Ran into Segfaults while trying to use pysnmp with 3.8.0rc1.
The code is running fine on 3.8.0b04.

$ python3.8
Python 3.8.0rc1 (default, Oct  2 2019, 14:15:18)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-36)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> class in6_addr_U(ctypes.Union):
...     _fields_ = [
...         ('__u6_addr8', ctypes.c_uint8 * 16),
...         ('__u6_addr16', ctypes.c_uint16 * 8),
...         ('__u6_addr32', ctypes.c_uint32 * 4),
...     ]
...
Segmentation fault

$ docker run -it python:3.8.0rc1-slim
Python 3.8.0rc1 (default, Oct  2 2019, 23:38:42)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> class in6_addr_U(ctypes.Union):
...     _fields_ = [
...         ('__u6_addr8', ctypes.c_uint8 * 16),
...         ('__u6_addr16', ctypes.c_uint16 * 8),
...         ('__u6_addr32', ctypes.c_uint32 * 4),
...     ]
...
$

The code is from here: https://github.com/etingof/pysnmp/blob/master/pysnmp/carrier/sockmsg.py#L47-L52

I am adding 3.8 regression since the report says 3.8.0b4 works and segfaults with 3.8.0RC1.

Also crashes on Windows (x64 and x86), and on 3.7.5rc1, so this is likely in our code.

I get this assertion in a debug build:

Assertion failed: actual_type_index <= MAX_ELEMENTS, file c:\projects\cpython\modules\_ctypes\stgdict.c, line 718

Increasing MAX_ELEMENTS fixes it, but I'm not sure what other impacts there are from doing that.

+Vinay who added the array handling that's hitting the limit.

Given this limit can easily be hit by user code, I'd like to see it turned into a proper check with an exception (or a dynamic array) rather than just an assertion. We should not segfault here.

while (length > 0) {
    actual_types[actual_type_index++] = &edict->ffi_type_pointer;
    assert(actual_type_index <= MAX_ELEMENTS);
    length--;
}

> We should not segfault here.

Agreed, MAX_ELEMENTS was set to be an upper bound which shouldn't be hit. I'll investigate with OP's example data and see where the bug is.

New changeset e8bedbddadaa86be6bd86dc32dbdbd53933a4988 by Vinay Sajip in branch 'master':
bpo-38368: Added fix for ctypes crash when handling arrays in structs… (GH-16589)
https://github.com/python/cpython/commit/e8bedbddadaa86be6bd86dc32dbdbd53933a4988

New changeset d004a5b082d0770682e6efbf03105a67469d4820 by Vinay Sajip in branch '3.8':
bpo-38368: Added fix for ctypes crash when handling arrays in structs/unions. (GH-16589) (GH-16671)
https://github.com/python/cpython/commit/d004a5b082d0770682e6efbf03105a67469d4820

New changeset 129c2b3d44a962fc5964ef8fd4e6b22dbb691a6c by Vinay Sajip in branch '3.7':
bpo-38368: Added fix for ctypes crash when handling arrays in structs/unions. (GH-16589) (GH-16672)
https://github.com/python/cpython/commit/129c2b3d44a962fc5964ef8fd4e6b22dbb691a6c

New changeset 1c61f2cc7ab44591b1832307a4022c927502d446 by Ned Deily (Vinay Sajip) in branch '3.7':
bpo-38368: Added fix for ctypes crash when handling arrays in structs/unions. (GH-16589) (GH-16672)
https://github.com/python/cpython/commit/1c61f2cc7ab44591b1832307a4022c927502d446

(fix released in 3.8.0 and 3.7.5)