bugfix: Avoid PETSc IS size query during boundary rebuild

[gthyagi]

Summary

Fixes #285 on the development branch.

This applies the same boundary-label rebuild fix as the release-targeted PR: _dm_unstack_bcs() now tests the returned PETSc IS object by truthiness instead of calling getSize() before setting the stratum.

Root cause

Imported DMPlex meshes can include non-boundary labels such as Elements / cell-region labels. During UW_Boundaries reconstruction, label.getStratumIS() can return a PETSc IS object that should not be dereferenced with getSize() just to decide whether it is usable. On the issue #285 reproducer, that size query aborts the process before the Stokes system is assembled.

Using the PETSc object truthiness check avoids the crash while preserving the existing label reconstruction behavior for valid strata.

Validation

• ./uw build
• PYTHONFAULTHANDLER=1 ./uw python -X faulthandler -m pytest tests/test_1012_stokesImportedDMPlex.py::test_stokes_essential_bc_imported_dmplex -ra -vv -s passed: 1 passed, 3 warnings

Related

• Release branch PR: bugfix: Avoid PETSc IS size query during boundary rebuild #287