bug#56774: 29.0.50; tramp-error: ‘tramp_readlink_file_truename /’ does not return a valid Lisp expression: ‘""/

[Po Lu]

Open the root of any remote host in tramp, like so:

  C-x C-f /scp:root@emerald:/

Tramp will complain:

  tramp-error: ‘tramp_readlink_file_truename /’ does not return a valid Lisp 
expression: ‘""/

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, cairo version 1.17.6)
 of 2022-07-26 built on trinity
Repository revision: 06cec5ee0bd2c2a8fbc761bd5eb0254a4ecf10f4
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 36 (Workstation Edition)

Configured using:
 'configure --with-x --with-x-toolkit=no --with-dumping=unexec
 --cache-file=/tmp/ccache'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NOTIFY INOTIFY OLDXMENU PNG
RSVG SECCOMP SOUND SQLITE3 THREADS TIFF UNEXEC WEBP X11 XDBE XIM XINPUT2
XPM ZLIB

Important settings:
  value of $LANG: en_GB.UTF-8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix

Major mode: Messages

Minor modes in effect:
  shell-dirtrack-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  buffer-read-only: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr dired-aux emacsbug message mailcap yank-media
puny rfc822 mml mml-sec epa derived epg rfc6068 epg-config gnus-util
text-property-search mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils dired dired-loaddefs tramp-cache tramp-sh
tramp tramp-loaddefs trampver tramp-integration cus-edit pp cus-start
cus-load wid-edit files-x tramp-compat shell pcomplete comint ansi-color
ring parse-time iso8601 time-date ls-lisp format-spec auth-source cl-seq
eieio eieio-core cl-macs cl-loaddefs cl-lib password-cache json subr-x
map byte-opt gv bytecomp byte-compile cconv rmc iso-transl tooltip eldoc
paren electric uniquify ediff-hook vc-hooks lisp-float-type elisp-mode
mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
faces cus-face macroexp files window text-properties overlay sha1 md5
base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo xinput2 x
multi-tty make-network-process emacs)

Memory information:
((conses 16 152849 12533)
 (symbols 48 25453 0)
 (strings 32 44478 2177)
 (string-bytes 1 1284574)
 (vectors 16 23346)
 (vector-slots 8 625741 16477)
 (floats 8 89 98)
 (intervals 56 595 0)
 (buffers 992 12))