ElementSubdomainModifier should not modify SubdomainIDs during the element loop

[dschwen]

Reason

ElementSubdomainModifier is currently changing element->subdomain_id() in execute(). This will cause UOs in the same execution stage operating on the same element (or face of the same element), see a subdomain ID that is inconsistent with the current subdomain set in onSubdomainChange. This will trip this assertion for example.

Design

Store all subdomain id reassignments in a list first and only apply them to the elements in finalize.

Impact

More robust capability. Ping @dewenyushu

[dewenyushu]

Nice finding!

[hugary1995]

Caching is a fix. The ultimate fix in my opinion is to set up a dependency graph for runtime mesh modifiers (including this element subdomain modifier), pretty much like the current MeshGenerators.

[dschwen]

Yeah, multiple options here

• A runtime mesh modifier system that runs independently of UOs
• Multiple UO loops
• A custom execute_on for a second UO loop :-p

I was going for the lowest hanging fruit.

[hugary1995]

Several things we frequently use at hand could be unified in to the runtime mesh modifier system, such as XFEM and CZM. I know I'm dreaming right now, but if we are heading towards that direction, I'd be happy to help.

[dschwen]

I like that idea. The scope is not small though and would require some funding. Maybe we can propose a workpackage that could cover this activity for next FY?

[hugary1995]

Sure, let's talk before FY24 planning.