> I think this is reasonable.  We're already setting epg-gpg-program in
> test/test-lib.sh, and in debian, epg-gpg-program is provided by
> epg-config.el, which is part of emacs$VERSION-el, which is a dependency
> of emacs$VERSION-common.

I believe that epg is part of the standard emacs packages now.  It
should be available wherever Emacs is.

> If we wanted to be extra careful, we could try to make it fall back to
> plain "gpg" if epg-gpg-program is unset.

This isn't a bad idea. My elisp-fu is limited, but I think that I can
probably figure out how to do this.  It looks like epg-gpg-program is
set with this in epg-config.el.

https://github.com/emacs-mirror/emacs/blob/65eee8392ff95f58f7b0bd036e1fe065523658c6/lisp/epg-config.el

(defcustom epg-gpg-program (if (executable-find "gpg2")
                               "gpg2"
                                                            "gpg")
                                                              "The `gpg'
                               executable.
                               Setting this variable directly does not
                               take effect;
                               instead use \\[customize] (see the info
                               node `Easy Customization')."
                                 :version "25.1"
                                   :group 'epg
                                     :type 'string)



> fwiw, debian will be shipping gpg2 as /usr/bin/gpg in stretch, and the
> old 1.4 branch will be /usr/bin/gpg1 -- is there a reason that NixOS
> isn't shipping gpg2 as /usr/bin/gpg ?

I actually have no idea.  I'm usually a Debian user, but liked the idea
of having a reproducible setup which was easy to move between
machines. Nix works pretty well, but there are a lot of rough edges --
like not having gpg linked to gpg2.

Best regards,
John