bug#74438: 29.1; global-mark-ring does not work as described

[Sean McAfee <eefacm@gmail.com>]

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On Thu, Nov 21, 2024 at 12:32 PM Nikolay Kudryavtsev <
nikolay.kudryavtsev@gmail.com> wrote:

> I still don't understand why do you need more than one mark for your use
> case?
>
> You are comparing two sexps. You put the foo buffer sexp position into
> the mark ring. Now you go to the bar buffer and select the sexp there.
> Why do you need to put the bar into the mark ring? Why do you need the
> second foo mark? You can just use the first(and only) foo mark and be
> done with it.
>

Er...yeah.  That's exactly what I want to do.  Not sure how I gave a
different impression.

Here's what I have now, minus some defensive coding to keep it short:

    ;; Helper function:
    (defun write-sexp-at (pos dest-file)
      (write-region pos (save-excursion (goto-char pos) (forward-sexp)
(point)) dest-file))

    (defun json-diff ()
      (interactive)
      (let ((file-a (make-temp-file "json-diff"))
            (file-b (make-temp-file "json-diff")))
        (with-current-buffer (marker-buffer last-global-mark)
          (write-sexp-at (marker-position last-global-mark) file-a))
        (write-sexp-at (point) file-b)
        (shell-command (format "~/bin/json-diff %s %s" file-a file-b))))

Originally I tried using (car global-mark-ring) instead of
last-global-mark, but that doesn't work, per my original bug report.  I
also tried saying (mark t) instead of (marker-position (car
global-mark-ring)).  That would have returned the correct position, but it
doesn't matter because (marker-buffer (car global-mark-ring)) can't be
relied upon to return the right buffer.

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