Using 3.5.2-2ubuntu0~16.04.3 (Xenial) we see an occasional segfault during garbage collection of a generator object

A full backtrace is attached, but the crash appears to be triggered inside gen_traverse during gc

---
(gdb) info args
gen = 0x7f22385f0150
visit = 0x50eaa0 <visit_decref>
arg = 0x0

(gdb) print *gen
$109 = {ob_base = {ob_refcnt = 1, ob_type = 0xa35760 <PyGen_Type>}, gi_frame = 0x386aed8, gi_running = 1 '\001', gi_code = <code at remote 0x7f223bb42f60>, gi_weakreflist = 0x0, gi_name = 'linesplit', gi_qualname = 'linesplit'}
---

I believe gen_traverse is doing the following

---
static int
gen_traverse(PyGenObject *gen, visitproc visit, void *arg)
{
    Py_VISIT((PyObject *)gen->gi_frame);
    Py_VISIT(gen->gi_code);
    Py_VISIT(gen->gi_name);
    Py_VISIT(gen->gi_qualname);
    return 0;
}
---

The problem here being that this generator's gen->gi_frame has managed to acquire a NULL object type but still has references

---
(gdb) print *gen->gi_frame
$112 = {ob_base = {ob_base = {ob_refcnt = 2, ob_type = 0x0}, ob_size = 0}, f_back = 0x0, f_code = 0xca3e4fd8950fef91, ...
---

Thus it gets visited and it doesn't go well.

I have attached the py-bt as well, it's very deep with ansible, multiprocessing forking, imp.load_source() importing ... basically a nightmare.  I have not managed to get it down to any sort of minimal test case unfortunately.  This happens fairly infrequently, so suggests a race.  The generator in question has a socket involved:

---
def linesplit(socket):
    buff = socket.recv(4096).decode("utf-8")
    buffering = True
    while buffering:
        if "\n" in buff:
            (line, buff) = buff.split("\n", 1)
            yield line + "\n"
        else:
            more = socket.recv(4096).decode("utf-8")
            if not more:
                buffering = False
            else:
                buff += more
    if buff:
        yield buff
---

Wild speculation but maybe something to do with finalizing generators with file-descriptors across fork()?

At this point we are trying a work-around of not having the above socket reading routine in a generator but just a "regular" loop.  As it triggers as part of a production roll-out I'm not sure we can do too much more debugging.  Unless this rings any immediate bells for people, we can probably just have this for tracking at this point.  [1] is the original upstream issue
 
[1] https://storyboard.openstack.org/#!/story/2001186#comment-17441