Manually clean up PETSc objects (#899)

Garbage collection works differently (worse) in petsc4py 3.18.3
than in 3.17.4.

Fixes #896

fipy/matrices/petscMatrix.py

@@ -24,6 +24,9 @@ def __init__(self, matrix):
         """
         self.matrix = matrix
 
+    def __del__(self):
+        self.matrix.destroy()
+
     def copy(self):
         return _PETScMatrix(matrix=self.matrix.copy())
 
@@ -547,6 +550,11 @@ def copy(self):
         copy.matrix = tmp.matrix
         return copy
 
+    def __del__(self):
+        if hasattr(self, "_ao_"):
+            self._ao_.destroy()
+        super(_PETScBaseMeshMatrix, self).__del__()
+
     @property
     def _ao(self):
         """Application Ordering to relate FiPy matrix rows to PETSc matrix rows

fipy/solvers/petsc/petscKrylovSolver.py

@@ -67,3 +67,5 @@ def _solve_(self, L, x, b):
         self._log.debug('iterations: %d / %d', ksp.its, self.iterations)
         self._log.debug('norm: %s', ksp.norm)
         self._log.debug('norm_type: %s', ksp.norm_type)
+
+        ksp.destroy()

fipy/solvers/petsc/petscSolver.py

@@ -38,7 +38,7 @@ def _globalMatrixAndVectors(self):
         return self.globalVectors
 
     def _deleteGlobalMatrixAndVectors(self):
-        self.matrix.flush()
+        del self.matrix
         if hasattr(self, "globalVectors"):
             globalMatrix, overlappingVector, overlappingRHSvector = self._globalMatrixAndVectors
             overlappingVector.destroy()
@@ -106,5 +106,6 @@ def _calcRHSNorm(self):
     def __del__(self):
         if hasattr(self, "globalVectors"):
             globalMatrix, overlappingVector, overlappingRHSvector = self._globalMatrixAndVectors
+            del globalMatrix
             overlappingVector.destroy()
             overlappingRHSvector.destroy()

fipy/solvers/solver.py

@@ -169,3 +169,6 @@ def __enter__(self):
 
     def __exit__(self, exc_type, exc_value, traceback):
         pass
+
+    def __del__(self):
+        pass