bug#583: Use XDG basedir spec for configuration files?

[Eli Zaretskii <eliz@gnu.org>]

> From: Glenn Morris <rgm@gnu.org>
> Cc: 583@debbugs.gnu.org,  ferkiwi+a@gmail.com,  eggert@cs.ucla.edu
> Date: Wed, 28 Aug 2019 22:17:55 -0400
> 
> IMO a true conversion of Emacs to the XDG spec would involve the
> addition of new variables (user-config-directory, user-data-directory,
> user-cache-directory, etc). Each use of user-emacs-directory would have
> to be reviewed to see where it actually belongs. For some, it would
> probably be ambiguous.

I'm afraid almost all of them will be ambiguous.  Moreover, defining a
clear enough set of rules for where to put files that will be added in
the future is a non-trivial job, and enforcing it will not be easy.  I
predict confusion and arguments.

My gut feeling is that Emacs should have one, at most 2 XDG-derived
directories, certainly not 3.  And if we want to support that
correctly, these environment variables should be accessed much earlier
in the startup process, somewhere in init_callproc and friends,
because setting user-emacs-directory in startup is too late and
creates problems, as we saw already.

We've made a small step in that direction; if we want to continue that
way, someone should volunteer to do the job of defining what files
will go were.