bug#454: file-precious-flag overwrites symlinks

[Chong Yidong <cyd@stupidchicken.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.

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?