bug#18141: 24.4.50; saving .gz file breaks file coding

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: rgm@gnu.org,  18141@debbugs.gnu.org,  vincent@vinc17.net,  yamaoka@jpl.org
> Date: Thu, 07 Aug 2014 15:08:41 -0400
> 
> > Why not move the call to backup-buffer (and surrounding code that
> > deals with backup complications) from basic-save-buffer-2 to a
> > separate function, and then call that function directly from
> > write-region, right before it is about to write the new contents?
> 
> That's the hook I suggested.
> It's indeed a valid approach.  But it suffers from two problems:
> - currently write-region doesn't itself perform backups, so if we make
>   it do them always, it's an incompatible change.
> - if write-region sometimes performs backups, it'd probably be
>   controlled by some dynamic-scoped var (e.g. a hook), in which case
>   we could get "spurious" backups if write-region gets called during the
>   execution of write-region.

We could add a new optional argument to write-region to control
whether backup is performed.

> > While at that, we should IMO make the backup-then-write sequence a
> > transaction, by using the suitable unwind-protect functions, and
> > perhaps also make sure that unwind-protect function runs if Emacs is
> > killed half-way through the sequence, to keep the transaction promise.
> 
> I'm not terribly worried about this "time window" especially since
> I think it's pretty clear that we can't completely eliminate it anyway.

I think users generally expect this to be a transaction.  And at least
on Posix platforms, where rename is an atomic operation, we can
achieve this, or come pretty close.