Move ~/.Xauthority to ${XDG_RUNTIME_DIR} if XDG_RUNTIME_DIR is set

Set user-authority-in-system-dir=false in config.
Tested with ConsoleKit2. Not sure that will work with logind without additional code.

Licence: BSD2CLAUSE

data/lightdm.conf

@@ -6,7 +6,7 @@
 # minimum-display-number = Minimum display number to use for X servers
 # minimum-vt = First VT to run displays on
 # lock-memory = True to prevent memory from being paged to disk
-# user-authority-in-system-dir = True if session authority should be in the system location
+# user-authority-in-system-dir = True if session authority should be in the system location, else XDG_RUNTIME_DIR or home dir will be used
 # guest-account-script = Script to be run to setup guest account
 # logind-check-graphical = True to on start seats that are marked as graphical by logind
 # log-directory = Directory to log information to

src/session-child.c

@@ -602,6 +602,13 @@ session_child_run (int argc, char **argv)
     /* Write X authority */
     if (x_authority)
     {
+        /* If XDG_RUNTIME_DIR is set and user-authority-in-system-dir=false than use
+         * XDG_RUNTIME_DIR to store .Xauthority file. */
+        const gchar *runtime_dir = pam_getenv (pam_handle, "XDG_RUNTIME_DIR");
+        if (runtime_dir && x_authority_filename && g_str_has_suffix (x_authority_filename, ".Xauthority")) {
+            x_authority_filename = g_build_filename (runtime_dir, ".Xauthority", NULL);
+        }
+
         gboolean drop_privileges = geteuid () == 0;
         if (drop_privileges)
             privileges_drop (user_get_uid (user), user_get_gid (user));