And compare with the event sequence after the proposed change:
1. send set_foo_value request (riverctl)
2. receive set_foo_value request (river)
3. send set_foo_value event (river)
4. send layout_demand (river)
This requires *much* less back and forth between the server and clients
and is clearly much simpler.
Options are now all global but may be overridden per-output. If an
output local value is requested but none has been set, the global value
is provided instead. This makes for much better ergonomics when
configuring layout related options in particular.
Passing an empty string as the value argument for riverctl set-option or
declare-option will set the value to null. The riverctl get-option
command produces no output for both null and empty string values.
This is not perfect as it is unable to distinguish between null and
empty strings through the riverctl CLI. I don't see a better alternative
here however. Forbidding null strings in the river-options protocol
would be one solution, however null strings are useful and more pleasant
to use from code despite being problematic on the CLI.
