bug#54130: 29.0.50; TRAMP SSHFS can't visit remote files using absolute symlinks

bug#54130: 29.0.50; TRAMP SSHFS can't visit remote files using absolute symlinks

[Philipp Stephani]

On a remote host REMOTE, create an arbitrary file and an absolute
symlink to it:

REMOTE$ echo hi > /tmp/a.txt
REMOTE$ ln -svi /tmp/{a,b}.txt

Visiting the symlink from another host using SSH works as expected:

emacs -Q /ssh:REMOTE:/tmp/b.txt

But the same fails with SSHFS:

emacs -Q /sshfs:REMOTE:/tmp/b.txt

TRAMP prints a message "File exists, but cannot be read", and the buffer
stays empty.


In GNU Emacs 29.0.50 (build 59, x86_64-pc-linux-gnu, GTK+ Version 3.24.31, cairo version 1.16.0)
 of 2022-02-23
Repository revision: 85ad8616007e286c237bb2906d1928bb551462e7
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
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:
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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:
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  auto-composition-mode: t
  auto-encryption-mode: t
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  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug sendmail phst skeleton pcase ffap
thingatpt url url-proxy url-privacy url-expand url-methods url-history
url-cookie url-domsuf url-util url-parse auth-source cl-seq eieio
eieio-core cl-macs eieio-loaddefs json map url-vars rx message mailcap
yank-media rmc dired dired-loaddefs rfc822 mml mml-sec password-cache
epa derived epg rfc6068 epg-config gnus-util time-date mm-decode
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compile text-property-search comint ansi-color ring cl-loaddefs cl-lib
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mule custom widget keymap 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 67529 10057)
 (symbols 48 8204 1)
 (strings 32 23696 1693)
 (string-bytes 1 761048)
 (vectors 16 15597)
 (vector-slots 8 208626 48149)
 (floats 8 28 30)
 (intervals 56 230 0)
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bug#54130: 29.0.50; TRAMP SSHFS can't visit remote files using absolute symlinks

[Michael Albinus]

Version: 28.2

Philipp Stephani <p.stephani2@gmail.com> writes:

Hi Philipp,

> On a remote host REMOTE, create an arbitrary file and an absolute
> symlink to it:
>
> REMOTE$ echo hi > /tmp/a.txt
> REMOTE$ ln -svi /tmp/{a,b}.txt
>
> Visiting the symlink from another host using SSH works as expected:
>
> emacs -Q /ssh:REMOTE:/tmp/b.txt
>
> But the same fails with SSHFS:
>
> emacs -Q /sshfs:REMOTE:/tmp/b.txt
>
> TRAMP prints a message "File exists, but cannot be read", and the buffer
> stays empty.

SSHFS interprets symbolic links on the local host. An absolute file name
cannot work, therefore.

I've added ("-o" "transform_symlinks") to the tramp-mount-args of the
sshfs Tramp method. This shall fix it. Pls unmount any sshfs mounts
prior testing, otherwise it wouldn't work.

An alternative would be to use ("-o" "follow_symlinks"). But this has
the disadvantage that symbolic links look like regular files then, so I
haven't used it.

Pushed to the repositories. You'll find it on the master branch, and in
the upcoming GNU ELPA Tramp 2.5.2.2. Closing the bug.

Best regards, Michael.

bug#54130: 29.0.50; TRAMP SSHFS can't visit remote files using absolute symlinks

[Philipp Stephani]

Am Do., 24. Feb. 2022 um 16:19 Uhr schrieb Michael Albinus
<michael.albinus@gmx.de>:
>
> Version: 28.2
>
> Philipp Stephani <p.stephani2@gmail.com> writes:
>
> Hi Philipp,
>
> > On a remote host REMOTE, create an arbitrary file and an absolute
> > symlink to it:
> >
> > REMOTE$ echo hi > /tmp/a.txt
> > REMOTE$ ln -svi /tmp/{a,b}.txt
> >
> > Visiting the symlink from another host using SSH works as expected:
> >
> > emacs -Q /ssh:REMOTE:/tmp/b.txt
> >
> > But the same fails with SSHFS:
> >
> > emacs -Q /sshfs:REMOTE:/tmp/b.txt
> >
> > TRAMP prints a message "File exists, but cannot be read", and the buffer
> > stays empty.
>
> SSHFS interprets symbolic links on the local host. An absolute file name
> cannot work, therefore.
>
> I've added ("-o" "transform_symlinks") to the tramp-mount-args of the
> sshfs Tramp method. This shall fix it. Pls unmount any sshfs mounts
> prior testing, otherwise it wouldn't work.

Thanks for the quick fix! Confirmed that it works now on master.

>
> An alternative would be to use ("-o" "follow_symlinks"). But this has
> the disadvantage that symbolic links look like regular files then, so I
> haven't used it.

Yes, agreed.