> +#ifdef HAVE_UTMPX_H
> +#define PATH_UTMPX _PATH_WTMPX
Should be called PATH_WTMP, since it's the wtmp file not the utmp file

> +#ifdef HAVE_UTMPX_H
> +        file = g_file_new_for_path (PATH_UTMPX);
> +        daemon->priv->utmpx_monitor = g_file_monitor_file (file,
> +                                                           G_FILE_MONITOR_NONE,
> +                                                           NULL,
> +                                                           &error);
> +        g_object_unref (file);
> +#endif
This highlights an existing problem in the code that would be good to fix in a prerequisite commit.  The various monitors aren't checked for error until way later in the function and so their errors can stomp on top of each other.

>  
>          if (daemon->priv->passwd_monitor != NULL) {
>                  g_signal_connect (daemon->priv->passwd_monitor,
> @@ -801,6 +815,17 @@ daemon_init (Daemon *daemon)
>                  g_warning ("Unable to monitor %s: %s", PATH_GDM_CUSTOM, error->message);
>                  g_error_free (error);
>          }
> +#ifdef HAVE_UTMPX_H
> +        if (daemon->priv->utmpx_monitor != NULL) {
> +                g_signal_connect (daemon->priv->utmpx_monitor,
> +                                  "changed",
> +                                  G_CALLBACK (on_passwd_monitor_changed),
This highlights another preexisting problem with the code. the function is named on_passwd_monitor_changed, but it's called for all the monitors.  It would be good if there was a preliminary commit that renamed it (maybe on_users_monitor_changed ?).

Created attachment 79756 [details] [review]
rename on_passwd_monitor_changed

Created attachment 79757 [details] [review]
check for monitor error immediately

Created attachment 79758 [details] [review]
add wtmp file monitor

Created attachment 80014 [details] [review]
Emit "changed" signal when setting information from wtmp.

I also found this patch to be necessary when reloading users on wtmp changes.

Comment on attachment 79756 [details] [review]
rename on_passwd_monitor_changed

Review of attachment 79756 [details] [review]:
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looks good