Why is `after-change-functions' reset?

[Michael Heerdegen <michael_heerdegen@web.de>]

Hello,

I'm working on a library that adds a function to
`after-change-functions'.

After some time, the thing stops working.  I look at the value of
`after-change-functions' and my added function is missing.

So I thought there would have been some error that caused
`after-change-functions' to be reset.  But I can preclude that - there
is no error, no error message, etc.

I had a hard time to find some recipe for emacs -Q.  Here it comes:

--8<---------------cut here---------------start------------->8---
(progn
  
  (defun foo (&rest _) ())
  
  (add-hook 'after-change-functions 'foo)
  
  (defun complain ()
    (when (not (memq 'foo (default-value 'after-change-functions)))
      (message "Why does this happen?")
      (cancel-timer complain-timer)))
  
  (defvar complain-timer
    (run-with-idle-timer .03 .03 #'complain))

  (require 'cl-lib)
  (completing-read "Hit tab and C-g:  "
		   (cl-loop for i from 1 to 100000
			    collect (format "%d" i))))
--8<---------------cut here---------------end--------------->8---

Evaluate it and hit tab (for completion) and C-g C-g in succession.
When you repeat this a few times, you get "Why does this happen?".

Is this expected behavior?  In my original situation, quitting was not
involved, the above recipe was just the first reproducible program I had
discovered.


Thanks,

Michael.