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

[Eli Zaretskii]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Date: Thu, 29 Sep 2022 07:29:25 +0200
> 
> In its current form, interval tree iteration works like this:
> 
> 1. Call begin_iteration(T) to iterate over tree T
> 2. do stuff
> 3. Call end_iteration(T)
> 
> with the following rules:
> 
> - Begin_iteration and end_iteration must be paired.
> 
> - There can be only one iteration per tree at a time.  Nested iteration
>   over the same tree is not supported (abort).
> 
> - 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).
That is not very elegant, and might even cause memory pressure in some
(hopefully rare) situations, but we do have some users of this in the
sources.

What higher-level operations require "interval tree iteration" that
you describe?  Which primitives end up doing such iterations?