Safer handling of PETSc destruction in high-level classes (#3951)

* Safer handling of PETSc destruction

* Fix across the board

* Follow @schnellerhase's logic and fix solver wrapper test

* Ruff

* Rename o->obj

Author: jorgensd

python/dolfinx/fem/petsc.py

@@ -879,12 +879,10 @@ def __init__(
             self.solver.getPC().setFieldSplitIS(*fieldsplit_IS)
 
     def __del__(self):
-        self._solver.destroy()
-        self._A.destroy()
-        self._b.destroy()
-        self._x.destroy()
-        if self._P_mat is not None:
-            self._P_mat.destroy()
+        for obj in filter(
+            lambda obj: obj is not None, (self._solver, self._A, self._b, self._x, self._P_mat)
+        ):
+            obj.destroy()
 
     def solve(self) -> _Function | Sequence[_Function]:
         """Solve the problem.
@@ -1359,12 +1357,10 @@ def solve(self) -> _Function | Sequence[_Function]:
         return self.u
 
     def __del__(self):
-        self._snes.destroy()
-        self._x.destroy()
-        self._A.destroy()
-        self._b.destroy()
-        if self._P_mat is not None:
-            self._P_mat.destroy()
+        for obj in filter(
+            lambda obj: obj is not None, (self._snes, self._A, self._b, self._x, self._P_mat)
+        ):
+            obj.destroy()
 
     @property
     def F(self) -> Form | Sequence[Form]:

python/dolfinx/nls/petsc.py

@@ -64,8 +64,8 @@ def __init__(self, comm: MPI.Intracomm, problem: NewtonSolverNonlinearProblem):
         self.set_form(problem.form)
 
     def __del__(self):
-        self._A.destroy()
-        self._b.destroy()
+        for obj in filter(lambda obj: obj is not None, (self._A, self._b)):
+            obj.destroy()
 
     def solve(self, u: fem.Function):
         """Solve non-linear problem into function u. Returns the number

python/test/unit/fem/test_petsc_solver_wrappers.py

@@ -125,6 +125,8 @@ def test_compare_solution_linear_vs_nonlinear_problem(self, mode):
     def test_mixed_system(self, mode, kind):
         from petsc4py import PETSc
 
+        import dolfinx.fem.petsc
+
         msh = dolfinx.mesh.create_unit_square(
             MPI.COMM_WORLD, 12, 12, ghost_mode=mode, dtype=PETSc.RealType
         )