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

[martin rudalics <rudalics@gmx.at>]

 >> I cannot confirm that, no.  I can reproduce the issue in a newly built
 >> Emacs 27 as well as in the 27 that comes with Debian.  "It" again being
 >> `count-markers-in-buffer-undo-list'.
 >
 > The issue disappears (marker count stays constantly less than 3) if I
 > comment out these two expressions in `push-mark':
 >
 >    (add-to-history 'mark-ring (copy-marker (mark-marker)) mark-ring-max t)
 >
 >    (add-to-history
 >      'global-mark-ring (copy-marker (mark-marker)) global-mark-ring-max t)
 >
 > I think we really have a problem here: when those newly created markers
 > are thrown out of the rings, they are not invalidated, and are also
 > still referenced by buffer-undo-list, so they will live forever, AFAIU.

So you mean that we really have two problems here, both caused by
`push-mark'?  IIUC a marker is "thrown out of the rings" when its buffer
gets deleted or a marker is pushed on a "full" ring.  I suppose that
"invalidating" markers in these cases should be safe.  But how comes
that `add-to-history' affects `buffer-undo-list'?

martin