bug#13522: 24.2; save-buffer removes edited file under some conditions

[Vincent Lefevre]

1. Create a file with: printf "\x80" > file
2. Open the file under X Window with: emacs -Q file
3. Modify the file e.g. by adding a space.
4. Type C-x C-s
   At this point, Emacs asks the user to select a coding system.
5. Type C-c in the terminal to kill Emacs.

The result is that the file "file" is no longer there!

Actually there is a backup. Here it is easy to see (file~), but
if the user has defined find-backup-file-name, he may not be
aware that there is a backup (as this is not the normal use of
backups since the file hasn't been saved) and may think that the
file has been lost (it took me some time to find out...).

I think that Emacs makes the backup too soon. It should do it only
just before saving.

This might be a variant of the old bug:

  http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=194171


In GNU Emacs 24.2.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.10)
 of 2012-09-11 on xvii, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11204000
Configured using:
 `configure '--build' 'x86_64-linux-gnu' '--build' 'x86_64-linux-gnu'
 '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
 '--localstatedir=/var/lib' '--infodir=/usr/share/info'
 '--mandir=/usr/share/man' '--with-pop=yes'
 
'--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.2/site-lisp:/usr/share/emacs/site-lisp'
 '--with-crt-dir=/usr/lib/x86_64-linux-gnu' '--with-x=yes'
 '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars'
 'build_alias=x86_64-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
 --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall'
 'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: POSIX
  value of $LC_CTYPE: en_US.UTF-8
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: en_DK
  value of $LANG: POSIX
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-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

Recent input:
<help-echo> <escape> x r e p o r t - b u <tab> <re
turn>

Recent messages:
Loading /etc/emacs/site-start.d/50html-helper-mode.el (source)...done
Loading /etc/emacs/site-start.d/50latex-cjk-common.el (source)...
Loading cjk-enc...done
Loading /etc/emacs/site-start.d/50latex-cjk-common.el (source)...done
Loading /etc/emacs/site-start.d/50latex-cjk-thai.el (source)...done
Loading /etc/emacs/site-start.d/50psvn.el (source)...done
Loading /etc/emacs/site-start.d/50rnc-mode.el (source)...done
Loading /etc/emacs/site-start.d/50thailatex.el (source)...done
Loading /etc/emacs/site-start.d/50w3m-el.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
/usr/share/emacs24/site-lisp/css-mode/css-mode hides 
/usr/share/emacs/site-lisp/css-mode/css-mode
/usr/share/emacs/24.2/site-lisp/debian-startup hides 
/usr/share/emacs/site-lisp/debian-startup
/usr/share/emacs/site-lisp/autoconf/autotest-mode hides 
/usr/share/emacs/site-lisp/autotest-mode
/usr/share/emacs/24.2/site-lisp/cmake-data/cmake-mode hides 
/usr/share/emacs/site-lisp/cmake-mode
/usr/share/emacs24/site-lisp/html-helper-mode/tempo hides 
/usr/share/emacs/24.2/lisp/tempo
/usr/share/emacs24/site-lisp/flim/hex-util hides 
/usr/share/emacs/24.2/lisp/hex-util
/usr/share/emacs24/site-lisp/flim/md4 hides /usr/share/emacs/24.2/lisp/md4
/usr/share/emacs24/site-lisp/dictionaries-common/flyspell hides 
/usr/share/emacs/24.2/lisp/textmodes/flyspell
/usr/share/emacs24/site-lisp/dictionaries-common/ispell hides 
/usr/share/emacs/24.2/lisp/textmodes/ispell
/usr/share/emacs24/site-lisp/css-mode/css-mode hides 
/usr/share/emacs/24.2/lisp/textmodes/css-mode
/usr/share/emacs24/site-lisp/flim/hmac-md5 hides 
/usr/share/emacs/24.2/lisp/net/hmac-md5
/usr/share/emacs24/site-lisp/flim/sasl-ntlm hides 
/usr/share/emacs/24.2/lisp/net/sasl-ntlm
/usr/share/emacs24/site-lisp/flim/sasl-cram hides 
/usr/share/emacs/24.2/lisp/net/sasl-cram
/usr/share/emacs24/site-lisp/flim/ntlm hides /usr/share/emacs/24.2/lisp/net/ntlm
/usr/share/emacs24/site-lisp/flim/sasl hides /usr/share/emacs/24.2/lisp/net/sasl
/usr/share/emacs24/site-lisp/flim/hmac-def hides 
/usr/share/emacs/24.2/lisp/net/hmac-def
/usr/share/emacs24/site-lisp/flim/sasl-digest hides 
/usr/share/emacs/24.2/lisp/net/sasl-digest
/usr/share/emacs24/site-lisp/latex-cjk-thai/thai-word hides 
/usr/share/emacs/24.2/lisp/language/thai-word
/usr/share/emacs24/site-lisp/html-helper-mode/html-helper-mode hides 
/usr/share/emacs/site-lisp/html-helper-mode/html-helper-mode
/usr/share/emacs24/site-lisp/html-helper-mode/hhm-config hides 
/usr/share/emacs/site-lisp/html-helper-mode/hhm-config
/usr/share/emacs24/site-lisp/html-helper-mode/tempo hides 
/usr/share/emacs/site-lisp/html-helper-mode/tempo
/usr/share/emacs24/site-lisp/html-helper-mode/visual-basic-mode hides 
/usr/share/emacs/site-lisp/html-helper-mode/visual-basic-mode

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
easymenu mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail regexp-opt rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils w3m-load jabber-autoloads
time-date tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd
tool-bar dnd fontset image fringe lisp-mode register page menu-bar
rfn-eshadow timer select scroll-bar mouse jit-lock font-lock syntax
facemenu font-core frame cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese hebrew greek romanian slovak
czech european ethiopic indian cyrillic chinese case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer loaddefs button faces
cus-face files text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget hashtable-print-readable backquote
make-network-process dbusbind dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)