Fix reference counting for KSP from NewtonSolver in Python (#3190)

* Fix reference counting for KSP from NewtonSolver in Python

* Test reference count

python/dolfinx/wrappers/petsc.cpp

@@ -353,8 +353,7 @@ void petsc_nls_module(nb::module_& m)
                    {
                      KSP ksp = self.get_krylov_solver().ksp();
                      PyObject* obj = PyPetscKSP_New(ksp);
-                     PetscObjectDereference((PetscObject)ksp);
-                     return nb::borrow(obj);
+                     return nb::steal(obj);
                    })
       .def("setF", &dolfinx::nls::petsc::NewtonSolver::setF, nb::arg("F"),
            nb::arg("b"))

python/test/unit/nls/test_newton.py

@@ -104,7 +104,7 @@ def J(self, snes, x, J, P):
 @pytest.mark.petsc4py
 class TestNLS:
     def test_linear_pde(self):
-        """Test Newton solver for a linear PDE"""
+        """Test Newton solver for a linear PDE."""
         from petsc4py import PETSc
 
         # Create mesh and function space
@@ -144,6 +144,12 @@ def update(solver, dx, x):
         assert converged
         assert n == 1
 
+        # Check reference counting
+        ksp = solver.krylov_solver
+        assert ksp.refcount == 2
+        del solver
+        assert ksp.refcount == 1
+
     def test_nonlinear_pde(self):
         """Test Newton solver for a simple nonlinear PDE"""
         from petsc4py import PETSc