bug#41218: 26.3; Windows: file-writable-p returns t for a file owned by the Administrators group but not writable by the user, a member of the Administrators group

[Michael Hoffman]

1. Start elevated Powershell (Win+X,A) and create file C:\foo.txt owned by the Administrators group

PS C:\> $null > C:\foo.txt
PS C:\> takeown /A /F C:\foo.txt

SUCCESS: The file (or folder): "C:\foo.txt" now owned by the administrators group.

2. Open `emacs -Q`

3. C-x C-f C:/foo.txt

4. Type `(file-writable-p "foo.txt")` C-x C-e
t

5. C-x C-s
Saving file c:/foo.txt...
Cannot write backup file; backing up in ~/.emacs.d/%backup%~
Error: (file-error "Setting ACL" "Operation not permitted" "c:/Users/mhoffman/.emacs.d/%backup%~")
basic-save-buffer-2: Opening output file: Permission denied, c:/foo.txt

Expected that `(file-writable-p "foo.txt")` would return nil.


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29 built on CIRROCUMULUS
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor 'Microsoft Corp.', version 10.0.18363
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark activated

Configured using:
 'configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS THREADS LCMS2

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252

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 puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel dos-w32 ls-lisp disp-table term/w32-win w32-win
w32-vars term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame 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 minibuffer 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 w32notify w32 lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 97495 11932)
 (symbols 48 20211 1)
 (miscs 40 42 143)
 (strings 32 29766 1358)
 (string-bytes 1 773975)
 (vectors 16 13889)
 (vector-slots 8 494823 7634)
 (floats 8 51 193)
 (intervals 56 251 11)
 (buffers 992 11))