bug#454: file-precious-flag overwrites symlinks

[Max Mikhanosha <max@openchat.com>]

I keep my .emacs file under version control as part of overall
collection of config files, and as result ~/.emacs is a symbolic
link.

`backup-by-copying-when-linked' is set to t, so visiting ~/.emacs
and then editing and saving it correctly saves the file that
symlink is pointing too.

For a long time I was trying to track down why sometimes, my ~/.emacs
symbolic link is suddenly overwritten by the actual file.

Finally found that this happens when one uses "Save for future
session" in customize package, and farther tracked it down to a
behavior of `basic-save-buffer-2' when file-precious-flag is set.

Test case:

-rw-r--r-- 1 max users 4 Jun 20 12:57 file
lrwxrwxrwx 1 max users 4 Jun 20 12:58 link -> file

backup-by-copying-when-linked => t
file-precious-flag => nil

Visit "link" edit and save

-rw-r--r-- 1 max users 14 Jun 20 12:59 file
lrwxrwxrwx 1 max users  4 Jun 20 12:58 link -> file

Everything fine..

backup-by-copying-when-linked => t
file-precious-flag => t

Visit "link" edit and save

-rw-r--r-- 1 max users 14 Jun 20 12:59 file
-rw-r--r-- 1 max users 24 Jun 20 13:00 link

Overwrites symlink

The offending code in basic-save-buffer-2 is:
            ;; We succeeded in writing the temp file
            ;; so rename it.
            (rename-file tempname buffer-file-name t))

Above should refer to buffer-file-truename IMHO. The documentation for
file-precious-flag mentiones that it will break hardlinks, but does
not mention that it will break symlinks.

If this behavour is not a bug, then maybe its a bug for
custom-save-all to set file-precious-flag, causing an suprise
to the user as to why ~/.emacs symlink is getting overwritten.

In GNU Emacs 22.2.1 (x86_64-suse-linux-gnu, GTK+ Version 2.10.6)
 of 2008-04-09 on momoland
configured using `configure  '--with-gcc' '--with-pop' '--without-hesiod' '--with-kerberos' '--with-kerberos5' '--with-leim' '--with-xim' '--with-system-malloc' '--prefix=/usr' '--infodir=/usr/share/info' '--mandir=/usr/share/man' '--localstatedir=/var' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib' '--with-x' '--with-sound' '--with-xpm' '--with-jpeg' '--with-tiff' '--with-gif' '--with-png' '--with-x-toolkit=gtk' '--x-includes=/usr/include' '--x-libraries=/usr/lib64:/usr/share/X11' '--build=x86_64-suse-linux-gnu' 'build_alias=x86_64-suse-linux-gnu' 'CC=gcc' 'CFLAGS=-O2 -g -fmessage-length=0 -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -std=gnu89 -pipe -fno-strict-aliasing -Wno-pointer-sign -Wno-unused-variable -Wno-unused-label -DSYSTEM_PURESIZE_EXTRA=55000 	 -DSITELOAD_PURESIZE_EXTRA=10000 ' 'L
 DFLAGS=''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: C
  locale-coding-system: nil
  default-enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  unify-8859-on-encoding-mode: t
  utf-translate-cjk-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
C-x C-f C-g i i C-x C-f C-g C-x C-b * C-x o ESC O B 
RET ( s e t q SPC f i l e DEL DEL DEL DEL DEL DEL DEL 
DEL DEL DEL M-x - s e DEL DEL DEL s e t - v a r i TAB 
RET b a c k u p - b y - c o p TAB - TAB l TAB RET t 
RET C-x C-f l i n k RET o l a l DEL DEL DEL ESC [ 4 
~ ESC [ 4 ~ ESC O B b DEL s o m e SPC t e x t C-x C-x 
M-x s a v e - b u f DEL DEL DEL DEL TAB - b u f f e 
r RET M-x s e t - v a r i a b l e RET f i l e - p r 
e TAB RET t RET RET m o r e SPC t e x t M-x M-p M-p 
RET C-x b TAB ESC O B ESC O B M-x s u b m i t - b u 
TAB DEL DEL DEL DEL DEL DEL DEL DEL DEL r e p o r t 
- e m TAB RET

Recent messages:
exchange-point-and-mark: No mark set in this buffer
Making completion list...
Wrote /home/max/link [2 times]
Making completion list...
next-history-element: End of history; no next item
Quit
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