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

From: Hauke Rehfeld <[email-masked]>
Date: Mon, 30 Aug 2021 18:04:31 +0200

Sorry, that was broken when called with an empty `pred' argument.
This should work (but testing all code paths is indeed not a
simple task):

    ;; Allow `pred' to be a function that returns a predicate
    ;; with lexical bindings in its original environment
    (bug#46374).
    (when (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)))))

Hauke Rehfeld <[email-masked]> writes:

> 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))