Ligther version of composite.c bug in emacs 27

ligature-bug-light.el

;;; ligature-bug-light.el --- Reproduce an autocomposition bug -*- lexical-binding: t; -*-
;;; Commentary:
;;
;;  Reproducer for the composition table bug
;;
;;;; Method
;;
;; Evaluate the file
;; Expected : "=> testing overlays" is displayed at some point and disappear
;; Actual : inifinite looping, trying to "shape unibyte text",
;;          which happens to be an empty string (from my earlier debugging)
;;
;;; Code:

(defvar ligature-font-with-big-arrow "Fira Code 11"
  "A valid font name for `set-frame-font'. A font with a => ligature is necessary to trigger the bug.")
(defvar bugged-overlay nil "Bugged overlay trying to compose an empty string.")

;;;; Setup
;; Requiring composite
(require 'composite)

;; Shaping => into a ligature
(set-char-table-range composition-function-table ?= '(["=>" 0 font-shape-gstring]))

;; Setting the font
;; =>
;; At this line
(set-frame-font ligature-font-with-big-arrow nil t)
;; Now you should see ligatures for =>
;; =>

;;;; Enabling the bug
(setq bugged-overlay (make-overlay (point-at-bol 4) (point-at-eol 4) (current-buffer)))
(overlay-put bugged-overlay 'category 'test-overlays)
;; C-x C-e at the end of this sexp will trigger infinite composition
(overlay-put bugged-overlay 'after-string "=> testing overlays")

(delete-overlay bugged-overlay)

(provide 'ligature-bug-light)
;;; ligature-bug-light.el ends here