bug#46908: 28.0.50; Dozens of spurious markers in buffer-undo-list

[Michael Heerdegen <michael_heerdegen@web.de>]

Hello,

after using `undo', `buffer-undo-list' often contains entries
referencing several or dozens of markers (seemingly unrelated to
overlays or such; I dunno from where they originate).  I know this
happens in current Emacs master, and I also found the problem in the
27.1 coming with Debian testing, build from 2020-11-08.  I tried in X.

I measure the count of different marker objects in a buffer's
`buffer-undo-list' with this function:

#+begin_src emacs-lisp
(defun count-markers-in-buffer-undo-list ()
  "Message number of (different) markers in `buffer-undo-list'."
  (interactive)
  (require 'cl-lib)
  (message "%d" (length
                 (cl-delete-duplicates
                  (seq-filter #'markerp (flatten-tree (copy-tree buffer-undo-list)))
                  :test #'eq))))
#+end_src

To reproduce the problem, open a fresh emacs -Q, and in *scratch*,
something like this should do it:

"C-5 C-0 a, RET, C-SPC, C-p, C-SPC, C-n, M-w,
C-y, C-y, C-y";

this recipe is by Yuan Fu, who first reported the problem in emacs-help,
and who is CC'd.  Then he calls the above command and gets a number much
larger than 1.

I can reproduce the problem without copy-and-paste, I just hold down the
"a" key and insert some random space chars in between, then undo
everything.  Sometimes I need to repeat once or twice to get a number
larger than "1", but very soon I see numbers like "40" which definitely
look pathological.  For me the appearance of the issue seems to behave a
bit randomly.


Thanks in advance,

Michael.



In GNU Emacs 28.0.50 (build 6, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0)
 of 2021-03-03 built on drachen
Repository revision: b28b8d83ac882415555fe02b73a26542c9fca781
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: Debian GNU/Linux bullseye/sid

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LCMS2 LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG SOUND
THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
  value of $LC_ALL: de_DE.utf8
  value of $LC_COLLATE: C
  value of $LC_TIME: C
  value of $LANG: de_DE.utf8
  locale-coding-system: utf-8-unix