bug#42903: 28.0.50; Emacs doesn't start if the current directory is inaccessible

[Philipp Stephani <p.stephani2@gmail.com>]

1. Create a FUSE mount where the FS driver runs in the foreground,
   e.g. using

   bindfs -f /bin /tmp/foo

2. In another shell, cd into the new mount point:

   cd /tmp/foo

3. Kill the bindfs process started in step (1) (e.g. using C-c).

4. In the shell (2), start Emacs:

   $ emacs -Q
   Error getting directory: Transport endpoint is not connected
   Symbol’s value as variable is void: auto-save-list-file-prefix

The "void variable" error is clearly a bug.  In general, Emacs should
fall back to / or ~ if the current directory is inaccessible.

The Lisp backtrace is

"default-toplevel-value" (0xffffb650)
"custom-initialize-reset" (0xffffbc00)
"custom-declare-variable" (0xffffc270)
"byte-code" (0xffffc740)
"display-warning" (0xffffcf30)
"normal-top-level" (0xffffd600)

i.e. it looks like `display-warning' is called too early.


In GNU Emacs 28.0.50 (build 84, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo version 1.16.0)
 of 2020-08-17
Repository revision: e5d4fae6797330d91e901c7ecb1412551db12f6a
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Debian GNU/Linux rodete

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

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=-O1 -ggdb3 -fno-omit-frame-pointer -fsanitize=address
 -fsanitize=undefined -fsanitize=pointer-compare
 -fsanitize=pointer-subtract''

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

Important settings:
  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
  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

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc dired dired-loaddefs rfc822
mml easymenu mml-sec epa epg epg-config gnus-util rmail rmail-loaddefs
text-property-search time-date mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils phst skeleton derived edmacro
kmacro 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
password-cache json map url-vars mailcap subr-x rx gnutls puny seq
byte-opt gv bytecomp byte-compile cconv dbus xml compile comint
ansi-color ring cl-loaddefs cl-lib tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type 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 elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu 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 charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray 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 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 68537 7755)
 (symbols 48 8604 1)
 (strings 32 23753 1822)
 (string-bytes 1 766201)
 (vectors 16 13662)
 (vector-slots 8 187551 5719)
 (floats 8 26 29)
 (intervals 56 221 0)
 (buffers 992 11))

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