bug#454: file-precious-flag overwrites symlinks

bug#454: file-precious-flag overwrites symlinks

[Max Mikhanosha]

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
Loading emacsbug...
Loading regexp-opt...done
Loading emacsbug...done
call-interactively: Text is read-only

bug#454: file-precious-flag overwrites symlinks

[Chong Yidong]

> 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.

Setting file-precious-flag to t in custom-save-all shouldn't be a
problem, because the custom file is visited using
find-file-visit-truename.  I tried to reproduce your problem, without
success:

$ touch foo
$ ln -s foo .emacs
$ emacs
  M-x customize-variable RET sentence-end-double-space RET
  [Click on Toggle]
  C-x C-s
  C-x C-c
$ ls -l foo .emacs

lrwxrwxrwx 1 cyd cyd   3 2008-07-30 18:54 .emacs -> foo
-rw-r--r-- 1 cyd cyd 528 2008-07-30 18:55 foo

Could you provide a recipe that shows this problem?

bug#454: file-precious-flag overwrites symlinks

[Max Mikhanosha]

After playing with it for a little bit, I found that the additional
condition is to have ~/.emacs opened in the buffer, it does not matter
if its modified or not.

So updated steps to reproduce the problem are:

1. Setup ~/.emacs being symlink to real file
2. M-x set-variable backup-by-copying-when-linked to t (this step is not nessesary to reproduce, just to show that his does not fix it)
3. Make sure sure ~/.emacs is not opened in any buffer, customize a variable, and do "Save for future sessions"
4. It saves changes to where file is pointing
5. Now do C-x C-f ~/.emacs
6. Customize variable again, do "Save for future sessions", it now overwrites ~/.emacs link with real file

I can reproduce this with 22.2.1 and with latest emacs from CVS.

Regards,
  Max

At Wed, 30 Jul 2008 18:56:16 -0400,
Chong Yidong wrote:
> 
> > 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.
> 
> Setting file-precious-flag to t in custom-save-all shouldn't be a
> problem, because the custom file is visited using
> find-file-visit-truename.  I tried to reproduce your problem, without
> success:
> 
> $ touch foo
> $ ln -s foo .emacs
> $ emacs
>   M-x customize-variable RET sentence-end-double-space RET
>   [Click on Toggle]
>   C-x C-s
>   C-x C-c
> $ ls -l foo .emacs
> 
> lrwxrwxrwx 1 cyd cyd   3 2008-07-30 18:54 .emacs -> foo
> -rw-r--r-- 1 cyd cyd 528 2008-07-30 18:55 foo
> 
> Could you provide a recipe that shows this problem?