bug#30931: 27.0.50; Crash in "Automatic GC"

[Michał Kondraciuk]

I don't know if it helps, but I was able to reproduce a crash in the 
same place (mark_object()) with bare Emacs, however only in 27.0.50.

I have no idea what this code does at all. The way I wrote this is I 
kept rerunning the previous recipe but each time I commented out some
part of yasnippet.el library. If Emacs crashed, I kept the change, 
otherwise I uncommented those parts and focused on another region, and 
so on until I was left with very simple functions. I hope someone can 
confirm that it crashes.

(progn
  (require 'cl-lib)
  (defvar overlays nil)

  (defun on-overlay-modification (&rest args)
    (push nil buffer-undo-list))

  (defun crash ()
    (save-restriction
      (narrow-to-region 1 1)

      (let (start end field)
        (goto-char 2)
        (insert " 123aaaa")
        (narrow-to-region 2 5)

        (setq start (set-marker (make-marker) 2))
        (setq end (set-marker (make-marker) 3))
        (indent-according-to-mode)

        (sit-for 0)
        (cond (overlays
               (move-overlay (nth 1 overlays) end (1+ end)))
              (t
               (setq overlays
                     (list (make-overlay (1- start) start nil t nil)
                           (make-overlay end (1+ end) nil t nil)))
               (dolist (ov (list (make-overlay (1- start) start nil t nil)
                           (make-overlay end (1+ end) nil t nil)))
                 (overlay-put ov 'modification-hooks 
'(on-overlay-modification))))))))

  (with-current-buffer "*scratch*"
    (let ((buffer-undo-list t))
      (while t
        (erase-buffer)
        (crash)))))