bug#56407: 29.0.50; desktop.el shouldn't be saving/restoring eglot--managed-mode, which is not for interactive use

[Eli Zaretskii <eliz@gnu.org>]

> Cc: Morten Welinder <terra@diku.dk>
> From: João Távora <joaotavora@gmail.com>
> Date: Tue, 05 Jul 2022 20:34:16 +0100
> 
> Hi Morten, maintainers,
> 
> Recently, in https://github.com/joaotavora/eglot/issues/990, we found
> out that users of both eglot.el and desktop.el were getting errors when
> restarting Emacs.  That's because desktop.el attempts to re-enable the
> eglot--managed-mode minor mode which was "on" when they saved the
> session (presumably on exit).
> 
> I confirmed this with a stack trace requested from the user and came up
> with this workaround in the user's config:
> 
>     (add-to-list 'desktop-minor-mode-handlers
>                  '(eglot--managed-mode . ignore))
> 
> This works, but we should come up with something better.

But why is the above not good enough?  You could also use
desktop-minor-mode-table, which is a defcustom.

IOW, desktop.el already has the machinery to not restore some modes,
and I see no need to make a general change like you suggest just
because the mode in your case happened to have a symbol which looks
like an internal one.  Your change also precludes anyone to have such
a mode restored, ever -- why?

> This probably works (though I haven't tested), but maybe we could come
> up with some other way around this, like having eglot.el propertize its
> 'eglot--managed-mode' symbol so that desktop.el doesn't even write it
> onto the user's save file.  Or something like that.

Why invent new machinery, when we already have more than enough to
handle this problem (and many similar ones)?