bpo-38787: C API for module state access from extension methods (PEP 573) (GH-19936)

Module C state is now accessible from C-defined heap type methods (PEP 573).
Patch by Marcel Plch and Petr Viktorin.

Co-authored-by: Marcel Plch <mplch@redhat.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Include/cpython/methodobject.h

@@ -0,0 +1,32 @@
+#ifndef Py_CPYTHON_METHODOBJECT_H
+#  error "this header file must not be included directly"
+#endif
+
+PyAPI_DATA(PyTypeObject) PyCMethod_Type;
+
+/* Macros for direct access to these values. Type checks are *not*
+   done, so use with care. */
+#define PyCFunction_GET_FUNCTION(func) \
+        (((PyCFunctionObject *)func) -> m_ml -> ml_meth)
+#define PyCFunction_GET_SELF(func) \
+        (((PyCFunctionObject *)func) -> m_ml -> ml_flags & METH_STATIC ? \
+         NULL : ((PyCFunctionObject *)func) -> m_self)
+#define PyCFunction_GET_FLAGS(func) \
+        (((PyCFunctionObject *)func) -> m_ml -> ml_flags)
+#define PyCFunction_GET_CLASS(func) \
+    (((PyCFunctionObject *)func) -> m_ml -> ml_flags & METH_METHOD ? \
+         ((PyCMethodObject *)func) -> mm_class : NULL)
+
+typedef struct {
+    PyObject_HEAD
+    PyMethodDef *m_ml; /* Description of the C function to call */
+    PyObject    *m_self; /* Passed as 'self' arg to the C func, can be NULL */
+    PyObject    *m_module; /* The __module__ attribute, can be anything */
+    PyObject    *m_weakreflist; /* List of weak references */
+    vectorcallfunc vectorcall;
+} PyCFunctionObject;
+
+typedef struct {
+    PyCFunctionObject func;
+    PyTypeObject *mm_class; /* Class that defines this method */
+} PyCMethodObject;