bug#60032: 30.0.50; [WISH] Customizing Info directories

[Eli Zaretskii <eliz@gnu.org>]

> From: Ihor Radchenko <yantar92@posteo.net>
> Cc: 60032@debbugs.gnu.org
> Date: Tue, 13 Dec 2022 13:43:50 +0000
> 
> The user attempted to customize `Info-default-directory-list' and got
> surprised that it was ignored.
> 
> Upon my further investigation, `Info-default-directory-list' is indeed
> ignored when INFOPATH is set by OS.

Yes.  But why didn't that user modify INFOPATH?

> However, `Info-default-directory-list' being ignored is rather
> surprising, especially given that setting the other defcustom
> (`Info-additional-directory-list') does not allow overriding system
> settings.

It is done for a good reason.  Setting up the list correctly is
difficult enough, what with the requirement that Emacs's own manuals
should sometimes be before the system-wide installation, and sometimes
after it (to support different use cases).  When INFOPATH is defined,
and used by the stand-alone Info reader, install-info, and other
stuff, it becomes impossible to DTRT.  So we punt, in effect saying:
if you have INFOPATH, you should already know what you are doing.

If you want the gory details (which I happily forgot), run "git log -L"
(or vc-region-history) on the area where Info-default-directory-list
is defined and used, and read the discussions related to the changes
and bugs reported against that.

> Changing environment variables is something I never ever needed to
> customize Emacs' behaviour. Once I see it, my thinking goes: "Surely,
> there should be some custom variable.", but no such variable exists.

Does this include PATH?  If not, why not?

IME, the *PATH variables are quite special: they should generally be
set from outside Emacs, and doing the same via customization options
is usually not a good idea.