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

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> 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).

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.

But yes, the current restriction to have a single iteration at a time is
a bit of a problem, especially because it's very "global".  I added
a comment yesterday describing how we could make it non-global (hence
getting rid of the `visited` flag in the nodes).

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


        Stefan