Difficult to bootstrap the display config option

[Saviq]

It's currently difficult to retrieve the defaults for frame.display. We point at /var/snap/ubuntu-frame/current/frame.display, but that's an internal detail.

It would be good to snapctl set display … with the default config, since that what ends up in there anyway?

[AlanGriffiths]

Once the display layout is set we don't handle subsequent changes to the physical outputs very well. For now, I'd rather we "opt in" by manually setting a configuration. But I'm very open to ideas for improving this.

[Saviq]

Well, since the file is written by ubuntu-frame anyway, it's not really opt-in, is it?

[AlanGriffiths]

Well, Mir won't write that file until it runs as a daemon for the first time. As an example, I don't have it on my system.

[Saviq]

Sure, but on Ubuntu Core, just installing ubuntu-frame makes the current setting the persistent one. But it's still not available through snap get until you manually snap set it.

[AlanGriffiths]

Right, but when do you envisage setting it?

There's no file until after Mir has run as a daemon for the first time, so dumping snapctl set display="$(cat ${display_file})" in hooks/configure would not take effect until a subsequent "configure".

We probably want Mir to do some better change management (e.g. adding any new devices it encounters). And maybe have a "dump out the display config and exit" option too.

[Saviq]

Maybe we don't set it, but also don't store the file? At least not in a place that makes it the config? Like, always save it on startup (and thus, deal with hardware changes).

[AlanGriffiths]

Ah! Yeah, it is a bit weird moving it out of the way only in the configure hook.

You'd be happy if we move the mv "${display_file}" "${display_file}.save" || true incantation from configure to run-frame? Want to PR that?

[Saviq]

But isn't ${display_file} used when you snap set ..., too? That doesn't sound right to move it about?

[AlanGriffiths]

Yes, if $(snapctl get display) returns something (in configure) we write the file.

At present, if $(snapctl get display) returns nothing (in configure) we move the file out of the way.

Hypothetically, "if $(snapctl get display) returns nothing (in run-frame) we move the file" would give behaviour you're happy with?