bug#25963: 25.2; tramp su fails

[Tyler Smith]

Hello,

I'm having a problem opening files as root. 

>From emacs -Q, I issue
`(require 'tramp)`, customize tramp-verbose to 10, and try to open a
file as root: `C-x C-f /su::/etc/fstab`. I'm then prompted for my root
password. Entering the password, Emacs pauses for ca. 10 seconds, then
appears to open a new file called fstab, but it's not the fstab that
already exists in /etc/fstab. The message about calling make-directory
to create the parent directories shows up, so I've apparently opened a
new file somewhere. I can't see where, exactly, as `C-x C-f`, or even
`M-x shell-commmand pwd` both prompt me for the root password, and then
fail. 

Despite having turned `tramp-verbose` to the maximum value, there's not
much in my *Messages* buffer after all this:


Creating customization setup...done  
Tramp: Opening connection for root@<myhostname> using su...  
Tramp: Sending command ‘exec su - root’  
Tramp: Waiting for prompts from remote shell...  
Tramp: Sending Password  
Tramp: Waiting for prompts from remote shell...failed  
Tramp: Opening connection for root@<myhostname> using su...failed  
Use M-x make-directory RET RET to create the directory and its parents  

Tramp: Checking ‘vc-registered’ for /su:root@<myhostname>: 
/etc/fstab...failed  
Use M-x make-directory RET RET to create the directory and its parents  
Tramp: Opening connection for root@<myhostname> using su...  
Tramp: Sending command ‘exec su - root’  
Tramp: Waiting for prompts from remote shell...  
Tramp: Sending Password  
Tramp: Waiting for prompts from remote shell...failed  
Tramp: Opening connection for root@<myhostname> using su...failed  
Quit  

This is all on a laptop, I'm not trying to connect to another machine. 

Emacs 25.1.91.1, and confirmed again with Emacs 25.2.1.

In GNU Emacs 25.2.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.8)
 of 2017-03-03 built on onottra672626p
Repository revision: 6e788ef0e262fafc014c21f4ad52cc5dc9f1715b
Windowing system distributor 'The X.Org Foundation', version
11.0.11901000
System Description:     Debian GNU/Linux 9.0 (stretch)

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: en_CA.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-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
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list... [2 times]
user-error: End of history; no default available

Load-path shadows:
/home/tws/research/programs/emacs-github/lisp/net/rfc2104 hides
/home/tws/research/programs/emacs-github/lisp/gnus/rfc2104
/home/tws/research/programs/emacs-github/lisp/play/landmark hides
/home/tws/research/programs/emacs-github/lisp/obsolete/landmark
/home/tws/research/programs/emacs-github/lisp/emacs-lisp/gulp hides
/home/tws/research/programs/emacs-github/lisp/obsolete/gulp
/home/tws/research/programs/emacs-github/lisp/net/eudcb-ph hides
/home/tws/research/programs/emacs-github/lisp/obsolete/eudcb-ph
/home/tws/research/programs/emacs-github/lisp/vc/vc-arch hides
/home/tws/research/programs/emacs-github/lisp/obsolete/vc-arch

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util help-fns help-mode easymenu
cl-loaddefs pcase cl-lib mail-prsvr mail-utils thingatpt time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel x-win term/common-win x-dnd tool-bar dnd fontset
image regexp-opt fringe tabulated-list newcomment elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame cl-generic 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 charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 87181 6288)
 (symbols 48 19950 0)
 (miscs 40 53 171)
 (strings 32 14702 4549)
 (string-bytes 1 420962)
 (vectors 16 11979)
 (vector-slots 8 434895 4455)
 (floats 8 165 25)
 (intervals 56 213 4)
 (buffers 976 21))


Best,

Tyler