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

From: Hauke Rehfeld <[email-masked]>
Date: Mon, 30 Aug 2021 13:43:23 +0200

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 <[email-masked]> 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))