Fix #127127: bpy.types sometimes reference their parent's "bl_rna"

Accessing `bl_rna` for type defined in `scripts/modules/bpy_types.py`
could return their parent-classes RNA when accessed via their parent
types __subclasses__() method.

source/blender/python/intern/bpy.cc

@@ -740,7 +740,8 @@ void BPy_init_modules(bContext *C)
   Py_DECREF(mod);
 
   /* needs to be first so bpy_types can run */
-  PyModule_AddObject(mod, "types", BPY_rna_types());
+  PyObject *bpy_types = BPY_rna_types();
+  PyModule_AddObject(mod, "types", bpy_types);
 
   /* needs to be first so bpy_types can run */
   BPY_library_load_type_ready();
@@ -752,6 +753,8 @@ void BPy_init_modules(bContext *C)
   bpy_import_test("bpy_types");
   PyModule_AddObject(mod, "data", BPY_rna_module()); /* imports bpy_types by running this */
   bpy_import_test("bpy_types");
+  BPY_rna_types_finalize_external_types(bpy_types);
+
   PyModule_AddObject(mod, "props", BPY_rna_props());
   /* ops is now a python module that does the conversion from SOME_OT_foo -> some.foo */
   PyModule_AddObject(mod, "ops", BPY_operator_module());

source/blender/python/intern/bpy_rna.cc

@@ -101,6 +101,8 @@ static PyObject *pyrna_prop_collection_values(BPy_PropertyRNA *self);
 static PyObject *pyrna_register_class(PyObject *self, PyObject *py_class);
 static PyObject *pyrna_unregister_class(PyObject *self, PyObject *py_class);
 
+static StructRNA *srna_from_ptr(PointerRNA *ptr);
+
 #define BPY_DOC_ID_PROP_TYPE_NOTE \
   "   .. note::\n" \
   "\n" \
@@ -8066,6 +8068,57 @@ PyObject *BPY_rna_types()
   return submodule;
 }
 
+void BPY_rna_types_finalize_external_types(PyObject *submodule)
+{
+  /* NOTE: Blender is generally functional without running this logic
+   * however failure set the classes `bl_rna` (via `pyrna_subtype_set_rna`)
+   * means *partially* initialized classes exist.
+   * It's simpler to avoid this altogether as it's a corner case Python developers should
+   * not have to concern themselves with as it could cause errors with RNA introspection.
+   *
+   * If the classes are accessed via `bpy.types` they will be initialized correctly
+   * however classes can also be accessed via `bpy.types.ID.__subclasses__()`
+   * which doesn't ensure the `bl_rna` is set. See: #127127. */
+
+  BPy_TypesModule_State *state = static_cast<BPy_TypesModule_State *>(
+      PyModule_GetState(submodule));
+
+  PyObject *arg_key, *arg_value;
+  Py_ssize_t arg_pos = 0;
+  while (PyDict_Next(bpy_types_dict, &arg_pos, &arg_key, &arg_value)) {
+    const char *key_str = PyUnicode_AsUTF8(arg_key);
+    if (key_str[0] == '_') {
+      continue;
+    }
+
+    BLI_assert_msg(
+        PyObject_IsSubclass(arg_value, (PyObject *)&pyrna_struct_Type),
+        "Members of bpy_types.py which are not StructRNA sub-classes must use a \"_\" prefix!");
+
+    PointerRNA newptr;
+    if (RNA_property_collection_lookup_string(&state->ptr, state->prop, key_str, &newptr)) {
+      StructRNA *srna = srna_from_ptr(&newptr);
+      /* Within the Python logic of `./scripts/modules/bpy_types.py`
+       * it's possible this was already initialized. */
+      if (RNA_struct_py_type_get(srna) == nullptr) {
+        pyrna_subtype_set_rna(arg_value, srna);
+      }
+    }
+#ifndef NDEBUG
+    else {
+      /* Avoid noisy warnings based on build-options. */
+#  ifndef WITH_USD
+      if (STREQ(key_str, "USDHook")) {
+        continue;
+      }
+#  endif
+      CLOG_WARN(
+          BPY_LOG_RNA, "bpy_types.py defines \"%.200s\" which is not a known RNA type!", key_str);
+    }
+#endif
+  }
+}
+
 /** \} */
 
 StructRNA *pyrna_struct_as_srna(PyObject *self, const bool parent, const char *error_prefix)

source/blender/python/intern/bpy_rna.h

@@ -189,6 +189,7 @@ PyObject *BPY_rna_module(void);
 void BPY_update_rna_module(void);
 // PyObject *BPY_rna_doc(void);
 PyObject *BPY_rna_types(void);
+void BPY_rna_types_finalize_external_types(PyObject *submodule);
 
 PyObject *pyrna_struct_CreatePyObject_with_primitive_support(PointerRNA *ptr);
 PyObject *pyrna_struct_CreatePyObject(PointerRNA *ptr);