gen_iternext(): repair subtle refcount problem.
NeilS, please check! This came from staring at your genbug.py, but I'm
not sure it plugs all possible holes. Without this, I caught a
frameobject refcount going negative, and it was also the cause (in debug
build) of _Py_ForgetReference's attempt to forget an object with already-
NULL _ob_prev and _ob_next pointers -- although I'm still not entirely
sure how! Part of the difficulty is that frameobjects are stored on a
free list that gets recycled very quickly, so if there's a stray pointer
to one of them it never looks like an insane frameobject (never goes
trough the free() mangling MS debug forces, etc).
diff --git a/Python/ceval.c b/Python/ceval.c
index cf917db..ea5c537 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -152,6 +152,11 @@
gen->running = 1;
result = eval_frame(f);
gen->running = 0;
+ /* The connection between this frame and its parent is over now, so
+ must NULL out f_back lest it get decref'ed when gen dies (note
+ that eval_frame sets f->f_back without bumping its refcount: we
+ never had a fully legit reference to it). */
+ f->f_back = NULL;
return result;
}