Re: creating backups in temporary directories

[Richard Stallman <rms@gnu.org>]

    This of course applies to the backup-by-copying case too: then just open
    /tmp/foo~ with O_EXCL when performing the copy, with /tmp/randomfile~ and
    "you have enemies" if it fails.

The code in backup-buffer-copy already does part of this;
it calls copy-file  in a way that uses O_EXCL.

    If /tmp/foo~ already exists, rename it first to /tmp/backupforthebackup~
    rather than unlinking it; we have to get it out of the way and use O_EXCL
    even if we own it in case the directory's owner is the attacker.

Why is this needed?  When we're doing the copy, the actual source file
also exists.