Use a POD struct defining the memory layout of wrappers to access the underlying cyobject

Cython/Compiler/CypclassWrapper.py

@@ -294,7 +294,7 @@ def NAME(self, ARGDECLS):
         stats = []
         if not cclass_bases.args:
             # the memory layout for the underlying cyobject should always be the same
-            # -> maybe use a single common base cclass in the future
+            # and match the memory layout of CyPyObject.
             underlying_cyobject = self.synthesize_underlying_cyobject_attribute(node)
             stats.append(underlying_cyobject)
 
@@ -344,7 +344,7 @@ def NAME(self, ARGDECLS):
         underlying_name_declarator = Nodes.CNameDeclaratorNode(
             node.pos,
             name=underlying_name,
-            cname=Naming.cypclass_attr_cname
+            cname=Naming.cypclass_wrapper_underlying_attr
         )
 
         underlying_cyobject = Nodes.CVarDefNode(
@@ -1023,18 +1023,9 @@ def generate_cyp_class_wrapper_definition(type, wrapper_entry, constructor_entry
     # __new__ can be defined by user and return another type
     is_new_return_type = not new_entry or new_entry.type.return_type == type
 
-    # initialise PyObject fields
+    # allocate and initialise PyObject fields
     if is_new_return_type and type.wrapper_type:
-        objstruct_cname = type.wrapper_type.objstruct_cname
-        cclass_wrapper_base = type.wrapped_base_type().wrapper_type
-        code.putln("if(self) {")
-        code.putln("%s * wrapper = (%s *) ::operator new(sizeof *wrapper);" % (objstruct_cname, objstruct_cname))
-        code.putln("((%s *)wrapper)->%s = self;" % (cclass_wrapper_base.objstruct_cname, Naming.cypclass_attr_cname))
-        code.putln("PyObject * wrapper_as_py = (PyObject *) wrapper;")
-        code.putln("wrapper_as_py->ob_refcnt = 0;")
-        code.putln("wrapper_as_py->ob_type = %s;" % type.wrapper_type.typeptr_cname)
-        code.putln("self->cy_pyobject = wrapper_as_py;")
-        code.putln("}")
+        generate_cypclass_wrapper_allocation(code, type.wrapper_type)
 
     if init_entry:
         init_entry = PyrexTypes.best_match(wrapper_arg_types,
@@ -1124,3 +1115,18 @@ def generate_cyp_class_wrapper_definition(type, wrapper_entry, constructor_entry
     code.putln("return self;")
     code.putln("}")
 
+def generate_cypclass_wrapper_allocation(code, wrapper_type):
+    """
+        Generate allocation and essential setup of the wrapper object.
+        The cname of the cyobject is assumed to be 'self'.
+        The cname 'wrapper' is assumed to be available.
+    """
+
+    objstruct_cname = wrapper_type.objstruct_cname
+    code.putln("if (self) {")
+    code.putln("%s * wrapper = (%s *) ::operator new(sizeof *wrapper);" % (objstruct_cname, objstruct_cname))
+    code.putln("((PyObject *)wrapper)->ob_refcnt = 0;")
+    code.putln("((PyObject *)wrapper)->ob_type = %s;" % wrapper_type.typeptr_cname)
+    code.putln("((%s *)wrapper)->%s = self;" % (Naming.cypclass_wrapper_layout_type, Naming.cypclass_wrapper_underlying_attr))
+    code.putln("self->cy_pyobject = (PyObject *) wrapper;")
+    code.putln("}")

Cython/Compiler/ModuleNode.py

@@ -1686,7 +1686,7 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
         # for cyp wrappers, just decrement the atomic counter of the underlying type
         parent_type = scope.parent_type
         if parent_type.is_cyp_wrapper:
-            underlying_attribute_name = Naming.cypclass_attr_cname
+            underlying_attribute_name = Naming.cypclass_wrapper_underlying_attr
             self.generate_self_cast(scope, code)
             code.putln(
                 "CyObject * p_nogil_cyobject = p->%s;"

Cython/Compiler/Naming.py

@@ -166,8 +166,12 @@ exc_vars = (exc_type_name, exc_value_name, exc_tb_name)
 
 api_name        = pyrex_prefix + "capi__"
 
-# c name for underlying cypclass attribute in cypclass wrappers
-cypclass_attr_cname = "nogil_cyobject"
+# cname for the type that defines the essential memory layout of a cypclass wrapper.
+cypclass_wrapper_layout_type = "CyPyObject"
+
+# cname for the underlying cypclass attribute in the memory layout of a cypclass wrapper.
+cypclass_wrapper_underlying_attr = "nogil_cyobject"
+
 
 # the h and api guards get changed to:
 #  __PYX_HAVE__FILENAME (for ascii filenames)

Cython/Compiler/PyrexTypes.py

@@ -4180,7 +4180,6 @@ class CypClassType(CppClassType):
     #  _mro               [CppClassType] or None       the Method Resolution Order of this cypclass according to Python
     #  support_wrapper    boolean                      whether this cypclass will be wrapped
     #  wrapper_type       PyExtensionType or None      the type of the cclass wrapper
-    #  _wrapped_base_type CypClassType or None         the type of the oldest wrapped cypclass base
 
     is_cyp_class = 1
     to_py_function = None
@@ -4195,20 +4194,6 @@ class CypClassType(CppClassType):
         self.wrapper_type = None
         self._wrapped_base_type = None
 
-    # return the oldest left-path superclass such that all intervening classes have a wrapper
-    def wrapped_base_type(self):
-        # if the result has already been computed, return it
-        if self._wrapped_base_type is not None:
-            return self._wrapped_base_type
-        # find the first wrapped base (if there is one) and take the same oldest superclass
-        for base_type in self.base_classes:
-            if base_type.is_cyp_class and base_type.support_wrapper:
-                self._wrapped_base_type = base_type.wrapped_base_type()
-                return self._wrapped_base_type
-        # if no wrapped base was found, this type is the oldest wrapped base
-        self._wrapped_base_type = self
-        return self
-
     # iterate over the direct bases that support wrapping
     def iter_wrapped_base_types(self):
         for base_type in self.base_classes:
@@ -4252,9 +4237,8 @@ class CypClassType(CppClassType):
     def create_from_py_utility_code(self, env):
         if not self.wrapper_type:
             return False
-        wrapper_objstruct = self.wrapped_base_type().wrapper_type.objstruct_cname
         underlying_type_name = self.cname
-        self.from_py_function = "__Pyx_PyObject_AsCyObject<%s, %s>" % (wrapper_objstruct, underlying_type_name)
+        self.from_py_function = "__Pyx_PyObject_AsCyObject<%s>" % underlying_type_name
         return True
 
     def from_py_call_code(self, source_code, result_code, error_pos, code,

Cython/Utility/CyObjects.cpp

@@ -75,6 +75,19 @@
           int CyObject_TRYWLOCK();
     };
 
+    /*
+     * A POD type that has a compatible memory layout with any wrapper for a cypclass.
+     *
+     * Serves as a:
+     *  - convenience type to cast a wrapper and access its underlying cyobject pointer.
+     *  - reference for the memory layout that all cypclass wrappers must respect.
+     */
+    struct CyPyObject {
+        PyObject_HEAD
+        CyObject * nogil_cyobject;
+    };
+
+
     /* All this is made available by member injection inside the module scope */
 
     struct ActhonResultInterface : public CyObject {
@@ -200,10 +213,9 @@
      *  - return NULL
      * 
      * template:
-     *  - W: the type of the extension type wrapper
      *  - U: the type of the underlying cypclass
      */
-    template <typename W, typename U>
+    template <typename U>
     static inline U* __Pyx_PyObject_AsCyObject(PyObject * ob, PyTypeObject * type) {
         // the PyObject is not of the expected type
         if (ob->ob_type != type) {
@@ -211,7 +223,7 @@
             return NULL;
         }
 
-        W * wrapper = (W *)ob;
+        CyPyObject * wrapper = (CyPyObject *)ob;
         U * underlying = dynamic_cast<U *>(wrapper->nogil_cyobject);
 
         // no underlying cyobject: shouldn't happen, playing it safe for now