runuser/su/sudo -u from a root shell leaks XDG_RUNTIME_DIR=/run/user/0 into
the child. _user_systemd_socket_ready() stat-ed sockets under it with a bare
Path.exists(), which only suppresses ENOENT/ENOTDIR/EBADF/ELOOP — EACCES on
the 0700 root-owned dir escaped as a raw PermissionError traceback instead of
the documented UserSystemdUnavailableError remediation path.
- _path_exists_safe(): Path.exists() that treats EACCES as absent; used at
both the readiness and DBUS-detection call sites.
- _ensure_user_systemd_env(): drop an XDG_RUNTIME_DIR that is unset or owned
by another user in favour of our own /run/user/{uid}, so the restart
actually succeeds after su/sudo -u instead of only failing cleanly.
Regression tests cover the EACCES readiness probe, foreign-dir replacement,
and that preflight raises UserSystemdUnavailableError (not PermissionError).
PRATHAMESH75