bug#46374: Regression: erronous calls to PRED switch major-mode of unrelated modified buffers

[Hauke Rehfeld]

I'm way not deep enough into this issue, but how about using a
temp buffer and checking for major mode equivalent (can pred ever
reasonably be fundamental-mode?):

   ;; Allow `pred' to be a function that returns a predicate
   ;; with lexical bindings in its original environment
   (bug#46374).
   (let ((pred-fun (and (functionp pred))))
     ;; don't use the result of `pred' if it returns the current
     buffer major-mode
     (with-temp-buffer
       (let ((pred-fun (funcall pred)))
         (when (and (functionp pred-fun) (not (eq major-mode
         pred-fun)))
         (setq pred pred-fun)))))


Juri Linkov <juri@linkov.net> writes:

> > This means we need to invent some ad-hoc format to distinguish
> > between
> > these cases.  For example, to create a lexically-bound
> > predicate
> > at the beginning, it could be called with e.g.
> >
> >   (save-some-buffers t '(eval . save-some-buffers-root))
> >
> > and defcustom will look like:
> >
> > (defcustom save-some-buffers-default-predicate nil
> >   :type '(choice (const :tag "Default" nil)
> >                  (function :tag "Only in subdirs of root"
> >                            (eval . save-some-buffers-root))
> >                  (function :tag "Custom function"))
>
> Or maybe simply '(save-some-buffers-root):
>
>   (defcustom save-some-buffers-default-predicate nil
>     :type '(choice (const :tag "Default" nil)
>                    (function :tag "Only in subdirs of root"
>                              (save-some-buffers-root))
>
> Then the following two calls both will have exactly the same
> effect:
>
>     (save-some-buffers t '(save-some-buffers-root))
>     (save-some-buffers t (save-some-buffers-root))