Pass XDG environment variables to xdg-autostart-generator

[ptrnine]

Component

systemd-xdg-autostart-generator

Is your feature request related to a problem? Please describe

The environment provided for xdg-autostart-generator is insufficient.

For example, xdg-autostart-generator cannot use autostart files in non-default locations, because
XDG_CONFIG_DIRS/XDG_DATA_DIRS is not provided (however, it already able to do that because xdg_user_dirs() in src/basic/path-lookup.c can use the XDG_* variables).
This problem is exacerbated by the fact that it is difficult to use environment generators to provide these variables, because they are usually set by some startup scripts/executables (one for each installed DE), which usually call org.freedesktop.systemd1.Manager.UnsetAndSetEnvironment and Reload for that.

Describe the solution you'd like

It would be great if generators running on user will be able to access client's XDG_* environment variables (and maybe PATH so that xdg-autostart-generator can support Exec= in non-default locations).

The change required for this can look like so (somewhere in src/core/manager.c in build_generator_environment(), or maybe better somewhere before running environment generators):

if (MANAGER_IS_USER(m)) {
    STRV_FOREACH(env, m->client_environment)
        if (startswith(*env, "XDG_"))
            strv_env_replace_strdup(ge, *env);
}

Describe alternatives you've considered

Alternative appoach is to provide some mechanism to pass the specified environment variables from m->client_environment to specific generator (but I have no idea how to implement this).

The systemd version you checked that didn't have the feature you are asking for

No response