bug#66313: Acknowledgement (29.1.50; process-mark sometimes does not yield the expected value)

[Markus Triska]

I have reduced this to the following problem: set-process-buffer
sometimes unexpectedly changes the position of process-mark.

For a shorter test case that exhibits the issue, please download
process-mark.el from:

    https://www.metalevel.at/ei/process-mark/process-mark.el

Its contents are:

   (let ((p (start-process "bash" nil "bash")))
     (set-marker (process-mark p) (point))
     (set-process-buffer p (current-buffer))
     (marker-position (process-mark p)))

   (message "hello there!")

Place point after the first form in the file, and evaluate the form with
C-x C-e.

The expected result is: 165. It is obtained with Emacs 26.1 and several
later versions.

However, with the Emacs version I used to report this problem, the
result I get is unexpectedly: 192.

I would greatly appreciate if the previous behaviour could be restored.

Thank you and all the best,
Markus