Fix the type value of 'bpy.types.*.bl_rna'

The type of an RNA types "bl_rna" should be `bpy.types.Struct`,
it was being set to the type it represented so:
`type(bpy.types.Object.bl_rna) == bpy.types.Object`
which wasn't correct.

Ref !126877

Co-authored-by: Bastien Montagne <bastien@blender.org>

source/blender/python/intern/bpy_rna.cc

@@ -90,6 +90,11 @@
 
 BPy_StructRNA *bpy_context_module = nullptr; /* for fast access */
 
+static PyObject *pyrna_struct_CreatePyObject_from_type(PointerRNA *ptr,
+                                                       PyTypeObject *tp,
+                                                       void **instance);
+
+static PyObject *pyrna_srna_Subtype(StructRNA *srna);
 static PyObject *pyrna_struct_Subtype(PointerRNA *ptr);
 static PyObject *pyrna_prop_collection_values(BPy_PropertyRNA *self);
 
@@ -7347,8 +7352,6 @@ static void pyrna_prop_collection_iter_dealloc(BPy_PropertyCollectionIterRNA *se
 
 static void pyrna_subtype_set_rna(PyObject *newclass, StructRNA *srna)
 {
-  PyObject *item;
-
   Py_INCREF(newclass);
 
   if (PyObject *oldclass = static_cast<PyObject *>(RNA_struct_py_type_get(srna))) {
@@ -7362,12 +7365,25 @@ static void pyrna_subtype_set_rna(PyObject *newclass, StructRNA *srna)
 
   /* Python deals with the circular reference. */
   PointerRNA ptr = RNA_pointer_create(nullptr, &RNA_Struct, srna);
-  item = pyrna_struct_CreatePyObject(&ptr);
 
-  /* NOTE: must set the class not the __dict__ else the internal slots are not updated correctly.
-   */
-  PyObject_SetAttr(newclass, bpy_intern_str_bl_rna, item);
-  Py_DECREF(item);
+  /* NOTE: using `pyrna_struct_CreatePyObject(&ptr)` is close to what is needed,
+   * however the it isn't correct because the result of:
+   * `type(bpy.types.Object.bl_rna) == bpy.types.Object`.
+   * In this case the type of `bl_rna` should be `bpy.types.Struct`.
+   * This is passed in explicitly, while #pyrna_struct_CreatePyObject could
+   * take this as an argument it's such a corner case that using a lower level
+   * function that takes the type is preferable. */
+  {
+    BLI_assert(RNA_struct_instance(&ptr) == nullptr);
+    PyTypeObject *tp = (PyTypeObject *)pyrna_srna_Subtype(&RNA_Struct);
+    PyObject *item = pyrna_struct_CreatePyObject_from_type(&ptr, tp, nullptr);
+    Py_DECREF(tp); /* `srna` owns, can't hold a reference. */
+
+    /* NOTE: must set the class not the `__dict__`
+     * else the internal slots are not updated correctly. */
+    PyObject_SetAttr(newclass, bpy_intern_str_bl_rna, item);
+    Py_DECREF(item);
+  }
 
   /* Add `staticmethod` and `classmethod` functions. */
   {
@@ -7392,8 +7408,6 @@ static void pyrna_subtype_set_rna(PyObject *newclass, StructRNA *srna)
   /* Done with RNA instance. */
 }
 
-static PyObject *pyrna_srna_Subtype(StructRNA *srna);
-
 /* Return a borrowed reference. */
 static PyObject *pyrna_srna_PyBase(StructRNA *srna)  //, PyObject *bpy_types_dict)
 {
@@ -7604,38 +7618,16 @@ static PyObject *pyrna_struct_Subtype(PointerRNA *ptr)
 
 /*-----------------------CreatePyObject---------------------------------*/
 
-PyObject *pyrna_struct_CreatePyObject(PointerRNA *ptr)
+/**
+ * A lower level version of #pyrna_struct_CreatePyObject,
+ * use this when type (`tp`) needs to be set to a non-standard value.
+ */
+static PyObject *pyrna_struct_CreatePyObject_from_type(PointerRNA *ptr,
+                                                       PyTypeObject *tp,
+                                                       void **instance)
 {
   BPy_StructRNA *pyrna = nullptr;
-
-  /* NOTE: don't rely on this to return None since nullptr data with a valid type can often crash.
-   */
-  if (ptr->data == nullptr && ptr->type == nullptr) { /* Operator RNA has nullptr data. */
-    Py_RETURN_NONE;
-  }
-
-  /* New in 2.8x, since not many types support instancing
-   * we may want to use a flag to avoid looping over all classes. - campbell */
-  void **instance = ptr->data ? RNA_struct_instance(ptr) : nullptr;
-  if (instance && *instance) {
-    pyrna = static_cast<BPy_StructRNA *>(*instance);
-
-    /* Refine may have changed types after the first instance was created. */
-    if (ptr->type == pyrna->ptr.type) {
-      Py_INCREF(pyrna);
-      return (PyObject *)pyrna;
-    }
-
-    /* Existing users will need to use 'type_recast' method. */
-    Py_DECREF(pyrna);
-    *instance = nullptr;
-/* Continue as if no instance was made. */
-#if 0 /* No need to assign, will be written to next... */
-    pyrna = nullptr;
-#endif
-  }
-
-  if (PyTypeObject *tp = (PyTypeObject *)pyrna_struct_Subtype(ptr)) {
+  if (tp) {
     pyrna = (BPy_StructRNA *)tp->tp_alloc(tp, 0);
 #ifdef USE_PYRNA_STRUCT_REFERENCE
     /* #PyType_GenericAlloc will have set tracking.
@@ -7644,7 +7636,6 @@ PyObject *pyrna_struct_CreatePyObject(PointerRNA *ptr)
       PyObject_GC_UnTrack(pyrna);
     }
 #endif
-    Py_DECREF(tp); /* srna owns, can't hold a reference. */
   }
   else {
     CLOG_WARN(BPY_LOG_RNA, "could not make type '%s'", RNA_struct_identifier(ptr->type));
@@ -7698,6 +7689,43 @@ PyObject *pyrna_struct_CreatePyObject(PointerRNA *ptr)
   return (PyObject *)pyrna;
 }
 
+PyObject *pyrna_struct_CreatePyObject(PointerRNA *ptr)
+{
+  BPy_StructRNA *pyrna = nullptr;
+
+  /* NOTE: don't rely on this to return None since nullptr data with a valid type can often crash.
+   */
+  if (ptr->data == nullptr && ptr->type == nullptr) { /* Operator RNA has nullptr data. */
+    Py_RETURN_NONE;
+  }
+
+  /* New in 2.8x, since not many types support instancing
+   * we may want to use a flag to avoid looping over all classes. - campbell */
+  void **instance = ptr->data ? RNA_struct_instance(ptr) : nullptr;
+  if (instance && *instance) {
+    pyrna = static_cast<BPy_StructRNA *>(*instance);
+
+    /* Refine may have changed types after the first instance was created. */
+    if (ptr->type == pyrna->ptr.type) {
+      Py_INCREF(pyrna);
+      return (PyObject *)pyrna;
+    }
+
+    /* Existing users will need to use 'type_recast' method. */
+    Py_DECREF(pyrna);
+    *instance = nullptr;
+/* Continue as if no instance was made. */
+#if 0 /* No need to assign, will be written to next... */
+    pyrna = nullptr;
+#endif
+  }
+
+  PyTypeObject *tp = (PyTypeObject *)pyrna_struct_Subtype(ptr);
+  pyrna = (BPy_StructRNA *)pyrna_struct_CreatePyObject_from_type(ptr, tp, instance);
+  Py_XDECREF(tp); /* `srna` owns, can't hold a reference. */
+  return (PyObject *)pyrna;
+}
+
 PyObject *pyrna_struct_CreatePyObject_with_primitive_support(PointerRNA *ptr)
 {
   if (ptr->type == &RNA_PrimitiveString) {