Re: Question about composite.c

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerry Agbobada <gagbobada@gmail.com>
> Date: Wed, 15 Apr 2020 00:43:59 +0200
> Cc: emacs-devel@gnu.org
> 
> It took me a good while, but I found a case where :
> - I can reproduce with =emacs -q=  (Reproducing infinite looping in
> composition_get_gstring with an empty non-nil string)
> - prettify-symbols-mode is not enabled
> 
> The example includes the code of eros.el package, which creates an
> overlay when calling eval-last-sexp. The overlay is supposed to start
> with `=>`, and when I have this in my composition-function-table it
> triggers the loop.
> As explained in the file linked below, with the cursor on `(+ 3 2)|`
> I'd expect `C-x C-e` to create an overlay with `=> 5`. Instead, it
> loops indefinitely and the only way out is to kill -9 emacs (kill
> -USR2 doesn't help)

Is the overlay a necessary part of the problem?  Also, is it necessary
to define a separate char-table, instead of using
composition-function-table?

The example looks quite complex, and I wonder whether you could
prepare a simplified variant that only included the parts that are
necessary?  That would make debugging this issue much easier.

> (let ((alist
>        '(
>          (?= . "=>") ; Matches =>
>          )))
>   (dolist (char-regexp alist)
>     (set-char-table-range other-composition-ligature-table (car char-regexp)
>                           `([,(cdr char-regexp) 0 font-shape-gstring]))))
> 
> (set-char-table-parent other-composition-ligature-table composition-function-table)
> 
> (setq-local composition-function-table other-composition-ligature-table)

From this, I don't think I understand why the problem is triggered by
a '+' character.  If I do the following in "emacs -Q":

  (set-char-table-range composition-function-table
			#x3d
			(list (vector "=>" 0 'compose-gstring-for-graphic)))

and then type (+ 2 3)C-x C-e, I don't get any loop, and no characters
related to the => ligatures anywhere.  What am I missing?

In what version of Emacs do you see these infloops, btw?