[WIP] Improve guarding of Py_DECREF, Py_XDECREF triggered from C++ dtors

include/pybind11/detail/type_caster_base.h

@@ -77,7 +77,7 @@ class loader_life_support {
         }
         frame = parent;
         for (auto *item : keep_alive) {
-            Py_DECREF(item);
+            Py_DECREF(item); // decref in C++ dtor: always safe?
         }
     }
 
@@ -727,7 +727,7 @@ struct shared_ptr_parent_life_support {
     // NOLINTNEXTLINE(readability-make-member-function-const)
     void operator()(void *) {
         gil_scoped_acquire gil;
-        Py_DECREF(parent);
+        Py_DECREF(parent); // decref in C++ dtor: NEEDS can_decref_now
     }
 };
 
@@ -741,7 +741,7 @@ struct shared_ptr_trampoline_self_life_support {
     // NOLINTNEXTLINE(readability-make-member-function-const)
     void operator()(void *) {
         gil_scoped_acquire gil;
-        Py_DECREF(self);
+        Py_DECREF(self); // decref in C++ dtor: NEEDS can_decref_now
     }
 };
 

include/pybind11/trampoline_self_life_support.h

@@ -42,7 +42,9 @@ struct trampoline_self_life_support {
                 v_h.value_ptr() = nullptr;
                 v_h.holder<smart_holder>().release_disowned();
                 detail::deregister_instance(v_h.inst, value_void_ptr, v_h.type);
-                Py_DECREF((PyObject *) v_h.inst); // Must be after deregister.
+                Py_DECREF(
+                    (PyObject *)
+                        v_h.inst); // Must be after deregister. // decref in C++ dtor: always safe?
                 PyGILState_Release(threadstate);
             }
         }