Add awaiter pointer to 3.12

Summary:
There's a good chance we're going to need to forward port the async stack walking implementation
in 3.10 to 3.12. That relies on the presence of the awaiter pointer in coroutines (and we'd like
to support async generators too). Let's get this in early, even if we don't end up needing it,
since it's an ABI breaking change.

Reviewed By: DinoV

Differential Revision: D56263282

fbshipit-source-id: 5539cdb6713beeb8374b473f82e0b227e5f65729

Include/cpython/genobject.h

@@ -21,6 +21,9 @@ extern "C" {
     PyObject *prefix##_qualname;                                            \
     _PyErr_StackItem prefix##_exc_state;                                    \
     PyObject *prefix##_origin_or_finalizer;                                 \
+    /* The awaiter (a coroutine or async generator), if any, if this is a   \
+       coroutine or async generator */                                      \
+    PyObject *prefix##_ci_awaiter;                                          \
     char prefix##_hooks_inited;                                             \
     char prefix##_closed;                                                   \
     char prefix##_running_async;                                            \

Lib/test/test_sys.py

@@ -1504,7 +1504,7 @@ def bar(cls):
             check(bar, size('PP'))
         # generator
         def get_gen(): yield 1
-        check(get_gen(), size('PP4P4c7P2ic??2P'))
+        check(get_gen(), size('PP5P4c7P2ic??2P'))
         # iterator
         check(iter('abc'), size('lP'))
         # callable-iterator

Objects/genobject.c

@@ -982,6 +982,7 @@ gen_new_with_qualname(PyTypeObject *type, PyFrameObject *f,
         gen->gi_qualname = Py_NewRef(qualname);
     else
         gen->gi_qualname = Py_NewRef(_PyGen_GetCode(gen)->co_qualname);
+    gen->gi_ci_awaiter = NULL;
     _PyObject_GC_TRACK(gen);
     return (PyObject *)gen;
 }