bug#62991: 30.0.50; normal-backup-enable-predicate does not consider /dev/shm

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 21 Apr 2023 12:49:01 +0200
> From: Vasilij Schneidermann <mail@vasilij.de>
> 
> I've found that the `pass` password-store creates temporary files under
> /dev/shm rather than /tmp and editing those files with emacs creates
> backup files (which may leak passwords outside that directory if
> `backup-directory-alist` is customized to place them elsewhere)`. This
> seems to be due to `normal-backup-enable-predicate` specifically testing
> for /tmp (and two other temporary directories), but not other
> directories backed by a tmpfs file system (such as /dev/shm).  There
> does not appear to be a customizable to extend the list of directories
> it's checking for either. One more candidate for exclusion is the
> /run/user/<uid> directory (it's backed by tmpfs as well, but has
> different uses).

We could indeed teach normal-backup-enable-predicate about /dev/shm on
systems where that is appropriate, but I'm curious why you say this is
not customizable: normal-backup-enable-predicate is called via a
variable, backup-enable-predicate, so you could customize that by
writing your own predicate function, and assigning
backup-enable-predicate to that function.  Or what am I missing?