Sometimes, I need to run multiprocessing tests multiple times in parallel to attempt to reproduce a race condition. Except that test_shared_memory_across_processes() fails in this case:

$ ./python -m test test_multiprocessing_spawn --fail-env-changed -v -j4 -F -m test_shared_memory_across_processes 

== CPython 3.9.0a5+ (heads/master:17b4733f2f, Apr 1 2020, 15:02:04) [GCC 9.2.1 20190827 (Red Hat 9.2.1-1)]
== Linux-5.5.9-200.fc31.x86_64-x86_64-with-glibc2.30 little-endian
== cwd: /home/vstinner/python/master/build/test_python_1133450
== CPU count: 8
== encodings: locale=UTF-8, FS=utf-8
0:00:00 load avg: 1.64 Run tests in parallel using 4 child processes
0:00:01 load avg: 1.64 [  1/1] test_multiprocessing_spawn failed
test_shared_memory_across_processes (test.test_multiprocessing_spawn.WithProcessesTestSharedMemory) ... ERROR

======================================================================
ERROR: test_shared_memory_across_processes (test.test_multiprocessing_spawn.WithProcessesTestSharedMemory)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/master/Lib/test/_test_multiprocessing.py", line 3862, in test_shared_memory_across_processes
    sms = shared_memory.SharedMemory('test02_tsmap', True, size=512)
  File "/home/vstinner/python/master/Lib/multiprocessing/shared_memory.py", line 100, in __init__
    self._fd = _posixshmem.shm_open(
FileExistsError: [Errno 17] File exists: '/test02_tsmap'

----------------------------------------------------------------------

Ran 1 test in 0.247s

FAILED (errors=1)
test test_multiprocessing_spawn failed
Kill <TestWorkerProcess #2 running test=test_multiprocessing_spawn pid=1133458 time=1.2 sec> process group
Kill <TestWorkerProcess #3 running test=test_multiprocessing_spawn pid=1133457 time=1.2 sec> process group
Kill <TestWorkerProcess #4 running test=test_multiprocessing_spawn pid=1133459 time=1.2 sec> process group

== Tests result: FAILURE ==

1 test failed:
    test_multiprocessing_spawn

Total duration: 1.2 sec
Tests result: FAILURE

Python 3.7 is not affected: it doesn't have test_shared_memory_across_processes().

The easy way to fix this test case is that don't define the shared memory block's name. `test_shared_memory_basics` has tested against the shared memory block's name, so canceling the shared memory block'name has no side-effect.

New changeset caa3ef284a2e5e5b9bdd6a9e619804122c842d80 by Hai Shi in branch 'master':
bpo-40135: Fix multiprocessing test_shared_memory_across_processes() (GH-19892)
https://github.com/python/cpython/commit/caa3ef284a2e5e5b9bdd6a9e619804122c842d80

Thanks for the fix Hai Shi!

New changeset 70fe95cdc9ac1b00d4f86b7525dca80caf7003e1 by Miss Islington (bot) in branch '3.8':
bpo-40135: Fix multiprocessing test_shared_memory_across_processes() (GH-19892)
https://github.com/python/cpython/commit/70fe95cdc9ac1b00d4f86b7525dca80caf7003e1