Fix T94708: negative reference count error with Python API callbacks

Regression in 7972785d7b90771f50534fe3e1101d8adb615fa3 that caused
Python callback arguments to be de-referenced twice - potentially
accessing freed memory. Making a new-file with a circle-select
tool active triggered this (for example).

Now arguments aren't de-referenced when Blender it's self has already
removed the callback handle.

1 files changed, 11 insertions, 5 deletions

diff --git a/source/blender/python/intern/bpy_rna_callback.c b/source/blender/python/intern/bpy_rna_callback.c
index 16968cebab4..16ea4bf3e5e 100644
--- a/source/blender/python/intern/bpy_rna_callback.c
+++ b/source/blender/python/intern/bpy_rna_callback.c
@@ -383,6 +383,7 @@ PyObject *pyrna_callback_classmethod_remove(PyObject *UNUSED(self), PyObject *ar
   void *handle;
   StructRNA *srna;
   bool capsule_clear = false;
+  bool handle_removed = false;
 
   if (PyTuple_GET_SIZE(args) < 2) {
     PyErr_SetString(PyExc_ValueError, "callback_remove(handler): expected at least 2 args");
@@ -406,7 +407,7 @@ PyObject *pyrna_callback_classmethod_remove(PyObject *UNUSED(self), PyObject *ar
             args, "OO!:WindowManager.draw_cursor_remove", &cls, &PyCapsule_Type, &py_handle)) {
       return NULL;
     }
-    WM_paint_cursor_end(handle);
+    handle_removed = WM_paint_cursor_end(handle);
     capsule_clear = true;
   }
   else if (RNA_struct_is_a(srna, &RNA_Space)) {
@@ -445,7 +446,7 @@ PyObject *pyrna_callback_classmethod_remove(PyObject *UNUSED(self), PyObject *ar
                    params.region_type_enum.value_orig);
       return NULL;
     }
-    ED_region_draw_cb_exit(art, handle);
+    handle_removed = ED_region_draw_cb_exit(art, handle);
     capsule_clear = true;
   }
   else {
@@ -453,9 +454,14 @@ PyObject *pyrna_callback_classmethod_remove(PyObject *UNUSED(self), PyObject *ar
     return NULL;
   }
 
-  /* The handle has been removed, so decrement its customdata. */
-  PyObject *handle_args = PyCapsule_GetContext(py_handle);
-  Py_DECREF(handle_args);
+  /* When `handle_removed == false`: Blender has already freed the data
+   * (freeing screen data when loading a new file for example).
+   * This will have already decremented the user, so don't decrement twice. */
+  if (handle_removed == true) {
+    /* The handle has been removed, so decrement its custom-data. */
+    PyObject *handle_args = PyCapsule_GetContext(py_handle);
+    Py_DECREF(handle_args);
+  }
 
   /* don't allow reuse */
   if (capsule_clear) {