bug#58158: 29.0.50; [overlay] Interval tree iteration considered harmful

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: 58158@debbugs.gnu.org
> Date: Thu, 29 Sep 2022 09:03:17 +0200
> 
> >> - No GC may happen in step 2.  This is because mark_buffer iterates over
> >>   buffer overlays.
> >> 
> >> I think this is an exceedingly dangerous design.
> >
> > Why, because of "no GC" requirement?  We could ensure that by calling
> > inhibit_garbage_collection (if the code doesn't do that already).
> 
> It doesn't.

Should be easy to fix, no?

> BTW, if anything signals in step 2, so that end_iteration isn't called,
> we're also hosed.

record_unwind_protect should fix that, right?
(inhibit_garbage_collection already employs this mechanism).

> > What higher-level operations require "interval tree iteration" that
> > you describe?  Which primitives end up doing such iterations?
> 
> What has to do with overlays.  To name a few: overlay-at, overlays-in,
> next-overlay-change, previous-overlay-change, overlay-lists, ...
> 
> I personally think this is a no-go.

Really?  Even if we take all the measures mentioned above?  Why so?