bug#3702: Emacs fails to respect the intent in `C-u C-u C-x C-s' of making a backup

[Taylor R Campbell]

Often, when saving a file, I want to make a backup.  The only way I
know to express this intent is `C-u C-u C-x C-s'.  When I type `C-u
C-u C-x C-s', I mean to tell Emacs:  `I want you to make a backup and
then save this file.'  The documentation for the `save-buffer' command
says:

   Prefixed by two C-u's,
    unconditionally makes the previous version into a backup file.
    ^^^^^^^^^^^^^^^
Unfortunately, Emacs fails to respect my intent in many circumstances.
First, I set `version-control' to t in order to keep numbered backups.
Then in order to make backups in directories such as /tmp, I set
`backup-enable-predicate' to be a function that always returns t.
Next, in order to make backups when `vc' is involved, I set
`vc-make-backup-files' to t, which confused me because it didn't take
effect immediately -- I had to reload all my buffers before it did.
This is because some vc hook set another variable, `backup-inhibited',
when I first loaded those buffers.

There are plenty of other documented ways for arbitrary modes to
interfere with my intent when I type `C-u C-u' -- there are several
hooks that are run during `save-buffer' before `backup-buffer' and
conditions under which backups will be made.  This contradicts the
documentation of `save-buffer' which explicitly says that `C-u C-u'
means a backup will be made *unconditionally*.  I don't mind that
there may be various heuristics for choosing whether to make a backup
file when the user has not indicated a preference either way (by
typing `C-u 0' or `C-u C-u' or whatnot), but it is very frustrating
when the documentation says `unconditionally' and the implementation
has many documented ways to interfere and add conditions.  Of course,
the documentation could simply omit the word `unconditionally', but I
don't want that either -- I want it really to be unconditional (as far
as the documented hooks in Emacs go; of course someone could redefine
`save-buffer', but it's the documented hooks and conditions such as
`backup-inhibited' that bother me).

I am using

GNU Emacs 22.2.1 (i386-apple-darwin8.11.1, GTK+ Version 2.10.13) of 2008-07-06 
on Oberon.local.

In my .emacs file, relevant to backups, I have

(defun always-backup (filename) filename t)
(setq-default backup-enable-predicate 'always-backup)
(setq-default version-control t)
(setq-default vc-make-backup-files t)
(setq-default backup-inhibited nil)
(setq-default make-backup-files t)