Re: guaranteed undo boundaries

[Stefan Monnier]

> Possibly this is already guaranteed, merely this could be spelled out
> more explicitly in some docstring / the elisp manual. Possibly, I am
> missing something else.

By default, undo boundaries are pushed after each command run by
the user.  But every command/function is free to adjust this behavior by
calling undo-boundary internally (or by removing the boundary, as does
self-insert-command).

So there's no such guarantee.
What you can do, OTOH, is something like

    (undo-boundary)
    (let ((bottom buffer-undo-list))
      <do-db-update>
      (remove-all-boundaries-between buffer-undo-list bottom))


-- Stefan