bug#46374: 28.0.50; Ask me to save buffers only if they are under callers dir

[Juri Linkov <juri@linkov.net>]

>>>> Why not simply add a new option to the existing variable
>>>> save-some-buffers-default-predicate?  For example:
>>>>
>>>> (defcustom save-some-buffers-default-predicate nil
>>>>   "Default predicate for `save-some-buffers'."
>>>>   :group 'auto-save
>>>>   ;; FIXME nil should not be a valid option, let alone the default,
>>>>   ;; eg so that add-function can be used.
>>>>   :type '(choice (const :tag "Default" nil)
>>>>                  (const :tag "Subdirs of default directory" default-directory)
>>>>                  (const :tag "Project root" project-root)
>>>>                  function)
>>>
>>> Indeed, this was my initial implemention.
>>> Then I moved to the one I shared here; I found my second version
>>> superior for the following reason:
>>>
>>> 1. Users can restrict the buffer default directory AND still pass a
>>>    predicate to filter by any other thing they wish;
>>>    i.e. you don't need to chose either one of the other.
>>>
>>> 2. I find it cleaner having it in a separated option.
>>
>> I'm not sure if there is a need for adding
>> another dimension with another option.
>
> This is  explained in my point 1. above.
> I might want to restrict to the caller subdir and still filter with
> a function.  You cannot get both things just with one option.
>
> For my normal worflow, this is a very important addition and I am looking
> forward to see it added in master soon.

Please consider the consequences of backward-incompatibility
of adding such a separate option.  For example, in the org package:

org-mobile-push:
      (save-some-buffers nil
			 (lambda () (memq (current-buffer) agenda-buffers)))

When a user customizes a new option to restrict saving of buffers to
the subdirectories only, this means it will skip saving agenda-buffers
since most of them usually are located outside of the current directory.
This change would have a drastic effect for external packages where
a new option can't be forced to be bound to nil.

More examples:

org-save-all-org-buffers:
  (save-some-buffers t (lambda () (derived-mode-p 'org-mode)))

etc.

This means reusing the existing save-some-buffers-default-predicate
would be still preferable that guarantees backward-compatibility.
When it's customized to a predicate to filter out non-current subdirs,
then such call '(save-some-buffers t (lambda () (derived-mode-p 'org-mode)))'
still overrides the customized value.  This is the right thing to do.

>> But if yes, then still shouldn't a new option
>> provide a wider choice of restricting predicates
>> like above?
>
> Yeah, we could made wider the new option as you've suggested:
>>>>                  (const :tag "Subdirs of default directory" default-directory)
>>>>                  (const :tag "Project root" project-root)
>
> Since Stefan mentioned `project-root` is not ready and I am not familiar
> with it, I suggest we add the option as it is, possibly with a modified
> name that fit well our future intention and a TODO comment.
> Then, me or someone else, can update the option when `project-root`
> become mature.
>
> Agreed?

Then the option could be named 'project-root-or-subdirs',
so when a command is called outside a project, it will
fall back to subdirs.