Subject: bug#53294: 29.0.50; Indirect font changes incorrectly affecting original buffer

From: Stefan Monnier <[email-masked]>
Date: Mon, 17 Jan 2022 21:47:30 -0500

> That sounds reasonable, I can do that. In fact, I just did this based on
> your suggestions, and it does work well.  The only weird thing is that I had
> to pull `clone-indirect-buffer-hook' into the c code, because that's where
> `make-indirect-buffer' is.

You could leave the `defvar` in Lisp and only pull the DEFSYM, but
yes, you need to pull at least part of it into C.

> Let me know if that seems wrong.

Nope.

> I've attached the patch, please let me know if there is an issue. I have
> commit access, so I can just commit it myself after your OK (if so, I'll
> wait for a week or so to see if Eli has a comment as well before checkin).

Comments below, thanks,

        Stefan

> diff --git a/lisp/face-remap.el b/lisp/face-remap.el
> index 00560f9d2e..fc0edb7119 100644
> --- a/lisp/face-remap.el
> +++ b/lisp/face-remap.el
> @@ -70,6 +70,14 @@ internal-lisp-face-attributes
>     :foreground :background :stipple :overline :strike-through :box
>     :font :inherit :fontset :distant-foreground :extend :vector])
>  
> +(defun face-attrs--make-indirect-safe ()
> +  "Do a deep copy of `face-remapping-alist' to keep the original buffer safe."
> +  (setq-local face-remapping-alist
> +              (copy-tree
> +               (buffer-local-value 'face-remapping-alist (buffer-base-buffer)))))

I think `mapcar #'copy-sequence` is slightly more correct than
`copy-tree`, and you shouldn't need
(buffer-local-value 'face-remapping-alist (buffer-base-buffer)) because
that buffer's value should already have been copied to the current buffer.

> @@ -912,6 +912,10 @@ DEFUN ("make-indirect-buffer", Fmake_indirect_buffer, Smake_indirect_buffer,
>        Fset (intern ("buffer-save-without-query"), Qnil);
>        Fset (intern ("buffer-file-number"), Qnil);
>        Fset (intern ("buffer-stale-function"), Qnil);
> +      /* Cloned buffers need extra setup, to do things such as deep
> +	 variable copies for list variables that might be mangled due
> +	 to destructive operations in the indirect buffer. */
> +      safe_run_hooks (Qclone_indirect_buffer_hook);

I think you want to use just `run_hook` here because there's no need to
catch errors.

More importantly, you need to remove the corresponding `run-hook` from
`clone-indirect-buffer`.

        Stefan