bug#61215: 29.0.60; font-lock broken in diff-mode with long lines

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: 61215@debbugs.gnu.org
> Date: Mon, 06 Feb 2023 19:15:49 +0200
> 
> >> If you think the problem is in diff-syntax-fontify-hunk,
> >> then I could try to find a working workaround local to diff-mode.
> >
> > Yes, now this is something for diff-mode to solve: it shouldn't assume
> > that it will always be able to go to the beginning and the end of a
> > hunk, because narrowing could interfere with that.  Diff-mode should
> > have some fallback plan for these cases.
> 
> After debugging I discovered that the problem is not caused by font-lock
> in diff-mode.  The problem is in easy-mmode-define-navigation.
> diff-mode uses it to navigate diff hunks.  It supports such a feature
> that when the user narrowed the diff buffer, it puts narrowing back
> after moving to the next hunk.  But this fails when the diff buffer
> is narrowed automatically by long-line-optimizations.  If there is
> no way to distinguish whether the narrowing is automatic by long-lines
> or manual by the user, this patch fixes the problem.  It works without
> errors, and even highlights first 250000 characters of the long line:

Thanks.

Maybe Gregory (CC'ed) can help here and tell whether there's a better
solution.

> diff --git a/lisp/emacs-lisp/easy-mmode.el b/lisp/emacs-lisp/easy-mmode.el
> index 5721470ad0d..d9176061434 100644
> --- a/lisp/emacs-lisp/easy-mmode.el
> +++ b/lisp/emacs-lisp/easy-mmode.el
> @@ -782,7 +782,10 @@ easy-mmode-define-navigation
>           (when-narrowed
>            (lambda (body)
>              (if (null narrowfun) body
> -              `(let ((was-narrowed (prog1 (buffer-narrowed-p) (widen))))
> +              `(let ((was-narrowed
> +                      (prog1 (unless (long-line-optimizations-p)
> +                               (buffer-narrowed-p))
> +                        (widen))))
>                   ,body
>                   (when was-narrowed (funcall #',narrowfun)))))))
>      (unless name (setq name base-name))
>