Add PyMethod_SetSelf API to let extensions can change method's im_self

field

Some extensions, like Zope2.ExtensionClass(PyECMethod_New_ function),
will change instance method's im_self field in some situation. They do
it by access the field of PyMethodObject directly. We can't do it, so
provide a new API to do it.

from_cpython/Include/classobject.h

@@ -75,6 +75,9 @@ PyAPI_FUNC(PyObject *) PyMethod_Function(PyObject *) PYSTON_NOEXCEPT;
 PyAPI_FUNC(PyObject *) PyMethod_Self(PyObject *) PYSTON_NOEXCEPT;
 PyAPI_FUNC(PyObject *) PyMethod_Class(PyObject *) PYSTON_NOEXCEPT;
 
+// Pyston change: add help API to allow extensions to set the fields of PyMethodObject
+ PyAPI_FUNC(int) PyMethod_SetSelf(PyObject *, PyObject*) PYSTON_NOEXCEPT;
+
 /* Look up attribute with name (a string) on instance object pinst, using
  * only the instance and base class dicts.  If a descriptor is found in
  * a class dict, the descriptor is returned without calling it.

src/runtime/classobj.cpp

@@ -1927,6 +1927,19 @@ extern "C" PyObject* PyMethod_Self(PyObject* im) noexcept {
     return ((BoxedInstanceMethod*)im)->obj;
 }
 
+extern "C" int PyMethod_SetSelf(PyObject* im, PyObject* instance) noexcept {
+    if (!PyMethod_Check(im)) {
+        PyErr_BadInternalCall();
+        return -1;
+    }
+
+    Box* old = ((BoxedInstanceMethod*)im)->obj;
+    Py_INCREF(instance);
+    ((BoxedInstanceMethod*)im)->obj = instance;
+    Py_XDECREF(old);
+    return 1;
+}
+
 extern "C" PyObject* PyMethod_Class(PyObject* im) noexcept {
     if (!PyMethod_Check(im)) {
         PyErr_BadInternalCall();

test/test_extension/api_test.c

@@ -15,9 +15,27 @@ test_attrwrapper_parse(PyObject *self, PyObject* args) {
     Py_RETURN_NONE;
 }
 
+static PyObject*
+change_self(PyObject* self, PyObject* args) {
+    PyObject *im, *inst;
+    if (!PyArg_ParseTuple(args, "OO", &im, &inst))
+        return NULL;
+
+#if defined(PYSTON_VERSION)
+    PyMethod_SetSelf(im, inst);
+#else
+    Py_XDECREF(((PyMethodObject*)im)->im_self);
+    Py_INCREF(inst);
+    ((PyMethodObject*)im)->im_self = inst;
+#endif
+    Py_RETURN_NONE;
+}
+
 static PyMethodDef TestMethods[] = {
     {"set_size",  set_size, METH_O, "Get set size by PySet_Size." },
     {"test_attrwrapper_parse",  test_attrwrapper_parse, METH_VARARGS, "Test PyArg_ParseTuple for attrwrappers." },
+    {"change_self",  change_self, METH_VARARGS,
+     "A function which the self point to its base class."},
     {NULL, NULL, 0, NULL}        /* Sentinel */
 };