Reproduce:
Right click python.exe -> properties -> compatibility tab -> enable compatibility mode (e.g. Windows 7)

Then run this test will get a segmentfault: test_multiprocessing_main_handling.py

CPython 3.6 is Ok.

Here is two traceback messages:

D:\git\cpython\PCbuild\win32>python.exe -X dev d:\git\cpython\Lib\test\test_multiprocess
ing_main_handling.py
ssssssssssssssssssssssssssFFFFFFFFFFFFF
======================================================================
FAIL: test_basic_script (__main__.SpawnCmdLineTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "d:\git\cpython\Lib\test\test_multiprocessing_main_handling.py", line 162, in test_basic_script
    self._check_script(script_name)
  File "d:\git\cpython\Lib\test\test_multiprocessing_main_handling.py", line 156, in _check_script
    rc, out, err = assert_python_ok(*run_args, __isolated=False)
  File "D:\git\cpython\lib\test\support\script_helper.py", line 151, in assert_python_ok

    return _assert_python(True, *args, **env_vars)
  File "D:\git\cpython\lib\test\support\script_helper.py", line 137, in _assert_python
    res.fail(cmd_line)
  File "D:\git\cpython\lib\test\support\script_helper.py", line 79, in fail
    err))
AssertionError: Process return code is 3221225477
command line: ['D:\\git\\cpython\\PCbuild\\win32\\python.exe', '-X', 'faulthandler', '-E
', 'C:\\Users\\anima\\AppData\\Local\\Temp\\tmplgfds7rc\\script.py', 'spawn']

stdout:
---

---

stderr:
---
Windows fatal exception: access violation

Current thread 0x00001930 (most recent call first):
  File "D:\git\cpython\lib\multiprocessing\popen_spawn_win32.py", line 48 in __init__
  File "D:\git\cpython\lib\multiprocessing\context.py", line 322 in _Popen
  File "D:\git\cpython\lib\multiprocessing\process.py", line 112 in start
  File "D:\git\cpython\lib\multiprocessing\pool.py", line 241 in _repopulate_pool
  File "D:\git\cpython\lib\multiprocessing\pool.py", line 176 in __init__
  File "D:\git\cpython\lib\multiprocessing\context.py", line 119 in Pool
  File "C:\Users\anima\AppData\Local\Temp\tmplgfds7rc\script.py", line 27 in <module>
---

======================================================================
FAIL: test_basic_script_no_suffix (__main__.SpawnCmdLineTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "d:\git\cpython\Lib\test\test_multiprocessing_main_handling.py", line 168, in test_basic_script_no_suffix
    self._check_script(script_name)
  File "d:\git\cpython\Lib\test\test_multiprocessing_main_handling.py", line 156, in _check_script
    rc, out, err = assert_python_ok(*run_args, __isolated=False)
  File "D:\git\cpython\lib\test\support\script_helper.py", line 151, in assert_python_ok

    return _assert_python(True, *args, **env_vars)
  File "D:\git\cpython\lib\test\support\script_helper.py", line 137, in _assert_python
    res.fail(cmd_line)
  File "D:\git\cpython\lib\test\support\script_helper.py", line 79, in fail
    err))
AssertionError: Process return code is 3221225477
command line: ['D:\\git\\cpython\\PCbuild\\win32\\python.exe', '-X', 'faulthandler', '-E
', 'C:\\Users\\anima\\AppData\\Local\\Temp\\tmpkovn5nfz\\script', 'spawn']

stdout:
---

---

stderr:
---
Windows fatal exception: access violation

Current thread 0x00001d00 (most recent call first):
  File "D:\git\cpython\lib\multiprocessing\popen_spawn_win32.py", line 48 in __init__
  File "D:\git\cpython\lib\multiprocessing\context.py", line 322 in _Popen
  File "D:\git\cpython\lib\multiprocessing\process.py", line 112 in start
  File "D:\git\cpython\lib\multiprocessing\pool.py", line 241 in _repopulate_pool
  File "D:\git\cpython\lib\multiprocessing\pool.py", line 176 in __init__
  File "D:\git\cpython\lib\multiprocessing\context.py", line 119 in Pool
  File "C:\Users\anima\AppData\Local\Temp\tmpkovn5nfz\script", line 27 in <module>
---

segmentfault conditions:
64bit Windows + 32bit Python + Compatibility Mode enabled.

some infomation:
1, 64bit Python doesn't segmentfault.
2, 3.7.0a3 is ok, 3.7.0a4 segmentfaults.
3, unrelated to Windows SDK, switch from 1709 SDK to 1703 SDK also segmentfaults.

Git bisect says this is the first bad commit:
https://github.com/python/cpython/commit/b2a6083eb0384f38839d3f1ed32262a3852026fa

3.7.0 beta2 still segmentfault.
Finer, please have a look.

segmentfault conditions:
64bit Windows 10 (1709) + 32bit Python 3.7.0 Beta2 + Compatibility Mode enabled.
(64bit Python doesn't segmentfault)

Windows seems to dereference lpStartupInfo->lpAttributeList, despite it being NULL since we had no attributes to set. Smells like a Windows bug in the compatibility shim, since it doesn't crash in non compatibility mode.

Not setting EXTENDED_STARTUPINFO_PRESENT when lpStartupInfo->lpAttributeList is NULL will fix this. Not sure about lpStartupInfo->cb, I think Windows only checks it's large enough.

Also attached is a crash dump with symbols and used source revision of a reproduction of this.

Stack Trace:
00 00fee690 5ad47c1c AcLayers!NS_ElevateCreateProcess::APIHook_CreateProcessW+0x110
01 00fee740 5ad45f79 python38_d!_winapi_CreateProcess_impl+0x15c [cpython\modules\_winapi.c @ 1046]
02 00fee798 5ad78e75 python38_d!_winapi_CreateProcess+0x79 [cpython\modules\clinic\_winapi.c.h @ 326]
03 00fee7f8 5ad77b64 python38_d!_PyMethodDef_RawFastCallKeywords+0x245 [cpython\objects\call.c @ 651]
04 00fee81c 5aed8f9f python38_d!_PyCFunction_FastCallKeywords+0x84 [cpython\objects\call.c @ 730]
05 00fee854 5aee038f python38_d!call_function+0x15f [cpython\python\ceval.c @ 4532]
06 00fee8f4 5ad7b336 python38_d!_PyEval_EvalFrameDefault+0x59bf [cpython\python\ceval.c @ 3128]
07 00fee92c 5ad77ce4 python38_d!function_code_fastcall+0xe6 [cpython\objects\call.c @ 283]
08 00fee994 5ad79be7 python38_d!_PyFunction_FastCallDict+0x134 [cpython\objects\call.c @ 322]
09 00fee9d8 5ad79aa6 python38_d!_PyObject_FastCallDict+0xc7 [cpython\objects\call.c @ 98]
0a 00feea14 5ae0b8f7 python38_d!_PyObject_Call_Prepend+0xf6 [cpython\objects\call.c @ 904]
0b 00feea50 5ae10a97 python38_d!slot_tp_init+0x47 [cpython\objects\typeobject.c @ 6597]
0c 00feea74 5ad7a28e python38_d!type_call+0x187 [cpython\objects\typeobject.c @ 949]
0d 00feeae0 5aed91d1 python38_d!_PyObject_FastCallKeywords+0x37e [cpython\objects\call.c @ 199]
0e 00feeb18 5aedc95b python38_d!call_function+0x391 [cpython\python\ceval.c @ 4576]

messages: + msg313094