[Python] Improved fix for cleanup of Solution change callbacks

Fixes a case not handled by 2158ff9 for "weak" wrappers where errors
could occur if the destructor for the weak wrapper was called after the
destructor for the owning Solution object.

Fixes #1489

interfaces/cython/cantera/ctcxx.pxd

@@ -11,7 +11,7 @@ from libcpp.cast cimport dynamic_cast
 from libcpp.pair cimport pair
 from libcpp cimport bool as cbool
 from libcpp.functional cimport function
-from libcpp.memory cimport shared_ptr, dynamic_pointer_cast
+from libcpp.memory cimport shared_ptr, weak_ptr, dynamic_pointer_cast
 from cpython cimport bool as pybool
 from cpython.ref cimport PyObject
 from cython.operator cimport dereference as deref, preincrement as inc

interfaces/cython/cantera/solutionbase.pxd

@@ -107,6 +107,7 @@ cdef object _wrap_Solution(shared_ptr[CxxSolution] cxx_soln)
 
 cdef class _SolutionBase:
     cdef shared_ptr[CxxSolution] _base
+    cdef weak_ptr[CxxSolution] weak_base
     cdef CxxSolution* base
     cdef CxxThermoPhase* thermo
     cdef CxxKinetics* kinetics

interfaces/cython/cantera/solutionbase.pyx

@@ -113,8 +113,9 @@ cdef class _SolutionBase:
             self.name = name
 
     def __del__(self):
-        if self.base:
-            self.base.removeChangedCallback(<PyObject*>self)
+        cdef shared_ptr[CxxSolution] shared = self.weak_base.lock()
+        if shared:
+            shared.get().removeChangedCallback(<PyObject*>self)
 
     property name:
         """
@@ -433,13 +434,18 @@ cdef class _SolutionBase:
 cdef _assign_Solution(_SolutionBase soln, shared_ptr[CxxSolution] cxx_soln,
                       pybool reset_adjacent, pybool weak=False):
     if not weak:
-        # When the main application isn't Python, we should only hold a weak reference
-        # here, since the C++ Solution object owns this Python Solution.
+        # _SolutionBase owns the C++ Solution object by holding the shared_ptr instance
         if soln._base.get() != NULL:
             soln._base.get().removeChangedCallback(<PyObject*>(soln))
         soln._base = cxx_soln
+    # Make a raw pointer available for most use cases, where existence of the C++
+    # Solution object is assured.
     soln.base = cxx_soln.get()
 
+    # Hold a weak_ptr for use in the _SolutionBase destructor, where the C++
+    # object may have already been destroyed depending on ownership.
+    soln.weak_base = weak_ptr[CxxSolution](cxx_soln)
+
     def assign_pointers():
         soln.thermo = soln.base.thermo().get()
         soln.kinetics = soln.base.kinetics().get()