The BM_mesh_bm_to_me_for_eval() cal be called on the same BMesh
from multiple threads. This adds a restriction that this function
should not modify the BMesh. This started to be violated quite
madly during the generic attributes changes.
This change makes it so that the BMesh is not modified.
The code looks less functional-like, but it solves the threading
conflict.
Ideally the BMesh will be const in the function but doing it now
is a bit tricky due to the other APIs.
The repro case for the crash is a bit tricky to reproduce from
scratch. For those who has access to the Heis production repo
/pro/lib/char/pack_bot/pack_bot.blend file can be used. Simply
add loop to "GEO-leg.R" object and use bevel operator on the
new loop.
There is still some write of the element indices happening in
this function. In theory those could be removed (together with
the dirty index tag clear) but it leads to obscure crashes in
area far away from this one. I've left it unchanged for now as
on 64bit platforms those assignments should not be causing real
issues.
Differential Revision: https://developer.blender.org/D16023
a2966f6477