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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Gerd Möllmann <gerd.moellmann@gmail.com>,
>   58158@debbugs.gnu.org
> Date: Thu, 29 Sep 2022 09:10:19 -0400
> 
> > I may be missing something, but it looks like the sole purpose of the
> > iter_start/iter_finish dance is to ensure only one iteration per tree
> > is running at any given time, and that's because the iteration uses
> > some state variable(s) of which there's only one instance per tree.
> >
> > Stefan, am I missing something?
> 
> One reason is that traversing a binary tree usually requires something
> like recursion, but that wouldn't fit very conveniently with the current
> code (nor with C in general since you can't make a local recursive
> closure which accesses local variables from the surrounding function).

I'm not sure I understand how recursion is related.  Are you saying
that recursion is replaced with iteration?  But then, if _start and
_finish are called by the same caller, we don't really need the
protection, since no one can start another iteration while the first
one runs.  Right?

> Another is the need to update the begin/end fields (these need updating
> because of insertions/deletions but they're updated lazily while
> traversing the tree to avoid an O(N) complexity during the
> insertions/deletions).  Hiding that behind 'some kind of "next node"
> function keeps the code more readable.

Where in the code do you see iteration that adds or deletes nodes?

> For now, I pushed a simple fix to traverse the tree "by hand" in the GC
> rather than via the iterator.

So this removes the restriction of not having GC during iteration?

Also, I take it that you don't consider the current code is as
"harmful" as Gerd thinks it is?  IOW, you don't share his opinion that
this implementation is a "no-go"?