bug#63988: 30.0.50; Recent header line format changes cause spin/seg fault with format-mode-line

[Aaron Jensen <aaronjensen@gmail.com>]

On Sat, Jun 10, 2023 at 2:47 AM Eli Zaretskii <eliz@gnu.org> wrote:
>
> > From: Aaron Jensen <aaronjensen@gmail.com>
> > Date: Fri, 09 Jun 2023 21:09:45 -0400
> >
> >
> > (setq header-line-format '(:eval (format-mode-line "")))
> >
> > This causes Emacs to spin as of commit:
> > 4f66cbbfe520ee31ef26676e09a926217d9736fe
> >
> > After some time, it will segfault.
>
> It causes infinite recursion, since format-mode-line also calls
> window_wants_header_line (indirectly).
>
> But what is the purpose of such a strange (to use a civilized word)
> setting of header-line-format?  Why do you need :eval at all in this
> case?
>
> IOW, why not say "don't do that" and be done?

It's a minimal repro for an issue I encountered with a package that does this:

https://github.com/rougier/nano-modeline/blob/master/nano-modeline.el#L532

It's how this modeline/header line adds line number/cursor position to
a more complicated line. As I understand it, it has to format it first
in order to use its width to properly right align it:

https://github.com/rougier/nano-modeline/blob/master/nano-modeline.el#L278

Is there a better way to do this?

> So maybe we should declare this feature a failed experiment and remove
> it?

I'll admit I don't really understand the change. Is it actually
evaluating the cdr of the eval form up to two additional times to in
order to determine whether or not to display the headerline at all?
Wouldn't this have performance implications?

Aaron