Handle markers properly

[jackkamm]

PR #21 by @takaxp fixes a problem with org-clock-marker, but this problem is a symptom of a deeper problem, that we don't currently handle markers in the undo-propose buffers.

References

• Elisp manual: Overview of markers
• Elisp manual: Undo, see the entry on the marker element
• primitive-undo in simple.el, in particular these lines which handle marker adjustments.

Additional Notes

There are entries in the undo-list which adjust marker positions. When in the undo-propose buffer, (eq (marker-buffer m) (current-buffer)) is false so these entries are skipped.

One possible way to handle markers would be to copy the markers over to the undo-propose buffer, and edit entries in the undo-list to use the new markers.

[jackkamm]

It looks like grabbing and copying all the markers is not possible:

• https://stackoverflow.com/questions/36926513/are-there-elisp-functions-that-list-markers-in-a-given-buffer
• https://groups.google.com/forum/#!topic/gnu.emacs.help/vNG9y5qCaxc/discussion

[jackkamm]

As a workaround until we've found a permanent solution, we could create a list undo-propose--markers-to-check that we could add markers to (e.g. org-clock-marker from #21), and undo-propose will check for those markers and make copies to keep track of & update them.

[takaxp]

Yes. The important thing is that we actually don't need to handle all markers in a buffer. It is also hard to identify which markers should be checked by us for all existing packages. I think it is a good choice that only registered markers will be handled properly.

In the case of org-clock-marker, we don't need to always copy the marker and also the original maker in the original buffer shouldn't be touched in the case of calling undo-propose-cancel.

In my understanding, the markers will not be actually copied but forked from the original marker for editing works in undo-propose buffer. If we commit the buffer with some undos, then the original marker shall be updated (normally updating the position). If we don't commit, then forked marker will be just trashed.