bug#53854: 28.0.91; Tramp SSHFS: process-file output to current buffer doesn't work right

[Philipp Stephani]

This works:

$ emacs -Q -batch -eval '(let ((default-directory "/")) (with-temp-buffer 
(process-file "hello" nil (list t "/tmp/stderr")) (princ (buffer-string))))'
Hello, world!

This works, too:

$ emacs -Q -batch -eval '(let ((default-directory "/ssh:localhost:/")) 
(with-temp-buffer (process-file "hello" nil (list t "/tmp/stderr")) (princ 
(buffer-string))))'
Tramp: Sending command ‘exec ssh -e none localhost’
Tramp: Found remote shell prompt on ‘localhost’
Hello, world!

But with SSHFS, it doesn't work:

$ emacs -Q -batch -eval '(let ((default-directory "/sshfs:localhost:/")) 
(with-temp-buffer (process-file "hello" nil (list t "/tmp/stderr")) (princ 
(buffer-string))))'

The output buffer stays empty.
The same behavior can be observed on the master branch.


In GNU Emacs 28.0.91 (build 14, x86_64-pc-linux-gnu, GTK+ Version 3.24.31, 
cairo version 1.16.0)
 of 2022-02-07
Repository revision: 0d46ee2ddebb74a5a6e1d55da3641d1cd2ba5beb
Repository branch: emacs-28
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: Debian GNU/Linux rodete

Configured using:
 'configure --enable-gcc-warnings=warn-only
 --enable-gtk-deprecation-warnings --without-pop --with-mailutils
 --enable-checking=all --enable-check-lisp-object-type --with-modules
 'CFLAGS=-O0 -ggdb3''

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG SECCOMP
SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
  value of $LC_TIME: en_DK.utf8
  value of $LANG: en_US.utf8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug sendmail phst skeleton edmacro kmacro
pcase ffap thingatpt url url-proxy url-privacy url-expand url-methods
url-history url-cookie url-domsuf url-util url-parse url-vars mailcap rx
message rmc dired dired-loaddefs rfc822 mml mml-sec epa derived epg
rfc6068 epg-config gnus-util rmail rmail-loaddefs auth-source cl-seq
eieio eieio-core cl-macs eieio-loaddefs password-cache json map
time-date mm-decode mm-bodies mm-encode mail-parse rfc2231 rfc2047
rfc2045 mm-util ietf-drums mail-prsvr mailabbrev mail-utils gmm-utils
mailheader gnutls puny elp dbus xml subr-x seq byte-opt gv bytecomp
byte-compile cconv compile text-property-search comint ansi-color ring
cl-loaddefs cl-lib 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 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 composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice button
loaddefs faces cus-face macroexp files window text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads dbusbind inotify
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 69599 7804)
 (symbols 48 8545 3)
 (strings 32 24813 2157)
 (string-bytes 1 801727)
 (vectors 16 15667)
 (vector-slots 8 207324 16747)
 (floats 8 29 46)
 (intervals 56 228 0)
 (buffers 992 11))

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