bug#53798: 27.1; OSX: (redisplay) does not reliably redisplay

[Markus Triska]

Eli Zaretskii <eliz@gnu.org> writes:

>
> If I modify the snippet as below, it behaves the same no matter
> whether you press a key or not.
>

Thank you a lot for looking into this issue! You are right: For the
snippet I originally posted, I can fix the issue as you outline.

Since the issue I faced in my original program cannot be solved in this
way, I have now constructed a snippet that shows the problem in a way
that apparently cannot be solved in this way, using the forms that
follow this message.

On OSX, if you save these forms in redisplay.el and then invoke Emacs
with:

    $ emacs -Q redisplay.el -f eval-buffer

then the forms will open UnicodeData.txt which ships with Emacs (please
adjust the path if necessary), and then will apply a certain
transformation on each line of the file.

You will see that each line is changed as intended, and we see each
change immediately as it takes effect. However, unexpectedly, when
several keys are pressed quickly in a row, the program apparently
stalls, and no longer performs any actions, for several seconds.

I found that the settings of interprogram-cut-function and
save-interprogram-paste-before-kill are relevant to reproduce this
issue. Could it be that how these settings are treated internally may
interfere with how Emacs redisplays things, and whether it stalls?

To forms to reproduce the issue follow below.

Thank you a lot!
Markus

;; please adapt this as necessary to find UnicodeData.txt
(find-file "~/emacs/admin/unidata/UnicodeData.txt")

(defun transform-line ()
  (insert "code_description(")
  (forward-word)
  (delete-char 1)
  (insert ", \"")
  (search-forward ";")
  (delete-char -1)
  (insert "\").")
  (kill-line)
  (forward-line))

(defun paste-to-osx (text &optional push)
  (let ((process-connection-type nil))
    (let ((proc (start-process "pbcopy" "*Messages*" "pbcopy")))
      (process-send-string proc text)
      (process-send-eof proc))))

(setq interprogram-cut-function 'paste-to-osx)
(setq save-interprogram-paste-before-kill t)

(while t
  (transform-line)
  (redisplay)
  (or (sit-for 0.1)
      (read-char)))