# io.BufferedRWPair doesn't clear its weakrefs on deallocation:
# 
# static void
# bufferedrwpair_dealloc(rwpair *self)
# {
#     _PyObject_GC_UNTRACK(self);
#     Py_CLEAR(self->reader);
#     Py_CLEAR(self->writer);
#     Py_CLEAR(self->dict);
#     Py_TYPE(self)->tp_free((PyObject *) self);
# }
# 
# Weakrefs to this object contain stale pointer after BufferedRWPair is freed. 
# Deleting such a weakref triggers use-after-free:
# 
# Program received signal SIGSEGV, Segmentation fault.
# 0x0817b65e in clear_weakref (self=0x40644034) at Objects/weakrefobject.c:56
# 56              PyWeakReference **list = GET_WEAKREFS_LISTPTR(self->wr_object);
# (gdb) print *self->wr_object
# $1 = {_ob_next = 0xdbdbdbdb, _ob_prev = 0xdbdbdbdb, ob_refcnt = -606348325,
#   ob_type = 0xdbdbdbdb}
# (gdb) bt
# #0  0x0817b65e in clear_weakref (self=0x40644034) at Objects/weakrefobject.c:56
# #1  0x0817b815 in weakref_dealloc (self=<weakref at remote 0x40644034>)
#     at Objects/weakrefobject.c:106
# #2  0x080f4882 in _Py_Dealloc (op=<weakref at remote 0x40644034>) at Objects/object.c:1749

import io
import weakref

fn = "file.txt"
w = io.FileIO(fn, "w")
r = io.FileIO(fn, "r")
brw = io.BufferedRWPair(r,w)
ref = weakref.ref(brw)
del brw
del ref