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

[João Távora]

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.

In Eglot, the eglot--managed-mode minor mode is an implementation
detail, it is NOT meant to be called by the user, since it requires a
number of preconditions (like firing up a successful server) to be met.

Therefore, I have named the symbol with the "internal symbol"
convention.  In this very simple patch, I teach desktop.el to watch out
for this convention and not restart that mode.

diff --git a/lisp/desktop.el b/lisp/desktop.el
index 1a4103e209..a93703a77e 100644
--- a/lisp/desktop.el
+++ b/lisp/desktop.el
@@ -1617,7 +1617,9 @@ desktop-create-buffer
                   (let ((handler (cdr (assq minor-mode 
desktop-minor-mode-handlers))))
                     (if handler
                         (funcall handler desktop-buffer-locals)
-                      (when (functionp minor-mode)
+                      (when (or (functionp minor-mode)
+                                 (and (symbolp minor-mode)
+                                      (not (string-match "^[^-]+--" 
(symbol-name minor-mode)))))
                         (funcall minor-mode 1)))))))
          ;; Even though point and mark are non-nil when written by
          ;; `desktop-save', they may be modified by handlers wanting to set


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.

Thanks,
João