bug#27871: 26.0.50; Bad handling of unmounted directory

[Philipp]

Create a FUSE mount point, e.g.

cd /tmp
mkdir a a/a b
touch a/a/a
bindfs -f a b

While bindfs(1) is running, cd to the mounted directory in a second
shell:

cd /tmp/b/a

Now kill bindfs, e.g. by hitting ^C in the first shell.  The second
shell will now be in an unmounted directory.  From that directory, start
Emacs:

$ emacs -Q -batch a
Apparent cycle of symbolic links for (unreachable)

or, with stack trace

$ emacs -Q -batch -f toggle-debug-on-error a
Debug on Error enabled globally
...
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)/a/(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a/(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)/a/(unreachable)/a/(unreachable)" (-1) (nil))
  file-truename("(unreachable)/a/(unreachable)/a/(unreachable)/a" (-1) (nil))
  file-truename("(unreachable)/a/(unreachable)/a/(unreachable)/a/(unreachable)" 
(-1) (nil))
  
file-truename("(unreachable)/a/(unreachable)/a/(unreachable)/a/(unreachable)/a" 
(-1) (nil))
  
file-truename("(unreachable)/a/(unreachable)/a/(unreachable)/a/(unreachable)/a/a")
  find-file-noselect("(unreachable)/a/(unreachable)/a/a")
  #f(compiled-function (displayable-buffers dir line column name) 
#<bytecode>)((nil) "(unreachable)/a/" (0) (0) "a")
  command-line-1(("-f" "toggle-debug-on-error" "a"))
  command-line()
  normal-top-level()

It seems like the logic for `default-directory' and/or `file-truename'
should be improved for unmounted filesystems.


In GNU Emacs 26.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.10.8)
 of 2017-07-29 built on unknown
Repository revision: 2c930d15f541761422a268cd2b5a7f5c11c9a00e
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:     Ubuntu 14.04.5 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

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

Configured features:
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TOOLKIT_SCROLL_BARS GTK3 X11 MODULES

Important settings:
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  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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Load-path shadows:
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Memory information:
((conses 16 94115 11926)
 (symbols 48 20077 1)
 (miscs 40 38 119)
 (strings 32 28435 1945)
 (string-bytes 1 754739)
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