sessions-directory needs to be set in the default config file

[Dariqq]

It seems to not be possible to specify the sessions-directory configuration option in the config file passed as an argument to lightdm. As pointed out in #333 this leads to a poor user experience because greeters are looking for the user sessions in the wrong directory despite adjusting the setting in the config file.

This is especially a problem on Guix and Nix where everything is installled in nondefault prefixes.

Reason is that lightdm-gobject's update_sessions (which gets called from lightdm_get_sessions) passes NULL as the config file path leading to it always searching for sessions in the directory described in the default configuration file (or if unset the default value).

Also I could not find any documentation mentioning that this setting is required to be set in the default configuration file.

Not sure how a fix would look like as the custom filepath is only known at runtime of lightdm.

Reproducible: Always

Steps to Reproduce:

1. Launch lightdm with a custom config path lightdm --config /path/to/custom-lightdm.conf setting sessions-directory key to a custom value
2. Observe that e.g. the gtk-greeter doesnt use the path specified in the custom config file leading to sessions not being found.

Expected Results:

I would assume that there is no difference between setting something in the default config file or a custom config file.
Use the sessions-directory from the custom file.

Version:
lightdm 1.32.0
Any version where lightdm-gobject passes NULL to config_load_from_standard_locations in update_session.