bug#68618: 29.1; Tramp remote process fails in Eshell

[Sean Devlin]

Hi folks,

I found a situation where a Tramp remote process invoked via Eshell
fails unexpectedly. I discovered this while developing a custom Tramp
method, but it could equally apply to built-in methods under certain
system configurations.

Here's a recipe to reproduce:

1. Create a symlink to the ssh program in an unusual location:

$ mkdir -p /tmp/bin
$ ln -s $(which ssh) /tmp/bin/myssh

2. Start Emacs with this directory in your path:

$ PATH=/tmp/bin:$PATH emacs -Q

3. Evaluate these forms:

(require 'tramp)
(add-to-list 'tramp-methods
             `("myssh"
               (tramp-login-program "myssh")
               ,@(cdr (assoc "ssh" tramp-methods))))

4. M-x eshell
5. In Eshell:

$ cd /myssh:someremote:
$ uname -a

6. The Eshell buffer will show some corrupt output something like:

sh: cd: /root/: No such file or directory
///61166bb64799e9d0ec9f78bc44cc9d3d#$

I did a little debugging, and I found that the "uname -a" process is
created under a let-binding of process-environment established in
eshell-gather-process-output. This binding changes the PATH environment
variable. I think maybe it's setting it to the remote path from the
Tramp session, since our local directory in Eshell is
/myssh:someremote:.

This is a problem for Tramp, since it needs to start up a new "/bin/sh
-i" process to create the connection to run "uname -a" on the remote
host. When it starts the local shell process, it is now using the wrong
executable path, and it cannot find myssh, since it is in a non-standard
location.

You can see this by tracing process-send-string and
internal-default-process-filter before running "uname -a" in
Eshell. When Tramp tries to send "myssh ..." to the /bin/sh process, it
will get back an error saying myssh cannot be found.

I think Tramp and/or Eshell need to make sure this /bin/sh subprocess is
started with the local executable path and not the path from the remote
host.

There is a simple workaround: update your Tramp method's login program
to use an absolute filename, e.g. /tmp/bin/myssh.

Thanks!

In GNU Emacs 29.1
Windowing system distributor 'Apple'
System Description:  macOS 14

Configured using:
'configure --with-ns '--enable-locallisppath=/Library/Application
Support/Emacs/${version}/site-lisp:/Library/Application
Support/Emacs/site-lisp' --with-modules 'CFLAGS=-DFD_SETSIZE=10000
-DDARWIN_UNLIMITED_SELECT' --with-x-toolkit=no'

Configured features:
ACL GLIB GMP GNUTLS JPEG JSON LIBXML2 MODULES NOTIFY KQUEUE NS PDUMPER
PNG RSVG SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER ZLIB

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

Major mode: Eshell

Minor modes in effect:
  eshell-prompt-mode: t
  eshell-hist-mode: t
  eshell-pred-mode: t
  eshell-cmpl-mode: t
  eshell-proc-mode: t
  eshell-arg-mode: t
  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
  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 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 em-unix em-term term disp-table ehelp
em-script em-prompt em-ls em-hist em-pred em-glob em-extpipe em-cmpl
em-dirs esh-var em-basic em-banner em-alias esh-mode eshell esh-cmd
generator esh-ext esh-opt esh-proc esh-io esh-arg esh-module esh-groups
esh-util dired-aux dired dired-loaddefs cl-extra pp cl-print tramp-adb
tramp-archive tramp-cache time-stamp tramp-cmds tramp-container
tramp-ftp tramp-gvfs url-util url-parse url-vars dbus xml tramp-sh
thingatpt help-fns radix-tree help-mode tramp tramp-loaddefs trampver
tramp-integration files-x tramp-compat rx parse-time iso8601 time-date
format-spec auth-source cl-seq eieio eieio-core cl-macs cl-loaddefs
cl-lib password-cache json map byte-opt gv bytecomp byte-compile shell
subr-x pcomplete comint ansi-osc ansi-color ring rmc iso-transl tooltip
cconv eldoc paren electric uniquify ediff-hook vc-hooks lisp-float-type
elisp-mode mwheel term/ns-win ns-win ucs-normalize mule-util
term/common-win 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 theme-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 kqueue cocoa ns multi-tty make-network-process emacs)

Memory information:
((conses 16 99526 10654)
(symbols 48 10616 0)
(strings 32 37029 1504)
(string-bytes 1 1188080)
(vectors 16 22340)
(vector-slots 8 297950 17878)
(floats 8 64 26)
(intervals 56 1230 0)
(buffers 984 16))